Internet Research Task Force (IRTF) S. Symington
Request for Comments: 6257 The MITRE Corporation
Category: Experimental S. Farrell
ISSN: 2070-1721 Trinity College Dublin
H. Weiss
P. Lovell
SPARTA, Inc.
May 2011
Bundle Security Protocol Specification
Abstract
This document defines the bundle security protocol, which provides
data integrity and confidentiality services for the Bundle Protocol.
Separate capabilities are provided to protect the bundle payload and
additional data that may be included within the bundle. We also
describe various security considerations including some policy
options.
This document is a product of the Delay-Tolerant Networking Research
Group and has been reviewed by that group. No objections to its
publication as an RFC were raised.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Delay-Tolerant Networking Research Group of the Internet Research
Task Force (IRTF). Documents approved for publication by the IRSG
are not a candidate for any level of Internet Standard; see Section 2
of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6257.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
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Table of Contents
1. Introduction ....................................................4
1.1. Related Documents ..........................................4
1.2. Terminology ................................................5
2. Security Blocks .................................................8
2.1. Abstract Security Block ....................................9
2.2. Bundle Authentication Block ...............................13
2.3. Payload Integrity Block ...................................15
2.4. Payload Confidentiality Block .............................16
2.5. Extension Security Block ..................................20
2.6. Parameters and Result Fields ..............................21
2.7. Key Transport .............................................23
2.8. PIB and PCB Combinations ..................................24
3. Security Processing ............................................25
3.1. Nodes as Policy Enforcement Points ........................26
3.2. Processing Order of Security Blocks .......................26
3.3. Security Regions ..........................................29
3.4. Canonicalization of Bundles ...............................31
3.5. Endpoint ID Confidentiality ...............................37
3.6. Bundles Received from Other Nodes .........................38
3.7. The At-Most-Once-Delivery Option ..........................39
3.8. Bundle Fragmentation and Reassembly .......................40
3.9. Reactive Fragmentation ....................................41
3.10. Attack Model .............................................42
4. Mandatory Ciphersuites .........................................42
4.1. BAB-HMAC ..................................................42
4.2. PIB-RSA-SHA256 ............................................43
4.3. PCB-RSA-AES128-PAYLOAD-PIB-PCB ............................44
4.4. ESB-RSA-AES128-EXT ........................................48
5. Key Management .................................................51
6. Default Security Policy ........................................51
7. Security Considerations ........................................53
8. Conformance ....................................................55
9. IANA Considerations ............................................56
9.1. Bundle Block Types ........................................56
9.2. Ciphersuite Numbers .......................................56
9.3. Ciphersuite Flags .........................................56
9.4. Parameters and Results ....................................57
10. References ....................................................58
10.1. Normative References .....................................58
10.2. Informative References ...................................59
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1. Introduction
This document defines security features for the Bundle Protocol
[DTNBP] intended for use in delay-tolerant networks, in order to
provide Delay-Tolerant Networking (DTN) security services.
The Bundle Protocol is used in DTNs that overlay multiple networks,
some of which may be challenged by limitations such as intermittent
and possibly unpredictable loss of connectivity, long or variable
delay, asymmetric data rates, and high error rates. The purpose of
the Bundle Protocol is to support interoperability across such
stressed networks. The Bundle Protocol is layered on top of
underlay-network-specific convergence layers, on top of network-
specific lower layers, to enable an application in one network to
communicate with an application in another network, both of which are
spanned by the DTN.
Security will be important for the Bundle Protocol. The stressed
environment of the underlying networks over which the Bundle Protocol
will operate makes it important for the DTN to be protected from
unauthorized use, and this stressed environment poses unique
challenges for the mechanisms needed to secure the Bundle Protocol.
Furthermore, DTNs may very likely be deployed in environments where a
portion of the network might become compromised, posing the usual
security challenges related to confidentiality, integrity, and
availability.
Different security processing applies to the payload and extension
blocks that may accompany it in a bundle, and different rules apply
to various extension blocks.
This document describes both the base Bundle Security Protocol (BSP)
and a set of mandatory ciphersuites. A ciphersuite is a specific
collection of various cryptographic algorithms and implementation
rules that are used together to provide certain security services.
The Bundle Security Protocol applies, by definition, only to those
nodes that implement it, known as "security-aware" nodes. There MAY
be other nodes in the DTN that do not implement BSP. All nodes can
interoperate with the exception that BSP security operations can only
happen at security-aware nodes.
1.1. Related Documents
This document is best read and understood within the context of the
following other DTN documents:
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"Delay-Tolerant Networking Architecture" [DTNarch] defines the
architecture for delay-tolerant networks, but does not discuss
security at any length.
The DTN Bundle Protocol [DTNBP] defines the format and processing
of the blocks used to implement the Bundle Protocol, excluding the
security-specific blocks defined here.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
We introduce the following terminology for purposes of clarity:
source - the bundle node from which a bundle originates
destination - the bundle node to which a bundle is ultimately
destined
forwarder - the bundle node that forwarded the bundle on its most
recent hop
intermediate receiver or "next hop" - the neighboring bundle node
to which a forwarder forwards a bundle.
path - the ordered sequence of nodes through which a bundle passes
on its way from source to destination
In the figure below, which is adapted from figure 1 in the Bundle
Protocol Specification [DTNBP], four bundle nodes (denoted BN1, BN2,
BN3, and BN4) reside above some transport layer(s). Three distinct
transport and network protocols (denoted T1/N1, T2/N2, and T3/N3) are
also shown.
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+---------v-| +->>>>>>>>>>v-+ +->>>>>>>>>>v-+ +-^---------+
| BN1 v | | ^ BN2 v | | ^ BN3 v | | ^ BN4 |
+---------v-+ +-^---------v-+ +-^---------v-+ +-^---------+
| T1 v | + ^ T1/T2 v | + ^ T2/T3 v | | ^ T3 |
+---------v-+ +-^---------v-+ +-^---------v + +-^---------+
| N1 v | | ^ N1/N2 v | | ^ N2/N3 v | | ^ N3 |
+---------v-+ +-^---------v + +-^---------v-+ +-^---------+
| >>>>>>>>^ >>>>>>>>>>^ >>>>>>>>^ |
+-----------+ +------------+ +-------------+ +-----------+
| | | |
|<-- An Internet --->| |<--- An Internet --->|
| | | |
BN = "Bundle Node" as defined in the Bundle Protocol Specification
Figure 1: Bundle Nodes Sit at the Application Layer
of the Internet Model
Bundle node BN1 originates a bundle that it forwards to BN2. BN2
forwards the bundle to BN3, and BN3 forwards the bundle to BN4. BN1
is the source of the bundle and BN4 is the destination of the bundle.
BN1 is the first forwarder, and BN2 is the first intermediate
receiver; BN2 then becomes the forwarder, and BN3 the intermediate
receiver; BN3 then becomes the last forwarder, and BN4 the last
intermediate receiver, as well as the destination.
If node BN2 originates a bundle (for example, a bundle status report
or a custodial signal), which is then forwarded on to BN3, and then
to BN4, then BN2 is the source of the bundle (as well as being the
first forwarder of the bundle) and BN4 is the destination of the
bundle (as well as being the final intermediate receiver).
We introduce the following security-specific DTN terminology:
security-source - a bundle node that adds a security block to a
bundle
security-destination - a bundle node that processes a security
block of a bundle
security path - the ordered sequence of security-aware nodes
through which a bundle passes on its way from the security-source
to the security-destination
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Referring to Figure 1 again:
If the bundle that originates at BN1 is given a security block by
BN1, then BN1 is the security-source of this bundle with respect to
that security block, as well as being the source of the bundle.
If the bundle that originates at BN1 is given a security block by
BN2, then BN2 is the security-source of this bundle with respect to
that security block, even though BN1 is the source.
If the bundle that originates at BN1 is given a security block by BN1
that is intended to be processed by BN3, then BN1 is the security-
source and BN3 is the security-destination with respect to this
security block. The security path for this block is BN1 to BN3.
A bundle MAY have multiple security blocks. The security-source of a
bundle, with respect to a given security block in the bundle, MAY be
the same as or different from the security-source of the bundle with
respect to a different security block in the bundle. Similarly, the
security-destination of a bundle, with respect to each of that
bundle's security blocks, MAY be the same or different. Therefore,
the security paths for various blocks MAY be, and often will be,
different.
If the bundle that originates at BN1 is given a security block by BN1
that is intended to be processed by BN3, and BN2 adds a security
block with security-destination BN4, the security paths for the two
blocks overlap but not completely. This problem is discussed further
in Section 3.3.
As required in [DTNBP], forwarding nodes MUST transmit blocks in a
bundle in the same order in which they were received. This
requirement applies to all DTN nodes, not just ones that implement
security processing. Blocks in a bundle MAY be added or deleted
according to the applicable specification, but those blocks that are
both received and transmitted MUST be transmitted in the same order
that they were received.
If a node is not security-aware, then it forwards the security blocks
in the bundle unchanged unless the bundle's block processing flags
specify otherwise. If a network has some nodes that are not
security-aware, then the block processing flags SHOULD be set such
that security blocks are not discarded at those nodes solely because
they cannot be processed there. Except for this, the non-security-
aware nodes are transparent relay points and are invisible as far as
security processing is concerned.
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The block sequence also indicates the order in which certain
significant actions have affected the bundle, and therefore the
sequence in which actions MUST occur in order to produce the bundle
at its destination.
2. Security Blocks
There are four types of security blocks that MAY be included in a
bundle. These are the Bundle Authentication Block (BAB), the Payload
Integrity Block (PIB), the Payload Confidentiality Block (PCB), and
the Extension Security Block (ESB).
The BAB is used to ensure the authenticity and integrity of the
bundle along a single hop from forwarder to intermediate receiver.
Since security blocks are only processed at security-aware nodes,
a "single hop" from a security-aware forwarder to the next
security-aware intermediate receiver might be more than one actual
hop. This situation is discussed further in Section 2.2.
The PIB is used to ensure the authenticity and integrity of the
payload from the PIB security-source, which creates the PIB, to
the PIB security-destination, which verifies the PIB
authenticator. The authentication information in the PIB MAY (if
the ciphersuite allows) be verified by any node in between the PIB
security-source and the PIB security-destination that has access
to the cryptographic keys and revocation status information
required to do so.
Since a BAB protects a bundle on a "hop-by-hop" basis and other
security blocks MAY be protecting over several hops or end-to-end,
whenever both are present, the BAB MUST form the "outer" layer of
protection -- that is, the BAB MUST always be calculated and added
to the bundle after all other security blocks have been calculated
and added to the bundle.
The PCB indicates that the payload has been encrypted, in whole or
in part, at the PCB security-source in order to protect the bundle
content while in transit to the PCB security-destination.
PIB and PCB protect the payload and are regarded as "payload-
related" for purposes of the security discussion in this document.
Other blocks are regarded as "non-payload" blocks. Of course, the
primary block is unique and has separate rules.
The ESB provides security for non-payload blocks in a bundle.
Therefore, ESB is not applied to PIBs or PCBs and, of course, is
not appropriate for either the payload block or primary block.
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Each of the security blocks uses the Canonical Bundle Block Format as
defined in the Bundle Protocol Specification. That is, each security
block is comprised of the following elements:
o Block-type code
o Block processing control flags
o Block EID-reference list (OPTIONAL)
o Block data length
o Block-type-specific data fields
Since the four security blocks have most fields in common, we can
shorten the description of the Block-type-specific data fields of
each security block if we first define an abstract security block
(ASB) and then specify each of the real blocks in terms of the fields
that are present/absent in an ASB. Note that no bundle ever contains
an actual ASB, which is simply a specification artifact.
2.1. Abstract Security Block
Many of the fields below use the "SDNV" type defined in [DTNBP].
SDNV stands for Self-Delimiting Numeric Value.
