Network Working Group R. Housley
Request for Comments: 4309 Vigil Security
Category: Standards Track December 2005
Using Advanced Encryption Standard (AES) CCM Mode
with IPsec Encapsulating Security Payload (ESP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes the use of Advanced Encryption Standard (AES)
in Counter with CBC-MAC (CCM) Mode, with an explicit initialization
vector (IV), as an IPsec Encapsulating Security Payload (ESP)
mechanism to provide confidentiality, data origin authentication, and
connectionless integrity.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. AES CCM Mode ....................................................2
3. ESP Payload .....................................................4
3.1. Initialization Vector (IV) .................................4
3.2. Encrypted Payload ..........................................4
3.3. Authentication Data ........................................5
4. Nonce Format ....................................................5
5. AAD Construction ................................................6
6. Packet Expansion ................................................7
7. IKE Conventions .................................................7
7.1. Keying Material and Salt Values ............................7
7.2. Phase 1 Identifier .........................................8
7.3. Phase 2 Identifier .........................................8
7.4. Key Length Attribute .......................................8
8. Test Vectors ....................................................8
9. Security Considerations .........................................8
10. Design Rationale ...............................................9
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11. IANA Considerations ...........................................11
12. Acknowledgements ..............................................11
13. References ....................................................11
13.1. Normative References .....................................11
13.2. Informative References ...................................12
1. Introduction
The Advanced Encryption Standard (AES) [AES] is a block cipher, and
it can be used in many different modes. This document describes the
use of AES in CCM (Counter with CBC-MAC) mode (AES CCM), with an
explicit initialization vector (IV), as an IPsec Encapsulating
Security Payload (ESP) [ESP] mechanism to provide confidentiality,
data origin authentication, and connectionless integrity.
This document does not provide an overview of IPsec. However,
information about how the various components of IPsec and the way in
which they collectively provide security services is available in
[ARCH] and [ROAD].
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [STDWORDS].
2. AES CCM Mode
CCM is a generic authenticate-and-encrypt block cipher mode [CCM].
In this specification, CCM is used with the AES [AES] block cipher.
AES CCM has two parameters:
M M indicates the size of the integrity check value (ICV). CCM
defines values of 4, 6, 8, 10, 12, 14, and 16 octets; However,
to maintain alignment and provide adequate security, only the
values that are a multiple of four and are at least eight are
permitted. Implementations MUST support M values of 8 octets
and 16 octets, and implementations MAY support an M value of 12
octets.
L L indicates the size of the length field in octets. CCM
defines values of L between 2 octets and 8 octets. This
specification only supports L = 4. Implementations MUST
support an L value of 4 octets, which accommodates a full
Jumbogram [JUMBO]; however, the length includes all of the
encrypted data, which also includes the ESP Padding, Pad
Length, and Next Header fields.
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There are four inputs to CCM originator processing:
key
A single key is used to calculate the ICV using CBC-MAC and to
perform payload encryption using counter mode. AES supports
key sizes of 128 bits, 192 bits, and 256 bits. The default key
size is 128 bits, and implementations MUST support this key
size. Implementations MAY also support key sizes of 192 bits
and 256 bits.
nonce
The size of the nonce depends on the value selected for the
parameter L. It is 15-L octets. Implementations MUST support
a nonce of 11 octets. The construction of the nonce is
described in Section 4.
payload
The payload of the ESP packet. The payload MUST NOT be longer
than 4,294,967,295 octets, which is the maximum size of a
Jumbogram [JUMBO]; however, the ESP Padding, Pad Length, and
Next Header fields are also part of the payload.
AAD
CCM provides data integrity and data origin authentication for
some data outside the payload. CCM does not allow additional
authenticated data (AAD) to be longer than
18,446,744,073,709,551,615 octets. The ICV is computed from
the ESP header, Payload, and ESP trailer fields, which is
significantly smaller than the CCM-imposed limit. The
construction of the AAD described in Section 5.