An ASB consists of the following mandatory and optional fields:
o Block-type code (one byte) - as in all bundle protocol blocks
except the primary bundle block. The block-type codes for the
security blocks are:
BundleAuthenticationBlock - BAB: 0x02
PayloadIntegrityBlock - PIB: 0x03
PayloadConfidentialityBlock - PCB: 0x04
ExtensionSecurityBlock - ESB: 0x09
o Block processing control flags (SDNV) - defined as in all bundle
protocol blocks except the primary bundle block (as described in
the Bundle Protocol Specification [DTNBP]). SDNV encoding is
described in the Bundle Protocol. There are no general
constraints on the use of the block processing control flags, and
some specific requirements are discussed later.
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o EID-references - composite field defined in [DTNBP] containing
references to one or two endpoint identifiers (EIDs). Presence of
the EID-reference field is indicated by the setting of the "Block
contains an EID-reference field" (EID_REF) bit of the block
processing control flags. If one or more references are present,
flags in the ciphersuite ID field, described below, specify which.
If no EID fields are present, then the composite field itself MUST
be omitted entirely and the EID_REF bit MUST be unset. A count
field of zero is not permitted.
o The possible EIDs are:
* (OPTIONAL) Security-source - specifies the security-source for
the block. If this is omitted, then the source of the bundle
is assumed to be the security-source unless otherwise
indicated.
* (OPTIONAL) Security-destination - specifies the security-
destination for the block. If this is omitted, then the
destination of the bundle is assumed to be the security-
destination unless otherwise indicated.
If two EIDs are present, security-source is first and security-
destination comes second.
o Block data length (SDNV) - as in all bundle protocol blocks except
the primary bundle block. SDNV encoding is described in the
Bundle Protocol.
o Block-type-specific data fields as follows:
* Ciphersuite ID (SDNV)
* Ciphersuite flags (SDNV)
* (OPTIONAL) Correlator - when more than one related block is
inserted, then this field MUST have the same value in each
related block instance. This is encoded as an SDNV. See the
note in Section 3.8 with regard to correlator values in bundle
fragments.
* (OPTIONAL) Ciphersuite-parameters - compound field of the next
two items
+ Ciphersuite-parameters length - specifies the length of the
following Ciphersuite-parameters data field and is encoded
as an SDNV.
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+ Ciphersuite-parameters data - parameters to be used with the
ciphersuite in use, e.g., a key identifier or initialization
vector (IV). See Section 2.6 for a list of potential
parameters and their encoding rules. The particular set of
parameters that is included in this field is defined as part
of the ciphersuite specification.
* (OPTIONAL) Security-result - compound field of the next two
items
+ Security-result length - contains the length of the next
field and is encoded as an SDNV.
+ Security-result data - contains the results of the
appropriate ciphersuite-specific calculation (e.g., a
signature, Message Authentication Code (MAC), or ciphertext
block key).
Although the diagram hints at a 32-bit layout, this is purely for the
purpose of exposition. Except for the "type" field, all fields are
variable in length.
+----------------+----------------+----------------+----------------+
| type | flags (SDNV) | EID-ref list(comp) |
+----------------+----------------+----------------+----------------+
| length (SDNV) | ciphersuite (SDNV) |
+----------------+----------------+----------------+----------------+
| ciphersuite flags (SDNV) | correlator (SDNV) |
+----------------+----------------+----------------+----------------+
|params len(SDNV)| ciphersuite params data |
+----------------+----------------+----------------+----------------+
|res-len (SDNV) | security-result data |
+----------------+----------------+----------------+----------------+
Figure 2: Abstract Security Block Structure
Some ciphersuites are specified in Section 4, which also specifies
the rules that MUST be satisfied by ciphersuite specifications.
Additional ciphersuites MAY be defined in separate specifications.
Ciphersuite IDs not specified are reserved. Implementations of the
Bundle Security Protocol decide which ciphersuites to support,
subject to the requirements of Section 4. It is RECOMMENDED that
implementations that allow additional ciphersuites permit ciphersuite
ID values at least up to and including 127, and they MAY decline to
allow larger ID values.
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The structure of the ciphersuite flags field is shown in Figure 3.
In each case, the presence of an optional field is indicated by
setting the value of the corresponding flag to one. A value of zero
indicates the corresponding optional field is missing. Presently,
there are five flags defined for the field; for convenience, these
are shown as they would be extracted from a single-byte SDNV. Future
additions may cause the field to grow to the left so, as with the
flags fields defined in [DTNBP], the description below numbers the
bit positions from the right rather than the standard RFC definition,
which numbers bits from the left.
src - bit 4 indicates whether the EID-reference field of the ASB
contains the optional reference to the security-source.
dest - bit 3 indicates whether the EID-reference field of the ASB
contains the optional reference to the security-destination.
parm - bit 2 indicates whether or not the ciphersuite-parameters
length and ciphersuite-parameters data fields are present.
corr - bit 1 indicates whether or not the ASB contains an optional
correlator.
res - bit 0 indicates whether or not the ASB contains the
security-result length and security-result data fields.
bits 5-6 are reserved for future use.
Bit Bit Bit Bit Bit Bit Bit
6 5 4 3 2 1 0
+-----+-----+-----+-----+-----+-----+-----+
| reserved | src |dest |parm |corr |res |
+-----+-----+-----+-----+-----+-----+-----+
Figure 3: Ciphersuite Flags
A little bit more terminology: if the block is a PIB, when we refer
to the PIB-source, we mean the security-source for the PIB as
represented by the EID-reference in the EID-reference field.
Similarly, we may refer to the "PCB-dest", meaning the security-
destination of the PCB, again as represented by an EID reference.
For example, referring to Figure 1 again, if the bundle that
originates at BN1 is given a Payload Confidentiality Block (PCB) by
BN1 that is protected using a key held by BN3, and it is given a
Payload Integrity Block (PIB) by BN1, then BN1 is both the PCB-source
and the PIB-source of the bundle, and BN3 is the PCB-destination of
the bundle.
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The correlator field is used to associate several related instances
of a security block. This can be used to place a BAB that contains
the ciphersuite information at the "front" of a (probably large)
bundle, and another correlated BAB that contains the security-result
at the "end" of the bundle. This allows even very memory-constrained
nodes to be able to process the bundle and verify the BAB. There are
similar use cases for multiple related instances of PIB and PCB as
will be seen below.
The ciphersuite specification MUST make it clear whether or not
multiple block instances are allowed, and if so, under what
conditions. Some ciphersuites can, of course, leave flexibility to
the implementation, whereas others might mandate a fixed number of
instances.
For convenience, we use the term "first block" to refer to the
initial block in a group of correlated blocks or to the single block
if there are no others in the set. Obviously, there can be several
unrelated groups in a bundle, each containing only one block or more
than one, and each having its own "first block".
2.2. Bundle Authentication Block
In this section, we describe typical BAB field values for two
scenarios -- where a single instance of the BAB contains all the
information and where two related instances are used, one "up front",
which contains the ciphersuite, and another following the payload,
which contains the security-result (e.g., a MAC).
For the case where a single BAB is used:
The block-type code field value MUST be 0x02.
The block processing control flags value can be set to whatever
values are required by local policy. Ciphersuite designers should
carefully consider the effect of setting flags that either discard
the block or delete the bundle in the event that this block cannot
be processed.
The ciphersuite ID MUST be documented as a hop-by-hop
authentication-ciphersuite that requires one instance of the BAB.
The correlator field MUST NOT be present.
The ciphersuite-parameters field MAY be present, if so specified
in the ciphersuite specification.
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An EID-reference to the security-source MAY be present. The
security-source can also be specified as part of key-information
described in Section 2.6 or another block such as the Previous-Hop
Insertion Block [PHIB]. The security-source might also be
inferred from some implementation-specific means such as the
convergence layer.
An EID-reference to the security-destination MAY be present and is
useful to ensure that the bundle has been forwarded to the correct
next-hop node.
The security-result MUST be present as it is effectively the
"output" from the ciphersuite calculation (e.g., the MAC or
signature) applied to the (relevant parts of the) bundle (as
specified in the ciphersuite definition).
For the case using two related BAB instances, the first instance is
as defined above, except the ciphersuite ID MUST be documented as a
hop-by-hop authentication ciphersuite that requires two instances of
the BAB. In addition, the correlator MUST be present and the
security-result length and security-result fields MUST be absent.
The second instance of the BAB MUST have the same correlator value
present and MUST contain security-result length and security-result
data fields. The other optional fields MUST NOT be present.
Typically, this second instance of a BAB will be the last block of
the bundle.
The details of key transport for BAB are specified by the particular
ciphersuite. In the absence of conflicting requirements, the
following should be noted by implementors:
o the key-information item in Section 2.6 is OPTIONAL, and if not
provided, then the key SHOULD be inferred from the source-
destination tuple, being the previous key used, a key created from
a key-derivation function, or a pre-shared key.
o if all the nodes are security-aware, the capabilities of the
underlying convergence layer might be useful for identifying the
security-source.
o depending upon the key mechanism used, bundles can be signed by
the sender, or authenticated for one or more recipients, or both.
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2.3. Payload Integrity Block
A PIB is an ASB with the following additional restrictions:
The block-type code value MUST be 0x03.
The block processing control flags value can be set to whatever
values are required by local policy. Ciphersuite designers should
carefully consider the effect of setting flags that either discard
the block or delete the bundle in the event that this block cannot
be processed.
The ciphersuite ID MUST be documented as an end-to-end
authentication-ciphersuite or as an end-to-end error-detection-
ciphersuite.
The correlator MUST be present if the ciphersuite requires that
more than one related instance of a PIB be present in the bundle.
The correlator MUST NOT be present if the ciphersuite only
requires one instance of the PIB in the bundle.
The ciphersuite-parameters field MAY be present.
An EID-reference to the security-source MAY be present. The
security-source can also be specified as part of key-information
described in Section 2.6.
An EID-reference to the security-destination MAY be present.
The security-result is effectively the "output" from the
ciphersuite calculation (e.g., the MAC or signature) applied to
the (relevant parts of the) bundle. As in the case of the BAB,
this field MUST be present if the correlator is absent. If more
than one related instance of the PIB is required, then this is
handled in the same way as described for the BAB above.
The ciphersuite MAY process less than the entire original bundle
payload. This might be because it is defined to process some
subset of the bundle, or perhaps because the current payload is a
fragment of an original bundle. For whatever reason, if the
ciphersuite processes less than the complete, original bundle
payload, the ciphersuite-parameters of this block MUST specify
which bytes of the bundle payload are protected.
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For some ciphersuites, (e.g., those using asymmetric keying to
produce signatures or those using symmetric keying with a group key),
the security information can be checked at any hop on the way to the
security-destination that has access to the required keying
information. This possibility is further discussed in Section 3.6.
The use of a generally available key is RECOMMENDED if custodial
transfer is employed and all nodes SHOULD verify the bundle before
accepting custody.
Most asymmetric PIB ciphersuites will use the PIB-source to indicate
who the signer is and will not require the PIB-dest field because the
key needed to verify the PIB authenticator will be a public key
associated with the PIB-source.
2.4. Payload Confidentiality Block
A typical confidentiality ciphersuite will encrypt the payload using
a randomly generated bundle encrypting key (BEK) and will use a key-
information item in the PCB security-parameters to carry the BEK
encrypted with some long-term key encryption key (KEK) or well-known
public key. If neither the destination nor security-destination
resolves the key to use for decryption, the key-information item in
the ciphersuite-parameters field can also be used to indicate the
decryption key with which the BEK can be recovered. If the bundle
already contains PIBs and/or PCBs, these SHOULD also be encrypted
using this same BEK, as described just below for "super-encryption".
The encrypted block is encapsulated into a new PCB that replaces the
original block at the same place in the bundle.
It is strongly RECOMMENDED that a data integrity mechanism be used in
conjunction with confidentiality, and that encryption-only
ciphersuites NOT be used. AES-Galois/Counter Mode (AES-GCM)
satisfies this requirement. The "authentication tag" or "integrity
check value" is stored into the security-result rather than being
appended to the payload as is common in some protocols since, as
described below, it is important that there be no change in the size
of the payload.
The payload is encrypted "in-place", that is, following encryption,
the payload block payload field contains ciphertext, not plaintext.
The payload block processing control flags are unmodified.
The "in-place" encryption of payload bytes is to allow bundle payload
fragmentation and reassembly, and custody transfer, to operate
without knowledge of whether or not encryption has occurred and, if
so, how many times.
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Fragmentation, reassembly, and custody transfer are adversely
affected by a change in size of the payload due to ambiguity about
what byte range of the original payload is actually in any particular
fragment. Ciphersuites SHOULD place any payload expansion, such as
authentication tags (integrity check values) and any padding
generated by a block-mode cipher, into an integrity check value item
in the security-result field (see Section 2.6) of the confidentiality
block.
Payload super-encryption is allowed, that is, encrypting a payload
that has already been encrypted, perhaps more than once.
Ciphersuites SHOULD define super-encryption such that, as well as re-
encrypting the payload, it also protects the parameters of earlier
encryption. Failure to do so may represent a vulnerability in some
circumstances.
Confidentiality is normally applied to the payload, and possibly to
additional blocks. It is RECOMMENDED to apply a Payload
Confidentiality ciphersuite to non-payload blocks only if these
SHOULD be super-encrypted with the payload. If super-encryption of
the block is not desired, then protection of the block SHOULD be done
using the Extension Security Block mechanism rather than PCB.
Multiple related PCB instances are required if both the payload and
PIBs and PCBs in the bundle are to be encrypted. These multiple PCB
instances require correlators to associate them with each other since
the key-information is provided only in the first PCB.
There are situations where more than one PCB instance is required but
the instances are not "related" in the sense that requires
correlators. One example is where a payload is encrypted for more
than one security-destination so as to be robust in the face of
routing uncertainties. In this scenario, the payload is encrypted
using a BEK. Several PCBs contain the BEK encrypted using different
KEKs, one for each destination. These multiple PCB instances are not
"related" and SHOULD NOT contain correlators.
The ciphersuite MAY apply different rules to confidentiality for non-
payload blocks.
A PCB is an ASB with the following additional restrictions:
The block-type code value MUST be 0x04.
The block processing control flags value can be set to whatever
values are required by local policy, except that a PCB "first
block" MUST have the "replicate in every fragment" flag set. This
flag SHOULD NOT be set otherwise. Ciphersuite designers should
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carefully consider the effect of setting flags that either discard
the block or delete the bundle in the event that this block cannot
be processed.
The ciphersuite ID MUST be documented as a confidentiality
ciphersuite.
The correlator MUST be present if there is more than one related
PCB instance. The correlator MUST NOT be present if there are no
related PCB instances.
If a correlator is present, the key-information MUST be placed in
the PCB "first block".
Any additional bytes generated as a result of encryption and/or
authentication processing of the payload SHOULD be placed in an
"integrity check value" field (see Section 2.6) in the security-
result of the first PCB.
The ciphersuite-parameters field MAY be present.
An EID-reference to the security-source MAY be present. The
security-source can also be specified as part of key-information
described in Section 2.6.
An EID-reference to the security-destination MAY be present.
The security-result MAY be present and normally contains fields
such as an encrypted bundle encryption key, authentication tag, or
the encrypted versions of bundle blocks other than the payload
block.
The ciphersuite MAY process less than the entire original bundle
payload, either because the current payload is a fragment of the
original bundle or just because it is defined to process some subset.
For whatever reason, if the ciphersuite processes less than the
complete, original bundle payload, the "first" PCB MUST specify, as
part of the ciphersuite-parameters, which bytes of the bundle payload
are protected.
PCB ciphersuites MUST specify which blocks are to be encrypted. The
specification MAY be flexible and be dependent upon block type,
security policy, various data values, and other inputs, but it MUST
be deterministic. The determination of whether or not a block is to
be encrypted MUST NOT be ambiguous.
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As was the case for the BAB and PIB, if the ciphersuite requires more
than one instance of the PCB, then the "first block" MUST contain any
optional fields (e.g., security-destination, etc.) that apply to all
instances with this correlator. These MUST be contained in the first
instance and MUST NOT be repeated in other correlated blocks. Fields
that are specific to a particular instance of the PCB MAY appear in
that PCB. For example, security-result fields MAY (and probably
will) be included in multiple related PCB instances, with each result
being specific to that particular block. Similarly, several PCBs
might each contain a ciphersuite-parameters field with an IV specific
to that PCB instance.
Put another way: when confidentiality will generate multiple blocks,
it MUST create a "first" PCB with the required ciphersuite ID,
parameters, etc., as specified above. Typically, this PCB will
appear early in the bundle. This "first" PCB contains the parameters
that apply to the payload and also to the other correlated PCBs. The
correlated PCBs follow the "first" PCB and MUST NOT repeat the
ciphersuite-parameters, security-source, or security-destination
fields from the first PCB. These correlated PCBs need not follow
immediately after the "first" PCB, and probably will not do so. Each
correlated block, encapsulating an encrypted PIB or PCB, is at the
same place in the bundle as the original PIB or PCB.
A ciphersuite MUST NOT mix payload data and a non-payload block in a
single PCB.
Even if a to-be-encrypted block has the "discard" flag set, whether
or not the PCB's "discard" flag is set is an implementation/policy
decision for the encrypting node. (The "discard" flag is more
properly called the "Discard if block can't be processed" flag.)
Any existing EID-list in the to-be-encapsulated original block
remains exactly as-is, and is copied to become the EID-list for the
replacing block. The encapsulation process MUST NOT replace or
remove the existing EID-list entries. This is critically important
for correct updating of entries at the security-destination.
At the security-destination, either the specific destination or the
bundle-destination, the processes described above are reversed. The
payload is decrypted "in-place" using the salt, IV, and key values in
the first PCB, including verification using the ICV. These values
are described in Section 2.6. Each correlated PCB is also processed
at the same destination, using the salt and key values from the first
PCB and the block-specific IV item. The encapsulated block item in
the security-result is decrypted and validated, using also the tag
that SHOULD have been appended to the ciphertext of the original
block data. Assuming the validation succeeds, the resultant
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plaintext, which is the entire content of the original block,
replaces the PCB at the same place in the bundle. The block type
reverts to that of the original block prior to encapsulation, and the
other block-specific data fields also return to their original
values. Implementors are cautioned that this "replacement" process
requires delicate stitchery, as the EID-list contents in the
decapsulated block are invalid. As noted above, the EID-list
references in the original block were preserved in the "replacing"
PCB, and will have been updated as necessary as the bundle has toured
the DTN. The references from the PCB MUST replace the references
within the EID-list of the newly decapsulated block. Caveat
implementor.
2.5. Extension Security Block
Extension security blocks provide protection for non-payload-related
portions of a bundle. ESBs MUST NOT be used for the primary block or
payload, including payload-related security blocks (PIBs and PCBs).
It is sometimes desirable to protect certain parts of a bundle in
ways other than those applied to the bundle payload. One such
example is bundle metadata that might specify the kind of data in the
payload but not the actual payload detail, as described in [DTNMD].
ESBs are typically used to apply confidentiality protection. While
it is possible to create an integrity-only ciphersuite, the block
protection is not transparent and makes access to the data more
difficult. For simplicity, this discussion describes the use of a
confidentiality ciphersuite.
The protection mechanisms in ESBs are similar to other security
blocks with two important differences:
o different key values are used (using the same key as that for
payload would defeat the purpose)
o the block is not encrypted or super-encrypted with the payload
A typical ESB ciphersuite will encrypt the extension block using a
randomly generated ephemeral key and will use the key-information
item in the security-parameters field to carry the key encrypted with
some long-term key encryption key (KEK) or well-known public key. If
neither the destination nor security-destination resolves the key to
use for decryption, the key-information item in the ciphersuite-
parameters field can be used also to indicate the decryption key with
which the BEK can be recovered.
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It is strongly RECOMMENDED that a data integrity mechanism be used in
conjunction with confidentiality, and that encryption-only
ciphersuites NOT be used. AES-GCM satisfies this requirement.
The ESB is placed in the bundle in the same position as the block
being protected. That is, the entire original block is processed
(encrypted, etc.) and encapsulated in a "replacing" ESB-type block,
and this appears in the bundle at the same sequential position as the
original block. The processed data is placed in the security-result
field.
The process is reversed at the security-destination with the
recovered plaintext block replacing the ESB that had encapsulated it.
Processing of EID-list entries, if any, is described in Section 2.4,
and this MUST be followed in order to correctly recover EIDs.
An ESB is an ASB with the following additional restrictions:
The block type is 0x09.
Ciphersuite flags indicate which fields are present in this block.
Ciphersuite designers should carefully consider the effect of
setting flags that either discard the block or delete the bundle
in the event that this block cannot be processed.
EID-references MUST be stored in the EID-reference list.
The security-source MAY be present. The security-source can also
be specified as part of key-information described in Section 2.6.
If neither is present, then the bundle-source is used as the
security-source.
The security-destination MAY be present. If not present, then the
bundle-destination is used as the security-destination.
The security-parameters MAY optionally contain a block-type code
field to indicate the type of the encapsulated block. Since this
replicates a field in the encrypted portion of the block, it is a
slight security risk, and its use is therefore OPTIONAL.
2.6. Parameters and Result Fields
Various ciphersuites include several items in the security-parameters
and/or security-result fields. Which items MAY appear is defined by
the particular ciphersuite description. A ciphersuite MAY support
several instances of the same type within a single block.
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Each item is represented as a type-length-value. Type is a single
byte indicating which item this is. Length is the count of data
bytes to follow, and is an SDNV-encoded integer. Value is the data
content of the item.
Item types are
0: reserved
1: initialization vector (IV)
2: reserved
3: key-information
4: fragment-range (offset and length as a pair of SDNVs)
5: integrity signature
6: unassigned
7: salt
8: PCB integrity check value (ICV)
9: reserved
10: encapsulated block
11: block type of encapsulated block
12 - 191: reserved
192 - 250: private use
251 - 255: reserved
The following descriptions apply to the usage of these items for all
ciphersuites. Additional characteristics are noted in the discussion
for specific suites.
o initialization vector (IV): random value, typically eight to
sixteen bytes.
o key-information: key material encoded or protected by the key
management system and used to transport an ephemeral key protected
by a long-term key. This item is discussed further in
Section 2.7.
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o fragment-range: pair of SDNV values (offset then length)
specifying the range of payload bytes to which a particular
operation applies. This is termed "fragment-range" since that is
its typical use, even though sometimes it describes a subset range
that is not a fragment. The offset value MUST be the offset
within the original bundle, which might not be the offset within
the current bundle if the current bundle is already a fragment.
o integrity signature: result of BAB or PIB digest or signing
operation. This item is discussed further in Section 2.7.
o salt: an IV-like value used by certain confidentiality suites.
o PCB integrity check value (ICV): output from certain
confidentiality ciphersuite operations to be used at the
destination to verify that the protected data has not been
modified.
o encapsulated block: result of confidentiality operation on certain
blocks, contains the ciphertext of the block and MAY also contain
an integrity check value appended to the ciphertext; MAY also
contain padding if required by the encryption mode; used for non-
payload blocks only.
o block type of encapsulated block: block-type code for a block that
has been encapsulated in ESB.
2.7. Key Transport
This specification endeavors to maintain separation between the
security protocol and key management. However, these two interact in
the transfer of key-information, etc., from security-source to
security-destination. The intent of the separation is to facilitate
the use of a variety of key management systems without needing to
tailor a ciphersuite to each individually.
The key management process deals with such things as long-term keys,
specifiers for long-term keys, certificates for long-term keys, and
integrity signatures using long-term keys. The ciphersuite itself
SHOULD NOT require a knowledge of these, and separation is improved
if it treats these as opaque entities to be handled by the key
management process.
The key management process deals specifically with the content of two
of the items defined in Section 2.6: key-information (item type 3)
and integrity signature (item type 5). The ciphersuite MUST define
the details and format for these items. To facilitate
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interoperability, it is strongly RECOMMENDED that the implementations
use the appropriate definitions from the Cryptographic Message Syntax
(CMS) [RFC5652] and related RFCs.
Many situations will require several pieces of key-information.