AES CCM requires the encryptor to generate a unique per-packet value
and to communicate this value to the decryptor. This per-packet
value is one of the component parts of the nonce, and it is referred
to as the initialization vector (IV). The same IV and key
combination MUST NOT be used more than once. The encryptor can
generate the IV in any manner that ensures uniqueness. Common
approaches to IV generation include incrementing a counter for each
packet and linear feedback shift registers (LFSRs).
AES CCM employs counter mode for encryption. As with any stream
cipher, reuse of the same IV value with the same key is catastrophic.
An IV collision immediately leaks information about the plaintext in
both packets. For this reason, it is inappropriate to use this CCM
with statically configured keys. Extraordinary measures would be
needed to prevent reuse of an IV value with the static key across
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power cycles. To be safe, implementations MUST use fresh keys with
AES CCM. The Internet Key Exchange (IKE) [IKE] protocol or IKEv2
[IKEv2] can be used to establish fresh keys.
3. ESP Payload
The ESP payload is composed of the IV followed by the ciphertext.
The payload field, as defined in [ESP], is structured as shown in
Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Encrypted Payload (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Authentication Data (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. ESP Payload Encrypted with AES CCM
3.1. Initialization Vector (IV)
The AES CCM IV field MUST be eight octets. The IV MUST be chosen by
the encryptor in a manner that ensures that the same IV value is used
only once for a given key. The encryptor can generate the IV in any
manner that ensures uniqueness. Common approaches to IV generation
include incrementing a counter for each packet and linear feedback
shift registers (LFSRs).
Including the IV in each packet ensures that the decryptor can
generate the key stream needed for decryption, even when some
datagrams are lost or reordered.
3.2. Encrypted Payload
The encrypted payload contains the ciphertext.
AES CCM mode does not require plaintext padding. However, ESP does
require padding to 32-bit word-align the authentication data. The
Padding, Pad Length, and Next Header fields MUST be concatenated
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with the plaintext before performing encryption, as described in
[ESP]. When padding is required, it MUST be generated and checked
in accordance with the conventions specified in [ESP].
3.3. Authentication Data
AES CCM provides an encrypted ICV. The ICV provided by CCM is
carried in the Authentication Data fields without further encryption.
Implementations MUST support ICV sizes of 8 octets and 16 octets.
Implementations MAY also support ICV 12 octets.
4. Nonce Format
Each packet conveys the IV that is necessary to construct the
sequence of counter blocks used by counter mode to generate the key
stream. The AES counter block is 16 octets. One octet is used for
the CCM Flags, and 4 octets are used for the block counter, as
specified by the CCM L parameter. The remaining octets are the
nonce. These octets occupy the second through the twelfth octets in
the counter block. Figure 2 shows the format of the nonce.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Nonce Format
The components of the nonce are as follows:
Salt The salt field is 24 bits. As the name implies, it contains
an unpredictable value. It MUST be assigned at the beginning
of the security association. The salt value need not be
secret, but it MUST NOT be predictable prior to the beginning
of the security association.
Initialization Vector The IV field is 64 bits. As described in
Section 3.1, the IV MUST be chosen by the encryptor in a manner
that ensures that the same IV value is used only once for a
given key.
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This construction permits each packet to consist of up to:
2^32 blocks = 4,294,967,296 blocks
= 68,719,476,736 octets
This construction provides more key stream for each packet than is
needed to handle any IPv6 Jumbogram [JUMBO].
5. AAD Construction
The data integrity and data origin authentication for the Security
Parameters Index (SPI) and (Extended) Sequence Number fields is
provided without encrypting them. Two formats are defined: one for
32-bit sequence numbers and one for 64-bit extended sequence numbers.
The format with 32-bit sequence numbers is shown in Figure 3, and the
format with 64-bit extended sequence numbers is shown in Figure 4.