Again, ciphersuites MUST define whether they accept these packed into
a single key-information item and/or separated into multiple
instances of key-information. For interoperability, it is
RECOMMENDED that ciphersuites accept these packed into a single key-
information item, and that they MAY additionally choose to accept
them sent as separate items.
2.8. PIB and PCB Combinations
Given the above definitions, nodes are free to combine applications
of PIB and PCB in any way they wish -- the correlator value allows
for multiple applications of security services to be handled
separately. Since PIB and PCB apply to the payload and ESB to non-
payload blocks, combinations of ESB with PIB and/or PCB are not
considered.
There are some obvious security problems that could arise when
applying multiple services. For example, if we encrypted a payload
but left a PIB security-result containing a signature in the clear,
payload guesses could be confirmed.
We cannot, in general, prevent all such problems since we cannot
assume that every ciphersuite definition takes account of every other
ciphersuite definition. However, we can limit the potential for such
problems by requiring that any ciphersuite that applies to one
instance of a PIB or PCB MUST be applied to all instances with the
same correlator.
We now list the PIB and PCB combinations that we envisage as being
useful to support:
Encrypted tunnels - a single bundle MAY be encrypted many times en
route to its destination. Clearly, it has to be decrypted an
equal number of times, but we can imagine each encryption as
representing the entry into yet another layer of tunnel. This is
supported by using multiple instances of PCB, but with the payload
encrypted multiple times, "in-place". Depending upon the
ciphersuite definition, other blocks can and should be encrypted,
as discussed above and in Section 2.4 to ensure that parameters
are protected in the case of super-encryption.
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Multiple parallel authenticators - a single security-source might
wish to protect the integrity of a bundle in multiple ways. This
could be required if the bundle's path is unpredictable and if
various nodes might be involved as security-destinations.
Similarly, if the security-source cannot determine in advance
which algorithms to use, then using all might be reasonable. This
would result in uses of PIB that, presumably, all protect the
payload, and which cannot in general protect one another. Note
that this logic can also apply to a BAB, if the unpredictable
routing happens in the convergence layer, so we also envisage
support for multiple parallel uses of BAB.
Multiple sequential authenticators - if some security-destination
requires assurance about the route that bundles have taken, then
it might insist that each forwarding node add its own PIB. More
likely, however, would be that outbound "bastion" nodes would be
configured to sign bundles as a way of allowing the sending
"domain" to take accountability for the bundle. In this case, the
various PIBs will likely be layered, so that each protects the
earlier applications of PIB.
Authenticated and encrypted bundles - a single bundle MAY require
both authentication and confidentiality. Some specifications
first apply the authenticator and follow this by encrypting the
payload and authenticator. As noted previously in the case where
the authenticator is a signature, there are security reasons for
this ordering. (See the PCB-RSA-AES128-PAYLOAD-PIB-PCB
ciphersuite defined in Section 4.3.) Others apply the
authenticator after encryption, that is, to the ciphertext. This
ordering is generally RECOMMENDED and minimizes attacks that, in
some cases, can lead to recovery of the encryption key.
There are, no doubt, other valid ways to combine PIB and PCB
instances, but these are the "core" set supported in this
specification. Having said that, as will be seen, the mandatory
ciphersuites defined here are quite specific and restrictive in terms
of limiting the flexibility offered by the correlator mechanism.
This is primarily designed to keep this specification as simple as
possible, while at the same time supporting the above scenarios.
3. Security Processing
This section describes the security aspects of bundle processing.
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3.1. Nodes as Policy Enforcement Points
All nodes are REQUIRED to have and enforce their own configurable
security policies, whether these policies be explicit or default, as
defined in Section 6.
All nodes serve as Policy Enforcement Points (PEPs) insofar as they
enforce polices that MAY restrict the permissions of bundle nodes to
inject traffic into the network. Policies MAY apply to traffic that
originates at the current node, traffic that terminates at the
current node, and traffic that is to be forwarded by the current node
to other nodes. If a particular transmission request, originating
either locally or remotely, satisfies the node's policy or policies
and is therefore accepted, then an outbound bundle can be created and
dispatched. If not, then in its role as a PEP, the node will not
create or forward a bundle. Error handling for such cases is
currently considered out of scope for this document.
Policy enforcing code MAY override all other processing steps
described here and elsewhere in this document. For example, it is
valid to implement a node that always attempts to attach a PIB.
Similarly, it is also valid to implement a node that always rejects
all requests that imply the use of a PIB.
Nodes MUST consult their security policy to determine the criteria
that a received bundle ought to meet before it will be forwarded.
These criteria MUST include a determination of whether or not the
received bundle MUST include a valid BAB, PIB, PCB, or ESB. If the
bundle does not meet the node's policy criteria, then the bundle MUST
be discarded and processed no further; in this case, a bundle status
report indicating the failure MAY be generated.
The node's policy MAY call for the node to add or subtract some
security blocks. For example, it might require that the node attempt
to encrypt (parts of) the bundle for some security-destination or
that it add a PIB. If the node's policy requires a BAB to be added
to the bundle, it MUST be added last so that the calculation of its
security-result MAY take into consideration the values of all other
blocks in the bundle.
3.2. Processing Order of Security Blocks
The processing order of security actions for a bundle is critically
important for the actions to complete successfully. In general, the
actions performed at the originating node MUST be executed in the
reverse sequence at the destination. There are variations and
exceptions, and these are noted below.
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The sequence is maintained in the ordering of security blocks in the
bundle. It is for this reason that blocks MUST NOT be rearranged at
forwarding nodes, whether or not they support the security protocols.
The only blocks that participate in this ordering are the primary and
payload blocks, and the PIB and PCB security blocks themselves. All
other extension blocks, including ESBs, are ignored for purposes of
determining the processing order.
The security blocks are added to and removed from a bundle in a last-
in-first-out (LIFO) manner, with the top of the stack immediately
after the primary block. A newly created bundle has just the primary
and payload blocks, and the stack is empty. As security actions are
requested for the bundle, security blocks are pushed onto the stack
immediately after the primary block. The early actions have security
blocks close to the payload, later actions have blocks nearer to the
primary block. The actions deal with only those blocks in the bundle
at the time, so, for example, the first to be added processes only
the payload and primary blocks, the next might process the first if
it chooses and the payload and primary, and so on. The last block to
be added can process all the blocks.
When the bundle is received, this process is reversed and security
processing begins at the top of the stack, immediately after the
primary block. The security actions are performed, and the block is
popped from the stack. Processing continues with the next security
block until finally only the payload and primary blocks remain.
The simplicity of this description is undermined by various real-
world requirements. Nonetheless, it serves as a helpful initial
framework for understanding the bundle security process.
The first issue is a very common one and easy to handle. The bundle
may be sent indirectly to its destination, requiring several
forwarding hops to finally arrive there. Security processing happens
at each node, assuming that the node supports bundle security. For
the following discussion, we assume that a bundle is created and that
confidentiality, then payload integrity, and finally bundle
authentication are applied to it. The block sequence would therefore
be primary-BAB-PIB-PCB-payload. Traveling from source to destination
requires going through one intermediate node, so the trip consists of
two hops.
When the bundle is received at the intermediate node, the receive
processing validates the BAB and pops it from the stack. However,
the PIBs and PCBs have the final destination as their security-
destination, so these cannot be processed and removed. The
intermediate node then begins the send process with the four
remaining blocks in the bundle. The outbound processing adds any
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security blocks required by local policy, and these are pushed on the
stack immediately after the primary block, ahead of the PIB. In this
example, the intermediate node adds a PIB as a signature that the
bundle has passed through the node.
The receive processing at the destination first handles the
intermediate node's PIB and pops it, next is the originator's PIB,
also popped, and finally the originator's confidentiality block that
allows the payload to be decrypted and the bundle handled for
delivery.
In practice, DTNs are likely to be more complex. The security policy
for a node specifies the security requirements for a bundle. The
policy will possibly cause one or more security operations to be
applied to the bundle at the current node, each with its own
security-destination. Application of policy at subsequent nodes
might cause additional security operations, each with a security-
destination. The list of security-destinations in the security
blocks (BAB, PIB and PCB, not ESB) creates a partial-ordering of
nodes that MUST be visited en route to the bundle-destination.
The bundle security scheme does not deal with security paths that
overlap partially but not completely. The security policy for a node
MUST avoid specifying, for a bundle, a security-destination that
causes a conflict with any existing security-destination in that
bundle. This is discussed further in Section 3.3.
The second issue relates to the reversibility of certain security
process actions. In general, the actions fall into two categories:
those that do not affect other parts of the bundle and those that are
fully reversible. Creating a bundle signature, for example, does not
change the bundle content except for the result. The encryption
performed as part of the confidentiality processing does change the
bundle, but the reverse processing at the destination restores the
original content.
The third category is the one where the bundle content has changed
slightly and in a non-destructive way, but there is no mechanism to
reverse the change. The simplest example is the addition of an EID-
reference to a security block. The addition of the reference causes
the text to be added to the bundle's dictionary. The text may also
be used by other references, so removal of the block and this
specific EID-reference does not cause removal of the text from the
dictionary. This shortcoming is of no impact to the "sequential" or
"wrapping" security schemes described above, but does cause failures
with "parallel" authentication mechanisms. Solutions for this
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problem are implementation specific and typically involve multi-pass
processing such that blocks are added at one stage and the security-
results calculated at a later stage of the overall process.
Certain ciphersuites have sequence requirements for their correct
operation, most notably the bundle authentication ciphersuites.
Processing for bundle authentication is required to happen after all
other sending operations, and prior to any receive operations at the
next-hop node. Therefore, it follows that BABs MUST always be pushed
onto the stack after all others.
Although we describe the security block list as a stack, there are
some blocks that are placed after the payload and therefore are not
part of the stack. The BundleAuthentication ciphersuite #1 ("BA1")
requires a second, correlated block to contain the security-result,
and this block is placed after the payload, usually as the last block
in the bundle. We can apply the stack rules even to these blocks by
specifying that they be added to the end of the bundle at the same
time that their "owner" or "parent" block is pushed on the stack. In
fact, they form a stack beginning at the payload but growing in the
other direction. Also, not all blocks in the main stack have a
corresponding entry in the trailing stack. The only blocks that MUST
follow the payload are those mandated by ciphersuites as correlated
blocks for holding a security-result. No other blocks are required
to follow the payload block and it is NOT RECOMMENDED that they do
so.
ESBs are effectively placeholders for the blocks they encapsulate
and, since those do not form part of the processing sequence
described above, ESBs themselves do not either. ESBs MAY be
correlated, however, so the "no reordering" requirement applies to
them as well.
3.3. Security Regions
Each security block has a security path, as described in the
discussion for Figure 1, and the paths for various blocks are often
different.
BABs are always for a single hop, and these restricted paths never
cause conflict.
The paths for PIBs and PCBs are often from bundle-source to bundle-
destination, to provide end-to-end protection. A bundle-source-to-
bundle-destination path likewise never causes a problem.
Another common scenario is for gateway-to-gateway protection of
traffic between two sub-networks ("tunnel-mode").
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Looking at Figure 1 and the simplified version shown in Figure 4, we
can regard BN2 and BN3 as gateways connecting the two sub-networks
labeled "An internet". As long as they provide security for the BN2-
BN3 path, all is well. Problems begin, for example, when BN2 adds
blocks with BN4 as the security-destination, and the originating node
BN1 has created blocks with BN3 as security-destination. We now have
two paths, and neither is a subset of the other.
This scenario should be prevented by node BN2's security policy being
aware of the already existing block with BN3 as the security-
destination. This policy SHOULD NOT specify a security-destination
that is further distant than any existing security-destination.
+---------v-| +->>>>>>>>>>v-+ +->>>>>>>>>>v-+ +-^---------+
| BN1 v | | ^ BN2 v | | ^ BN3 v | | ^ BN4 |
+---------v-+ +-^---------v-+ +-^---------v-+ +-^---------+
>>>>>>>>^ >>>>>>>>>>^ >>>>>>>>^
<------------- BN1 to BN3 path ------------>
<------------- BN2 to BN4 path ------------>
Figure 4: Overlapping Security Paths
Consider the case where the security concern is for data integrity,
so the blocks are PIBs. BN1 creates one ("PIa") along with the new
bundle, and BN2 pushes its own PIB "PIb" on the stack, with security-
destination BN4. When this bundle arrives at BN3, the bundle blocks
are
primary - PIb - PIa - payload
Block PIb is not destined for this node BN3, so it has to be
forwarded. This is the security-destination for block PIa so, after
validation, it should be removed from the bundle; however, that will
invalidate the PIb signature when the block is checked at the final
destination. The PIb signature includes the primary block, PIb
itself, PIa and the payload block, so PIa MUST remain in the bundle.
This is why security blocks are treated as a stack and add/remove
operations are permitted only at the top-of-stack.
The situation would be worse if the security concern is
confidentiality, and PCBs are employed, using the confidentiality
ciphersuite #3 ("PC3") described in Section 4.3. In this scenario,
BN1 would encrypt the bundle with BN3 as security-destination, BN2
would create an overlapping security path by super-encrypting the
payload and encapsulating the PC3 block for security-destination BN4.
BN3 forwards all the blocks without change. BN4 decrypts the payload
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from its super-encryption and decapsulates the PC3 block, only to
find that it should have been processed earlier. Assuming that BN4
has no access to BN3's key store, BN4 has no way to decrypt the
bundle and recover the original content.
As mentioned above, authors of security policy need to use care to
ensure that their policies do not cause overlaps. These guidelines
should prove helpful.
The originator of a bundle can always specify the bundle-
destination as the security-destination and should be cautious
about doing otherwise.
In the "tunnel-mode" scenario where two sub-networks are connected
by a tunnel through a network, the gateways can each specify the
other as security-destination and should be cautious about doing
otherwise.
BAB is never a problem because it is always only a single hop.
PIB for a bundle without PCB will usually specify the bundle-
destination as security-destination.
PIB for a bundle containing a PCB should specify as its security-
destination the security-destination of the PCB (outermost PCB if
there are more than one).
3.4. Canonicalization of Bundles
In order to verify a signature or MAC on a bundle, the exact same
bits, in the exact same order, MUST be input to the calculation upon
verification as were input upon initial computation of the original
signature or MAC value. Consequently, a node MUST NOT change the
encoding of any URI [RFC3986] in the dictionary field, e.g., changing
the DNS part of some HTTP URL from lower case to upper case. Because
bundles MAY be modified while in transit (either correctly or due to
implementation errors), a canonical form of any given bundle (that
contains a BAB or PIB) MUST be defined.
This section defines bundle canonicalization algorithms used in
Sections 4.1 and 4.2 ciphersuites. Other ciphersuites can use these
or define their own canonicalization procedures.
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3.4.1. Strict Canonicalization
The first algorithm that can be used permits no changes at all to the
bundle between the security-source and the security-destination. It
is mainly intended for use in BAB ciphersuites. This algorithm
conceptually catenates all blocks in the order presented, but omits
all security-result data fields in blocks of this ciphersuite type.
That is, when a BAB ciphersuite specifies this algorithm, we omit all
BAB security-results for all BAB ciphersuites. When a PIB
ciphersuite specifies this algorithm, we omit all PIB security-
results for all PIB ciphersuites. All security-result length fields
are included, even though their corresponding security-result data
fields are omitted.
Notes:
o In the above, we specify that security-result data is omitted.
This means that no bytes of the security-result data are input.
We do not set the security-result length to zero. Rather, we
assume that the security-result length will be known to the module
that implements the ciphersuite before the security-result is
calculated, and require that this value be in the security-result
length field even though the security-result data itself will be
omitted.
o The 'res' bit of the ciphersuite ID, which indicates whether or
not the security-result length and security-result data field are
present, is part of the canonical form.
o The value of the block data length field, which indicates the
length of the block, is also part of the canonical form. Its
value indicates the length of the entire bundle when the bundle
includes the security-result data field.
o BABs are always added to bundles after PIBs, so when a PIB
ciphersuite specifies this strict canonicalization algorithm and
the PIB is received with a bundle that also includes one or more
BABs, application of strict canonicalization as part of the PIB
security-result verification process requires that all BABs in the
bundle be ignored entirely.
3.4.2. Mutable Canonicalization
This algorithm is intended to protect parts of the bundle that SHOULD
NOT be changed in transit. Hence, it omits the mutable parts of the
bundle.
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The basic approach is to define a canonical form of the primary block
and catenate it with the security (PIBs and PCBs only) and payload
blocks in the order that they will be transmitted. This algorithm
ignores all other blocks, including ESBs, because it cannot be
determined whether or not they will change as the bundle transits the
network. In short, this canonicalization protects the payload,
payload-related security blocks, and parts of the primary block.
Many fields in various blocks are stored as variable-length SDNVs.
These are canonicalized in unpacked form, as eight-byte fixed-width
fields in network byte order. The size of eight bytes is chosen
because implementations MAY handle larger values as invalid, as noted
in [DTNBP].
The canonical form of the primary block is shown in Figure 5.
Essentially, it de-references the dictionary block, adjusts lengths
where necessary, and ignores flags that MAY change in transit.
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+----------------+----------------+----------------+----------------+
| Version | Processing flags (incl. COS and SRR) |
+----------------+----------------+---------------------------------+
| Canonical primary block length |
+----------------+----------------+---------------------------------+
| Destination endpoint ID length |
+----------------+----------------+---------------------------------+
| |
| Destination endpoint ID |
| |
+----------------+----------------+---------------------------------+
| Source endpoint ID length |
+----------------+----------------+----------------+----------------+
| |
| Source endpoint ID |
| |
+----------------+----------------+---------------------------------+
| Report-to endpoint ID length |
+----------------+----------------+----------------+----------------+
| |
| Report-to endpoint ID |
| |
+----------------+----------------+----------------+----------------+
| |
+ Creation Timestamp (2 x SDNV) +
| |
+---------------------------------+---------------------------------+
| Lifetime |
+----------------+----------------+----------------+----------------+
Figure 5: The Canonical Form of the Primary Bundle Block
The fields shown in Figure 5 are as follows:
The version value is the single-byte value in the primary block.
The processing flags value in the primary block is an SDNV, and
includes the class-of-service (COS) and status report request
(SRR) fields. For purposes of canonicalization, the SDNV is
unpacked into a fixed-width field, and some bits are masked out.
The unpacked field is ANDed with mask 0x0000 0000 0007 C1BE to set
to zero all reserved bits and the "bundle is a fragment" bit.
The canonical primary block length value is a four-byte value
containing the length (in bytes) of this structure, in network
byte order.
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The destination endpoint ID length and value are the length (as a
four-byte value in network byte order) and value of the
destination endpoint ID from the primary bundle block. The URI is
simply copied from the relevant part(s) of the dictionary block
and is not itself canonicalized. Although the dictionary entries
contain "null-terminators", the null-terminators are not included
in the length or the canonicalization.
The source endpoint ID length and value are handled similarly to
the destination.
The report-to endpoint ID length and value are handled similarly
to the destination.
The creation timestamp (2 x SDNV) and lifetime (SDNV) are simply
copied from the primary block, with the SDNV values being
represented as eight-byte unpacked values.
Fragment offset and total application data unit length are
ignored, as is the case for the "bundle is a fragment" bit
mentioned above. If the payload data to be canonicalized is less
than the complete, original bundle payload, the offset and length
are specified in the security-parameters.
For non-primary blocks being included in the canonicalization, the
block processing control flags value used for canonicalization is the
unpacked SDNV value with reserved and mutable bits masked to zero.
The unpacked value is ANDed with mask 0x0000 0000 0000 0077 to zero
reserved bits and the "last block" flag. The "last block" flag is
ignored because BABs and other security blocks MAY be added for some
parts of the journey but not others, so the setting of this bit might
change from hop to hop.
Endpoint ID references in security blocks are canonicalized using the
de-referenced text form in place of the reference pair. The
reference count is not included, nor is the length of the endpoint ID
text.
The block-length is canonicalized as an eight-byte unpacked value in
network byte order. If the payload data to be canonicalized is less
than the complete, original bundle payload, this field contains the
size of the data being canonicalized (the "effective block") rather
that the actual size of the block.
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Payload blocks are generally canonicalized as-is, with the exception
that, in some instances, only a portion of the payload data is to be
protected. In such a case, only those bytes are included in the
canonical form, and additional ciphersuite-parameters are required to
specify which part of the payload is protected, as discussed further
below.
Security blocks are handled likewise, except that the ciphersuite
will likely specify that the "current" security block security-result
field not be considered part of the canonical form. This differs
from the strict canonicalization case since we might use the mutable
canonicalization algorithm to handle sequential signatures such that
signatures cover earlier ones.
ESBs MUST NOT be included in the canonicalization.
Notes:
o The canonical form of the bundle is not transmitted. It is simply
an artifact used as input to digesting.
o We omit the reserved flags because we cannot determine if they
will change in transit. The masks specified above will have to be
revised if additional flags are defined and they need to be
protected.
o Our URI encoding does not preserve the null-termination convention
from the dictionary field, nor do we separate the scheme and the
scheme-specific part (SSP) as is done there.
o The URI encoding will cause errors if any node rewrites the
dictionary content (e.g., changing the DNS part of an HTTP URL
from lower case to upper case). This could happen transparently
when a bundle is synched to disk using one set of software and
then read from disk and forwarded by a second set of software.
Because there are no general rules for canonicalizing URIs (or
IRIs), this problem may be an unavoidable source of integrity
failures.
o All SDNV fields here are canonicalized as eight-byte unpacked
values in network byte order. Length fields are canonicalized as
four-byte values in network byte order. Encoding does not need
optimization since the values are never sent over the network.
If a bundle is fragmented before the PIB is applied, then the PIB
applies to a fragment and not the entire bundle. However, the
protected fragment could be subsequently further fragmented, which
would leave the verifier unable to know which bytes were protected
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by the PIB. Even in the absence of fragmentation, the same
situation applies if the ciphersuite is defined to allow
protection of less than the entire, original bundle payload.
For this reason, PIB ciphersuites that support applying a PIB to
less than the complete, original bundle payload MUST specify, as
part of the ciphersuite-parameters, which bytes of the bundle
payload are protected. When verification occurs, only the
specified range of the payload bytes are input to PIB
verification. It is valid for a ciphersuite to be specified so as
to only apply to entire bundles and not to fragments. A
ciphersuite MAY be specified to apply to only a portion of the
payload, regardless of whether the payload is a fragment or the
complete, original bundle payload.
The same fragmentation issue applies equally to PCB ciphersuites.
Ciphersuites that support applying confidentiality to fragments
MUST specify, as part of the ciphersuite-parameters, which bytes
of the bundle payload are protected. When decrypting a fragment,
only the specified bytes are processed. It is also valid for a
confidentiality ciphersuite to be specified so as to only apply to
entire bundles and not to fragments.
This definition of mutable canonicalization assumes that endpoint IDs
themselves are immutable and is unsuitable for use in environments
where that assumption might be violated.
The canonicalization applies to a specific bundle and not a specific
payload. If a bundle is forwarded in some way, the recipient is not
able to verify the original integrity signature since the source EID
will be different, and possibly other fields.
The solution for either of these issues is to define and use a PIB
ciphersuite having an alternate version of mutable canonicalization
any fields from the primary block.
3.5. Endpoint ID Confidentiality
Every bundle MUST contain a primary block that contains the source
and destination endpoint IDs, and possibly other EIDs (in the
dictionary field), and that cannot be encrypted. If endpoint ID
confidentiality is required, then bundle-in-bundle encapsulation can
solve this problem in some instances.
Similarly, confidentiality requirements MAY also apply to other parts
of the primary block (e.g., the current-custodian), and that is
supported in the same manner.
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3.6. Bundles Received from Other Nodes
Nodes implementing this specification SHALL consult their security
policy to determine whether or not a received bundle is required by
policy to include a BAB. If the bundle has no BAB, and one is not
required, then BAB processing on the received bundle is complete, and
the bundle is ready to be further processed for PIB/PCB/ESB handling
or delivery or forwarding.