Sequence Numbers are conveyed canonical network byte order. Extended
Sequence Numbers are conveyed canonical network byte order, placing
the high-order 32 bits first and the low-order 32 bits second.
Canonical network byte order is fully described in RFC 791, Appendix
B.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32-bit Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. AAD Format with 32-bit Sequence Number
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64-bit Extended Sequence Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. AAD Format with 64-bit Extended Sequence Number
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6. Packet Expansion
The initialization vector (IV) and the integrity check value (ICV)
are the only sources of packet expansion. The IV always adds 8
octets to the front of the payload. The ICV is added at the end of
the payload, and the CCM parameter M determines the size of the ICV.
Implementations MUST support M values of 8 octets and 16 octets, and
implementations MAY also support an M value of 12 octets.
7. IKE Conventions
This section describes the conventions used to generate keying
material and salt values for use with AES CCM using the Internet Key
Exchange (IKE) [IKE] protocol. The identifiers and attributes needed
to negotiate a security association that uses AES CCM are also
defined.
7.1. Keying Material and Salt Values
As previously described, implementations MUST use fresh keys with AES
CCM. IKE can be used to establish fresh keys. This section
describes the conventions for obtaining the unpredictable salt value
for use in the nonce from IKE. Note that this convention provides a
salt value that is secret as well as unpredictable.
IKE makes use of a pseudo-random function (PRF) to derive keying
material. The PRF is used iteratively to derive keying material of
arbitrary size, called KEYMAT. Keying material is extracted from the
output string without regard to boundaries.
The size of KEYMAT MUST be three octets longer than is needed for the
associated AES key. The keying material is used as follows:
AES CCM with a 128-bit key
The KEYMAT requested for each AES CCM key is 19 octets. The
first 16 octets are the 128-bit AES key, and the remaining
three octets are used as the salt value in the counter block.
AES CCM with a 192-bit key
The KEYMAT requested for each AES CCM key is 27 octets. The
first 24 octets are the 192-bit AES key, and the remaining
three octets are used as the salt value in the counter block.
AES CCM with a 256-bit key
The KEYMAT requested for each AES CCM key is 35 octets. The
first 32 octets are the 256-bit AES key, and the remaining
three octets are used as the salt value in the counter block.
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7.2. Phase 1 Identifier
This document does not specify the conventions for using AES CCM for
IKE Phase 1 negotiations. For AES CCM to be used in this manner, a
separate specification is needed, and an Encryption Algorithm
Identifier needs to be assigned.
7.3. Phase 2 Identifier
For IKE Phase 2 negotiations, IANA has assigned three ESP Transform
Identifiers for AES CCM with an explicit IV:
14 for AES CCM with an 8-octet ICV;
15 for AES CCM with a 12-octet ICV; and
16 for AES CCM with a 16-octet ICV.
7.4. Key Length Attribute
Because the AES supports three key lengths, the Key Length attribute
MUST be specified in the IKE Phase 2 exchange [DOI]. The Key Length
attribute MUST have a value of 128, 192, or 256.
8. Test Vectors
Section 8 of [CCM] provides test vectors that will assist
implementers with AES CCM mode.
9. Security Considerations
AES CCM employs counter (CTR) mode for confidentiality. If a counter
value is ever used for more that one packet with the same key, then
the same key stream will be used to encrypt both packets, and the
confidentiality guarantees are voided.
What happens if the encryptor XORs the same key stream with two
different packet plaintexts? Suppose two packets are defined by two
plaintext byte sequences P1, P2, P3 and Q1, Q2, Q3, then both are
encrypted with key stream K1, K2, K3. The two corresponding
ciphertexts are:
(P1 XOR K1), (P2 XOR K2), (P3 XOR K3)
(Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3)
If both of these two ciphertext streams are exposed to an attacker,
then a catastrophic failure of confidentiality results, because:
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(P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1
(P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2
(P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3
Once the attacker obtains the two plaintexts XORed together, it is
relatively straightforward to separate them. Thus, using any stream
cipher, including AES CTR, to encrypt two plaintexts under the same
key stream leaks the plaintext.