If the bundle is required to have a BAB but does not, then the bundle
MUST be discarded and processed no further. If the bundle is
required to have a BAB but all of its BABs identify a node other than
the receiving node as the BAB security-destination, then the bundle
MUST be discarded and processed no further.
If the bundle is required to have a BAB, and has one or more BABs
that identify the receiving node as the BAB security-destination, or
for which there is no security-destination, then the value in the
security-result field(s) of the BAB(s) MUST be verified according to
the ciphersuite specification. If, for all such BABs in the bundle,
either the BAB security source cannot be determined or the security-
result value check fails, the bundle has failed to authenticate, and
the bundle MUST be discarded and processed no further. If any of the
BABs present verify, or if a BAB is not required, the bundle is ready
for further processing as determined by extension blocks and/or
policy.
BABs received in a bundle MUST be stripped before the bundle is
forwarded. New BABs MAY be added as required by policy. This MAY
require correcting the "last block" field of the to-be-forwarded
bundle.
Further processing of the bundle MUST take place in the order
indicated by the various blocks from the primary block to the payload
block, except as defined by an applicable specification.
If the bundle has a PCB and the receiving node is the PCB-destination
for the bundle (either because the node is listed as the bundle's
PCB-destination or because the node is listed as the bundle-
destination and there is no PCB-dest), the node MUST decrypt the
relevant parts of the bundle in accordance with the ciphersuite
specification. The PCB SHALL be deleted. If the relevant parts of
the bundle cannot be decrypted (i.e., the decryption key cannot be
deduced or decryption fails), then the bundle MUST be discarded and
processed no further; in this case, a bundle deletion status report
(see the Bundle Protocol Specification [DTNBP]) indicating the
decryption failure MAY be generated. If the PCB security-result
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included the ciphertext of a block other than the payload block, the
recovered plaintext block MUST be placed in the bundle at the
location from which the PCB was deleted.
If the bundle has one or more PIBs for which the receiving node is
the bundle's PIB-destination (either because the node is listed in
the bundle's PIB-destination or because the node is listed as the
bundle-destination and there is no PIB-dest), the node MUST verify
the value in the PIB security-result field(s) in accordance with the
ciphersuite specification. If all the checks fail, the bundle has
failed to authenticate and the bundle SHALL be processed according to
the security policy. A bundle status report indicating the failure
MAY be generated. Otherwise, if the PIB verifies, the bundle is
ready to be processed for either delivery or forwarding. Before
forwarding the bundle, the node SHOULD remove the PIB from the
bundle, subject to the requirements of Section 3.2, unless it is
likely that some downstream node will also be able to verify the PIB.
If the bundle has a PIB and the receiving node is not the bundle's
PIB-dest, the receiving node MAY attempt to verify the value in the
security-result field. If it is able to check and the check fails,
the node SHALL discard the bundle and it MAY send a bundle status
report indicating the failure.
If the bundle has an ESB and the receiving node is the ESB-
destination for the bundle (either because the node is listed as the
bundle's ESB-destination or because the node is listed as the bundle-
destination and there is no ESB-destination), the node MUST decrypt
and/or decapsulate the encapsulated block in accordance with the
ciphersuite specification. The decapsulated block replaces the ESB
in the bundle block sequence, and the ESB is thereby deleted. If the
content cannot be decrypted (i.e., the decryption key cannot be
deduced or decryption fails), then the bundle MAY be discarded and
processed no further unless the security policy specifies otherwise.
In this case, a bundle deletion status report (see the Bundle
Protocol Specification [DTNBP]) indicating the decryption failure MAY
be generated.
3.7. The At-Most-Once-Delivery Option
An application MAY request (in an implementation-specific manner)
that a node be registered as a member of an endpoint and that
received bundles destined for that endpoint be delivered to that
application.
An option for use in such cases is known as "at-most-once-delivery".
If this option is chosen, the application indicates that it wants the
node to check for duplicate bundles, discard duplicates, and deliver
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at most one copy of each received bundle to the application. If this
option is not chosen, the application indicates that it wants the
node to deliver all received bundle copies to the application. If
this option is chosen, the node SHALL deliver at most one copy of
each received bundle to the application. If the option is not
chosen, the node SHOULD, subject to policy, deliver all bundles.
To enforce this, the node MUST look at the source/timestamp pair
value of each complete (reassembled, if necessary) bundle received
and determine if this pair, which uniquely identifies a bundle, has
been previously received. If it has, then the bundle is a duplicate.
If it has not, then the bundle is not a duplicate. The source/
timestamp pair SHALL be added to the list of pair values already
received by that node.
Each node implementation MAY decide how long to maintain a table of
pair value state.
3.8. Bundle Fragmentation and Reassembly
If it is necessary for a node to fragment a bundle and security
services have been applied to that bundle, the fragmentation rules
described in [DTNBP] MUST be followed. As defined there and repeated
here for completeness, only the payload MAY be fragmented; security
blocks, like all extension blocks, can never be fragmented. In
addition, the following security-specific processing is REQUIRED:
The security policy requirements for a bundle MUST be applied
individually to all the bundles resulting from a fragmentation event.
If the original bundle contained a PIB, then each of the PIB
instances MUST be included in some fragment.
If the original bundle contained one or more PCBs, then any PCB
instances containing a key-information item MUST have the "replicate
in every fragment" flag set, and thereby be replicated in every
fragment. This is to ensure that the canonical block-sequence can be
recovered during reassembly.
If the original bundle contained one or more correlated PCBs not
containing a key-information item, then each of these MUST be
included in some fragment, but SHOULD NOT be sent more than once.
They MUST be placed in a fragment in accordance with the
fragmentation rules described in [DTNBP].
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Note: various fragments MAY have additional security blocks added at
this or later stages, and it is possible that correlators will
collide. In order to facilitate uniqueness, ciphersuites SHOULD
include the fragment-offset of the fragment as a high-order component
of the correlator.
3.9. Reactive Fragmentation
When a partial bundle has been received, the receiving node SHALL
consult its security policy to determine if it MAY fragment the
bundle, converting the received portion into a bundle fragment for
further forwarding. Whether or not reactive fragmentation is
permitted SHALL depend on the security policy and the ciphersuite
used to calculate the BAB authentication information, if required.
(Some BAB ciphersuites, i.e., the mandatory BAB-HMAC (Hashed Message
Authentication Code) ciphersuite defined in Section 4.1, do not
accommodate reactive fragmentation because the security-result in the
BAB requires that the entire bundle be signed. It is conceivable,
however, that a BAB ciphersuite could be defined such that multiple
security-results are calculated, each on a different segment of a
bundle, and that these security-results could be interspersed between
bundle payload segments such that reactive fragmentation could be
accommodated.)
If the bundle is reactively fragmented by the intermediate receiver
and the BAB-ciphersuite is of an appropriate type (e.g., with
multiple security-results embedded in the payload), the bundle MUST
be fragmented immediately after the last security-result value in the
partial payload that is received. Any data received after the last
security-result value MUST be dropped.
If a partial bundle is received at the intermediate receiver and is
reactively fragmented and forwarded, only the part of the bundle that
was not received MUST be retransmitted, though more of the bundle MAY
be retransmitted. Before retransmitting a portion of the bundle, it
SHALL be changed into a fragment and, if the original bundle included
a BAB, the fragmented bundle MUST also, and its BAB SHALL be
recalculated.
This specification does not define any ciphersuite that can handle
this reactive fragmentation case.
An interesting possibility is a ciphersuite definition such that the
transmission of a follow-up fragment would be accompanied by the
signature for the payload up to the restart point.
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3.10. Attack Model
An evaluation of resilience to cryptographic attack necessarily
depends upon the algorithms chosen for bulk data protection and for
key transport. The mandatory ciphersuites described in the following
section use AES, RSA, and SHA algorithms in ways that are believed to
be reasonably secure against ciphertext-only, chosen-ciphertext,
known-plaintext, and chosen-plaintext attacks.
The design has carefully preserved the resilience of the algorithms
against attack. For example, if a message is encrypted, then any
message integrity signature is also encrypted so that guesses cannot
be confirmed.
4. Mandatory Ciphersuites
This section defines the mandatory ciphersuites for this
specification. There is currently one mandatory ciphersuite for use
with each of the security block types BAB, PIB, PCB, and ESB. The
BAB ciphersuite is based on shared secrets using HMAC. The PIB
ciphersuite is based on digital signatures using RSA with SHA-256.
The PCB and ESB ciphersuites are based on using RSA for key transport
and AES for bulk encryption.
In all uses of CMS eContent in this specification, the relevant
eContentType to be used is id-data as specified in [RFC5652].
The ciphersuites use the mechanisms defined in Cryptographic Message
Syntax (CMS) [RFC5652] for packaging the keys, signatures, etc., for
transport in the appropriate security block. The data in the CMS
object is not the bundle data, as would be the typical usage for CMS.
Rather, the "message data" packaged by CMS is the ephemeral key,
message digest, etc., used in the core code of the ciphersuite.
In all cases where we use CMS, implementations SHOULD NOT include
additional attributes whether signed or unsigned, authenticated or
unauthenticated.
4.1. BAB-HMAC
The BAB-HMAC ciphersuite has ciphersuite ID value 0x001.
BAB-HMAC uses the strict canonicalization algorithm in Section 3.4.1.
Strict canonicalization supports digesting of a fragment-bundle. It
does not permit the digesting of only a subset of the payload, but
only the complete contents of the payload of the current bundle,
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which might be a fragment. The fragment-range item for security-
parameters is not used to indicate a fragment, as this information is
digested within the primary block.
The variant of HMAC to be used is HMAC-SHA1 as defined in [RFC2104].
This ciphersuite requires the use of two related instances of the
BAB. It involves placing the first BAB instance (as defined in
Section 2.2) just after the primary block. The second (correlated)
instance of the BAB MUST be placed after all other blocks (except
possibly other BAB blocks) in the bundle.
This means that, normally, the BAB will be the second and last blocks
of the bundle. If a forwarder wishes to apply more than one
correlated BAB pair, then this can be done. There is no requirement
that each application "wrap" the others, but the forwarder MUST
insert all the "up front" BABs, and their "at back" "partners"
(without any security-result), before canonicalizing.
Inserting more than one correlated BAB pair would be useful if the
bundle could be routed to more than one potential "next hop" or if
both an old and a new key were valid at sending time, with no
certainty about the situation that will obtain at reception time.
The security-result is the output of the HMAC-SHA1 calculation with
the input being the result of running the entire bundle through the
strict canonicalization algorithm. Both required BAB instances MUST
be included in the bundle before canonicalization.
Security-parameters are OPTIONAL with this scheme, but if used, then
the only field that can be present is key-information (see
Section 2.6).
In the absence of key-information, the receiver is expected to be
able to find the correct key based on the sending identity. The
sending identity MAY be known from the security-source field or the
content of a previous-hop block in the bundle. It MAY also be
determined using implementation-specific means such as the
convergence layer.
4.2. PIB-RSA-SHA256
The PIB-RSA-SHA256 ciphersuite has ciphersuite ID value 0x02.
PIB-RSA-SHA256 uses the mutable canonicalization algorithm in
Section 3.4.2, with the security-result data field for only the
"current" block being excluded from the canonical form. The
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resulting canonical form of the bundle is the input to the signing
process. This ciphersuite requires the use of a single instance of
the PIB.
Because the signature field in SignedData SignatureValue is a
security-result field, the entire key-information item MUST be placed
in the block's security-result field, rather than security-
parameters.
If the bundle being signed has been fragmented before signing, then
we have to specify which bytes were signed in case the signed bundle
is subsequently fragmented for a second time. If the bundle is a
fragment, then the ciphersuite-parameters MUST include a fragment-
range field, as described in Section 2.6, specifying the offset and
length of the signed fragment. If the entire bundle is signed, then
these numbers MUST be omitted.
Implementations MUST support the use of the "SignedData" type as
defined in [RFC5652], Section 5.1, with SignerInfo type
SignerIdentifier containing the issuer and serial number of a
suitable certificate. The data to be signed is the output of the
SHA256 mutable canonicalization process.
RSA is used with SHA256 as specified for the id-sha256 signature
scheme in [RFC4055], Section 5. The output of the signing process is
the SignatureValue field for the PIB.
"Commensurate strength" cryptography is generally held to be a good
idea. A combination of RSA with SHA-256 is reckoned to require a
3076-bit RSA key according to this logic. Few implementations will
choose this length by default (and probably some just will not
support such long keys). Since this is an experimental protocol, we
expect that 1024- or 2048-bit RSA keys will be used in many cases,
and that this will be fine since we also expect that the hash
function "issues" will be resolved before any standard would be
derived from this protocol.
4.3. PCB-RSA-AES128-PAYLOAD-PIB-PCB
The PCB-RSA-AES128-PAYLOAD-PIB-PCB ciphersuite has ciphersuite ID
value 0x003.
This scheme encrypts PIBs, PCBs, and the payload. The key size for
this ciphersuite is 128 bits.
Encryption is done using the AES algorithm in Galois/Counter Mode
(GCM) as described in [RFC5084]. Note: parts of the following
description are borrowed from [RFC4106].
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The choice of GCM avoids expansion of the payload, which causes
problems with fragmentation/reassembly and custody transfer. GCM
also includes authentication, essential in preventing attacks that
can alter the decrypted plaintext or even recover the encryption key.
GCM is a block cipher mode of operation providing both
confidentiality and data integrity. The GCM encryption operation has
four inputs: a secret key, an initialization vector (IV), a
plaintext, and an input for additional authenticated data (AAD),
which is not used here. It has two outputs, a ciphertext whose
length is identical to the plaintext, and an authentication tag, also
known as the integrity check value (ICV).
For consistency with the description in [RFC5084], we refer to the
GCM IV as a nonce. The same key and nonce combination MUST NOT be
used more than once. The nonce has the following layout:
+----------------+----------------+----------------+----------------+
| salt |
+----------------+----------------+----------------+----------------+
| |
| initialization vector |
| |
+----------------+----------------+----------------+----------------+
Figure 6: Nonce Format for PCB-RSA-AES128-PAYLOAD-PIB-PCB
The salt field is a four-octet value, usually chosen at random. It
MUST be the same for all PCBs that have the same correlator value.
The salt need not be kept secret.
The initialization vector (IV) is an eight-octet value, usually
chosen at random. It MUST be different for all PCBs that have the
same correlator value. The value need not be kept secret.
The key (bundle encryption key, BEK) is a 16-octet (128 bits) value,
usually chosen at random. The value MUST be kept secret, as
described below.
The integrity check value is a 16-octet value used to verify that the
protected data has not been altered. The value need not be kept
secret.
This ciphersuite requires the use of a single PCB instance to deal
with payload confidentiality. If the bundle already contains PIBs or
PCBs, then the ciphersuite will create additional correlated blocks
to protect these PIBs and PCBs. These "additional" blocks replace
the original blocks on a one-to-one basis, so the number of blocks
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remains unchanged. All of these related blocks MUST have the same
correlator value. The term "first PCB" in this section refers to the
single PCB if there is only one or, if there are several, then to the
one containing the key-information. This MUST be the first of the
set.
First PCB - the first PCB MAY contain a correlator value, and MAY
specify security-source and/or security-destination in the EID-list.
If not specified, the bundle-source and bundle-destination,
respectively, are used for these values, as with other ciphersuites.
The block MUST contain security-parameters and security-result
fields. Each field MAY contain several items formatted as described
in Section 2.6.
Security-parameters
key-information
salt
IV (this instance applies only to payload)
fragment offset and length, if bundle is a fragment
Security-result
ICV
Subsequent PCBs MUST contain a correlator value to link them to the
first PCB. Security-source and security-destination are implied from
the first PCB; however, see the discussion in Section 2.4 concerning
EID-list entries. They MUST contain security-parameters and
security-result fields as follows:
Security-parameters
IV for this specific block
Security-result
encapsulated block
The security-parameters and security-result fields in the subsequent
PCBs MUST NOT contain any items other than these two. Items such as
key and salt are supplied in the first PCB and MUST NOT be repeated.
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Implementations MUST support use of "enveloped-data" type as defined
in [RFC5652], Section 6, with RecipientInfo type
KeyTransRecipientInfo containing the issuer and serial number of a
suitable certificate. They MAY support additional RecipientInfo
types. The "encryptedContent" field in EncryptedContentInfo contains
the encrypted BEK that protects the payload and certain security
blocks of the bundle.
The Integrity Check Value from the AES-GCM encryption of the payload
is placed in the security-result field of the first PCB.
If the bundle being encrypted is a fragment-bundle, we have to
specify which bytes are encrypted in case the bundle is subsequently
fragmented again. If the bundle is a fragment, the ciphersuite-
parameters MUST include a fragment-range field, as described in
Section 2.6, specifying the offset and length of the encrypted
fragment. Note that this is not the same pair of fields that appear
in the primary block as "offset and length". The "length" in this
case is the length of the fragment, not the original length. If the
bundle is not a fragment, then this field MUST be omitted.
The confidentiality processing for payload and other blocks is
different, mainly because the payload might be fragmented later at
some other node.
For the payload, only the bytes of the bundle payload field are
affected, being replaced by ciphertext. The salt, IV, and key values
specified in the first PCB are used to encrypt the payload, and the
resultant authentication tag (ICV) is placed in an ICV item in the
security-result field of that first PCB. The other bytes of the
payload block, such as type, flags, and length, are not modified.
For each PIB or PCB to be protected, the entire original block is
encapsulated in a "replacing" PCB. This replacing PCB is placed in
the outgoing bundle in the same position as the original block, PIB
or PCB. As mentioned above, this is one-to-one replacement, and
there is no consolidation of blocks or mixing of data in any way.
The encryption process uses AES-GCM with the salt and key values from
the first PCB, and an IV unique to this PCB. The process creates
ciphertext for the entire original block and an authentication tag
for validation at the security-destination. For this encapsulation
process, unlike the processing of the bundle payload, the
authentication tag is appended to the ciphertext for the block, and
the combination is stored into the encapsulated block item in the
security-result.
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The replacing block, of course, also has the same correlator value as
the first PCB with which it is associated. It also contains the
block-specific IV in security-parameters, and the combination of
original-block-ciphertext and authentication tag, stored as an
encapsulated block item in the security-result.
If the payload was fragmented after encryption, then all those
fragments MUST be present and reassembled before decryption. This
process might be repeated several times at different destinations if
multiple fragmentation actions have occurred.
The size of the GCM counter field limits the payload size to 2^39 -
256 bytes, about half a terabyte. A future revision of this
specification will address the issue of handling payloads in excess
of this size.
4.4. ESB-RSA-AES128-EXT
The ESB-RSA-AES128-EXT ciphersuite has ciphersuite ID value 0x004.
This scheme encrypts non-payload-related blocks. It MUST NOT be used
to encrypt PIBs, PCBs, or primary or payload blocks. The key size
for this ciphersuite is 128 bits.
Encryption is done using the AES algorithm in Galois/Counter Mode
(GCM) as described in [RFC5084]. Note: parts of the following
description are borrowed from [RFC4106].
GCM is a block cipher mode of operation providing both
confidentiality and data origin authentication. The GCM
authenticated encryption operation has four inputs: a secret key, an
initialization vector (IV), a plaintext, and an input for additional
authenticated data (AAD), which is not used here. It has two
outputs, a ciphertext whose length is identical to the plaintext, and
an authentication tag, also known as the Integrity Check Value (ICV).
For consistency with the description in [RFC5084], we refer to the
GCM IV as a nonce. The same key and nonce combination MUST NOT be
used more than once. The nonce has the following layout:
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+----------------+----------------+---------------------------------+
| salt |
+----------------+----------------+---------------------------------+
| |
| initialization vector |
| |
+----------------+----------------+---------------------------------+
Figure 7: Nonce Format for ESB-RSA-AES128-EXT
The salt field is a four-octet value, usually chosen at random. It
MUST be the same for all ESBs that have the same correlator value.
The salt need not be kept secret.
The initialization vector (IV) is an eight-octet value, usually
chosen at random. It MUST be different for all ESBs that have the
same correlator value. The value need not be kept secret.
The data encryption key is a 16-octet (128 bits) value, usually
chosen at random. The value MUST be kept secret, as described below.
The integrity check value is a 16-octet value used to verify that the
protected data has not been altered. The value need not be kept
secret.
This ciphersuite replaces each BP extension block to be protected
with a "replacing" ESB, and each can be individually specified.
If a number of related BP extension blocks are to be protected, they
can be grouped as a correlated set and protected using a single key.
These blocks replace the original blocks on a one-to-one basis, so
the number of blocks remains unchanged. All these related blocks
MUST have the same correlator value. The term "first ESB" in this
section refers to the single ESB if there is only one or, if there
are several, then to the one containing the key or key-identifier.
This MUST be the first of the set. If the blocks are individually
specified, then there is no correlated set and each block is its own
"first ESB".
First ESB - the first ESB MAY contain a correlator value, and MAY
specify security-source and/or security-destination in the EID-list.
If not specified, the bundle-source and bundle-destination,
respectively, are used for these values, as with other ciphersuites.
The block MUST contain security-parameters and security-result
fields. Each field MAY contain several items formatted as described
in Section 2.6.
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Security-parameters
key-information
salt
IV for this specific block
block type of encapsulated block (OPTIONAL)
Security-result
encapsulated block
Subsequent ESBs MUST contain a correlator value to link them to the
first ESB. Security-source and security-destination are implied from
the first ESB; however, see the discussion in Section 2.4 concerning
EID-list entries. Subsequent ESBs MUST contain security-parameters
and security-result fields as follows:
Security-parameters
IV for this specific block
block type of encapsulated block (OPTIONAL)
Security-result
encapsulated block
The security-parameters and security-result fields in the subsequent
ESBs MUST NOT contain any items other than those listed. Items such
as key-information and salt are supplied in the first ESB and MUST
NOT be repeated.
Implementations MUST support the use of "enveloped-data" type as
defined in [RFC5652], Section 6, with RecipientInfo type
KeyTransRecipientInfo containing the issuer and serial number of a
suitable certificate. They MAY support additional RecipientInfo
types. The "encryptedContent" field in EncryptedContentInfo contains
the encrypted BEK used to encrypt the content of the block being
protected.
For each block to be protected, the entire original block is
encapsulated in a "replacing" ESB. This replacing ESB is placed in
the outgoing bundle in the same position as the original block. As
mentioned above, this is one-to-one replacement, and there is no
consolidation of blocks or mixing of data in any way.
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The encryption process uses AES-GCM with the salt and key values from
the first ESB and an IV unique to this ESB. The process creates
ciphertext for the entire original block, and an authentication tag
for validation at the security-destination. The authentication tag
is appended to the ciphertext for the block and the combination is
stored into the encapsulated block item in security-result.
The replacing block, of course, also has the same correlator value as
the first ESB with which it is associated. It also contains the
block-specific IV in security-parameters, and the combination of
original-block-ciphertext and authentication tag, stored as an
encapsulated block item in security-result.
5. Key Management
Key management in delay-tolerant networks is recognized as a
difficult topic and is one that this specification does not attempt
to solve. However, solely in order to support implementation and
testing, implementations SHOULD support:
o The use of well-known RSA public keys for all ciphersuites.
o Long-term pre-shared-symmetric keys for the BAB-HMAC ciphersuite.
Since endpoint IDs are URIs and URIs can be placed in X.509 [RFC5280]
public key certificates (in the subjectAltName extension),
implementations SHOULD support this way of distributing public keys.
RFC 5280 does not insist that implementations include revocation
checking. In the context of a DTN, it is reasonably likely that some
nodes would not be able to use revocation checking services (either
Certificate Revocation Lists (CRLs) or the Online Certificate Status
Protocol (OCSP)) and deployments SHOULD take this into account when
planning any public key infrastructure to support this specification.
6. Default Security Policy
Every node serves as a Policy Enforcement Point insofar as it
enforces some policy that controls the forwarding and delivery of
bundles via one or more convergence layer protocol implementation.
Consequently, every node SHALL have and operate according to its own
configurable security policy, whether the policy be explicit or
default. The policy SHALL specify:
Under what conditions received bundles SHALL be forwarded.
Under what conditions received bundles SHALL be required to
include valid BABs.
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Under what conditions the authentication information provided in a
bundle's BAB SHALL be deemed adequate to authenticate the bundle.
Under what conditions received bundles SHALL be required to have
valid PIBs and/or PCBs.
Under what conditions the authentication information provided in a
bundle's PIB SHALL be deemed adequate to authenticate the bundle.
Under what conditions a BAB SHALL be added to a received bundle
before that bundle is forwarded.
Under what conditions a PIB SHALL be added to a received bundle
before that bundle is forwarded.
Under what conditions a PCB SHALL be added to a received bundle
before that bundle is forwarded.
Under what conditions an ESB SHALL be applied to one or more
blocks in a received bundle before that bundle is forwarded.
The actions that SHALL be taken in the event that a received
bundle does not meet the receiving node's security policy
criteria.
This specification does not address how security policies get
distributed to nodes. It only REQUIRES that nodes have and enforce
security policies.
If no security policy is specified at a given node, or if a security
policy is only partially specified, that node's default policy
regarding unspecified criteria SHALL consist of the following:
Bundles that are not well-formed do not meet the security policy
criteria.
The mandatory ciphersuites MUST be used.
All bundles received MUST have a BAB that MUST be verified to
contain a valid security-result. If the bundle does not have a
BAB, then the bundle MUST be discarded and processed no further; a
bundle status report indicating the authentication failure MAY be
generated.
No received bundles SHALL be required to have a PIB; if a received
bundle does have a PIB, however, the PIB can be ignored unless the
receiving node is the PIB-destination, in which case the PIB MUST
be verified.
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No received bundles SHALL be required to have a PCB; if a received
bundle does have a PCB, however, the PCB can be ignored unless the
receiving node is the PCB-destination, in which case the PCB MUST
be processed. If processing a PCB yields a PIB, that PIB SHALL be
processed by the node according to the node's security policy.
A PIB SHALL NOT be added to a bundle before sourcing or forwarding
it.
A PCB SHALL NOT be added to a bundle before sourcing or forwarding
it.
A BAB MUST always be added to a bundle before that bundle is
forwarded.
If a destination node receives a bundle that has a PIB-destination
but the value in that PIB-destination is not the EID of the
destination node, the bundle SHALL be delivered at that
destination node.
If a destination node receives a bundle that has an ESB-
destination but the value in that ESB-destination is not the EID
of the destination node, the bundle SHALL be delivered at that
destination node.
If a received bundle does not satisfy the node's security policy
for any reason, then the bundle MUST be discarded and processed no
further; in this case, a bundle deletion status report (see the
Bundle Protocol Specification [DTNBP]) indicating the failure MAY
be generated.
7. Security Considerations
The Bundle Security Protocol builds upon much work of others, in
particular, "Cryptographic Message Syntax (CMS)" [RFC5652] and
"Internet X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile" [RFC5280]. The security
considerations in these two documents apply here as well.
Several documents specifically consider the use of Galois/Counter
Mode (GCM) and of AES and are important to consider when building
ciphersuites. These are "The Use of Galois/Counter Mode (GCM) in
IPsec Encapsulating Security Payload (ESP)" [RFC4106] and "Using AES-
CCM and AES-GCM Authenticated Encryption in the Cryptographic Message
Syntax (CMS)" [RFC5084]. Although the BSP is not identical, many of
the security issues considered in these documents also apply here.
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Certain applications of DTN need to both sign and encrypt a message,
and there are security issues to consider with this.
If the intent is to provide an assurance that a message did, in fact,
come from a specific source and has not been changed, then it should
be signed first and then encrypted. A signature on an encrypted
message does not establish any relationship between the signer and
the original plaintext message.
On the other hand, if the intent is to reduce the threat of denial-
of-service attacks, then signing the encrypted message is
appropriate. A message that fails the signature check will not be
processed through the computationally intensive decryption pass. A
more extensive discussion of these points is in S/MIME 3.2 Message
Specification [RFC5751], especially in Section 3.6.
Additional details relating to these combinations can be found in
Section 2.8 where it is RECOMMENDED that the encrypt-then-sign
combination is usually appropriate for usage in a DTN.
In a DTN, encrypt-then-sign potentially allows intermediate nodes to
verify a signature (over the ciphertext) and thereby apply policy to
manage possibly scarce storage or other resources at intermediate
nodes in the path the bundle takes from source to destination EID.
An encrypt-then-sign scheme does not further expose identity in most
cases since the BP mandates that the source EID (which is commonly
expected to be the security-source) is already exposed in the primary
block of the bundle. Should exposure of either the source EID or the
signerInfo be considered an interesting vulnerability, then some form
of bundle-in-bundle encapsulation would be required as a mitigation.
If a BAB ciphersuite uses digital signatures but doesn't include the
security-destination (which for a BAB is the next host), then this
allows the bundle to be sent to some node other than the intended
adjacent node. Because the BAB will still authenticate, the
receiving node might erroneously accept and forward the bundle. When
asymmetric BAB ciphersuites are used, the security-destination field
SHOULD therefore be included in the BAB.
If a bundle's PIB-destination is not the same as its destination,
then some node other than the destination (the node identified as the
PIB-destination) is expected to validate the PIB security-result
while the bundle is en route. However, if for some reason the PIB is
not validated, there is no way for the destination to become aware of
this. Typically, a PIB-destination will remove the PIB from the
bundle after verifying the PIB and before forwarding it. However, if
there is a possibility that the PIB will also be verified at a
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downstream node, the PIB-destination will leave the PIB in the
bundle. Therefore, if a destination receives a bundle with a PIB
that has a PIB-destination (which isn't the destination), this might,
but does not necessarily, indicate a possible problem.
If a bundle is fragmented after being forwarded by its PIB-source but
before being received by its PIB-destination, the payload in the
bundle MUST be reassembled before validating the PIB security-result
in order for the security-result to validate correctly. Therefore,
if the PIB-destination is not capable of performing payload
reassembly, its utility as a PIB-destination will be limited to
validating only those bundles that have not been fragmented since
being forwarded from the PIB-source. Similarly, if a bundle is
fragmented after being forwarded by its PIB-source but before being
received by its PIB-destination, all fragments MUST be received at
that PIB-destination in order for the bundle payload to be able to be
reassembled. If not all fragments are received at the PIB-
destination node, the bundle will not be able to be authenticated,
and will therefore never be forwarded by this PIB-destination node.
Specification of a security-destination other than the bundle-
destination creates a routing requirement that the bundle somehow be
directed to the security-destination node on its way to the final
destination. This requirement is presently private to the
ciphersuite, since routing nodes are not required to implement
security processing.
If a security target were to generate reports in the event that some
security validation step fails, then that might leak information
about the internal structure or policies of the DTN containing the
security target. This is sometimes considered bad security practice,
so it SHOULD only be done with care.
8. Conformance
As indicated above, this document describes both BSP and
ciphersuites. A conformant implementation MUST implement both BSP
support and the four ciphersuites described in Section 4. It MAY
also support other ciphersuites.
Implementations that support BSP but not all four mandatory
ciphersuites MUST claim only "restricted compliance" with this
specification, even if they provide other ciphersuites.
All implementations are strongly RECOMMENDED to provide at least a
BAB ciphersuite. A relay node, for example, might not deal with end-
to-end confidentiality and data integrity, but it SHOULD exclude
unauthorized traffic and perform hop-by-hop bundle verification.
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9. IANA Considerations
This protocol has fields that have been registered by IANA.
9.1. Bundle Block Types
This specification allocates four codepoints from the existing
"Bundle Block Types" registry defined in [RFC6255].
Additional Entries for the Bundle Block-Type Codes Registry:
+-------+--------------------------------------+----------------+
| Value | Description | Reference |
+-------+--------------------------------------+----------------+
| 2 | Bundle Authentication Block | This document |
| 3 | Payload Integrity Block | This document |
| 4 | Payload Confidentiality Block | This document |
| 9 | Extension Security Block | This document |
+-------+--------------------------------------+----------------+
9.2. Ciphersuite Numbers
This protocol has a ciphersuite number field and certain ciphersuites
are defined. An IANA registry has been set up as follows.
The registration policy for this registry is: Specification Required
The Value range is: Variable Length
Ciphersuite Numbers Registry:
+-------+--------------------------------------+----------------+
| Value | Description | Reference |
+-------+--------------------------------------+----------------+
| 0 | unassigned | This document |
| 1 | BAB-HMAC | This document |
| 2 | PIB-RSA-SHA256 | This document |
| 3 | PCB-RSA-AES128-PAYLOAD-PIB-PCB | This document |
| 4 | ESB-RSA-AES128-EXT | This document |
| >4 | Reserved | This document |
+-------+--------------------------------------+----------------+
9.3. Ciphersuite Flags
This protocol has a ciphersuite flags field and certain flags are
defined. An IANA registry has been set up as follows.
The registration policy for this registry is: Specification Required
The Value range is: Variable Length
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Ciphersuite Flags Registry:
+-----------------+----------------------------+----------------+
| Bit Position | Description | Reference |
| (right to left) | | |
+-----------------+----------------------------+----------------+
| 0 | Block contains result | This document |
| 1 | Block contains correlator | This document |
| 2 | Block contains parameters | This document |
| 3 | Destination EIDref present | This document |
| 4 | Source EIDref present | This document |
| >4 | Reserved | This document |
+-----------------+----------------------------+----------------+
9.4. Parameters and Results
This protocol has fields for ciphersuite-parameters and results. The
field is a type-length-value triple and a registry is required for
the "type" sub-field. The values for "type" apply to both the
ciphersuite-parameters and the ciphersuite results fields. Certain
values are defined. An IANA registry has been set up as follows.
The registration policy for this registry is: Specification Required
The Value range is: 8-bit unsigned integer
Ciphersuite-Parameters and Results Type Registry:
+---------+------------------------------------+----------------+
| Value | Description | Reference |
+---------+------------------------------------+----------------+
| 0 | reserved | This document |
| 1 | initialization vector (IV) | This document |
| 2 | reserved | This document |
| 3 | key-information | This document |
| 4 | fragment-range (pair of SDNVs) | This document |
| 5 | integrity signature | This document |
| 6 | unassigned | This document |
| 7 | salt | This document |
| 8 | PCB integrity check value (ICV) | This document |
| 9 | reserved | This document |
| 10 | encapsulated block | This document |
| 11 | block type of encapsulated block | This document |
| 12-191 | reserved | This document |
| 192-250 | private use | This document |
| 251-255 | reserved | This document |
+-------+--------------------------------------+----------------+
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10. References
10.1. Normative References
[DTNBP] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
[DTNMD] Symington, S., "Delay-Tolerant Networking Metadata
Extension Block", RFC 6258, May 2011.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
June 2005.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC6255] Blanchet, M., "Delay-Tolerant Networking (DTN) Bundle
Protocol IANA Registries", RFC 6255, May 2011.
10.2. Informative References
[DTNarch] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, April 2007.
[PHIB] Symington, S., "Delay-Tolerant Networking Previous-Hop
Insertion Block", RFC 6259, May 2011.
Symington, et al. Experimental [Page 58]
RFC 6257 Bundle Security Protocol May 2011
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
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Authors' Addresses
Susan Flynn Symington
The MITRE Corporation
7515 Colshire Drive
McLean, VA 22102
US
Phone: +1 (703) 983-7209
EMail: susan@mitre.org
URI: http://mitre.org/
Stephen Farrell
Trinity College Dublin
Distributed Systems Group
Department of Computer Science
Trinity College
Dublin 2
Ireland
Phone: +353-1-896-2354
EMail: stephen.farrell@cs.tcd.ie
Howard Weiss
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
US
Phone: +1-443-430-8089
EMail: howard.weiss@sparta.com
Peter Lovell
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
US
Phone: +1-443-430-8052
EMail: dtnbsp@gmail.com
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