Therefore, AES CCM should not be used with statically configured
keys. Extraordinary measures would be needed to prevent the reuse of
a counter block value with the static key across power cycles. To be
safe, implementations MUST use fresh keys with AES CCM. The IKE
[IKE] protocol can be used to establish fresh keys.
When IKE is used to establish fresh keys between two peer entities,
separate keys are established for the two traffic flows. If a
different mechanism is used to establish fresh keys, one that
establishes only a single key to encrypt packets, then there is a
high probability that the peers will select the same IV values for
some packets. Thus, to avoid counter block collisions, ESP
implementations that permit use of the same key for encrypting and
decrypting packets with the same peer MUST ensure that the two peers
assign different salt values to the security association (SA).
Regardless of the mode used, AES with a 128-bit key is vulnerable to
the birthday attack after 2^64 blocks are encrypted with a single
key. Since ESP with Extended Sequence Numbers allows for up to 2^64
packets in a single SA, there is real potential for more than 2^64
blocks to be encrypted with one key. Implementations SHOULD generate
a fresh key before 2^64 blocks are encrypted with the same key, or
implementations SHOULD make use of the longer AES key sizes. Note
that ESP with 32-bit Sequence Numbers will not exceed 2^64 blocks
even if all of the packets are maximum-length Jumbograms.
10. Design Rationale
In the development of this specification, the use of the ESP sequence
number field instead of an explicit IV field was considered. This
section documents the rationale for the selection of an explicit IV.
This selection is not a cryptographic security issue, as either
approach will prevent counter block collisions.
The use of the explicit IV does not dictate the manner that the
encryptor uses to assign the per-packet value in the counter block.
This is desirable for several reasons.
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1. Only the encryptor can ensure that the value is not used for
more than one packet, so there is no advantage to selecting a
mechanism that allows the decryptor to determine whether counter
block values collide. Damage from the collision is done, whether
the decryptor detects it or not.
2. The use of explicit IVs allows adders, LFSRs, and any other
technique that meets the time budget of the encryptor, so long as
the technique results in a unique value for each packet. Adders
are simple and straightforward to implement, but due to carries,
they do not execute in constant time. LFSRs offer an alternative
that executes in constant time.
3. Complexity is in control of the implementer. Furthermore, the
decision made by the implementer of the encryptor does not make
the decryptor more (or less) complex.
4. The assignment of the per-packet counter block value needs to
be inside the assurance boundary. Some implementations assign the
sequence number inside the assurance boundary, but others do not.
A sequence number collision does not have the dire consequences,
but, as described in Section 6, a collision in counter block
values has disastrous consequences.
5. Using the sequence number as the IV is possible in those
architectures where the sequence number assignment is performed
within the assurance boundary. In this situation, the sequence
number and the IV field will contain the same value.
6. By decoupling the IV and the sequence number, architectures
where the sequence number assignment is performed outside the
assurance boundary are accommodated.
The use of an explicit IV field directly follows from the decoupling
of the sequence number and the per-packet counter block value. The
additional overhead (64 bits for the IV field) is acceptable. This
overhead is significantly less overhead associated with Cipher Block
Chaining (CBC) mode. As normally employed, CBC requires a full block
for the IV and, on average, half of a block for padding. AES CCM
confidentiality processing with an explicit IV has about one-third of
the overhead as AES CBC, and the overhead is constant for each
packet.
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11. IANA Considerations
IANA has assigned three ESP transform numbers for use with AES CCM
with an explicit IV:
14 for AES CCM with an 8-octet ICV;
15 for AES CCM with a 12-octet ICV; and
16 for AES CCM with a 16-octet ICV.
12. Acknowledgements
Doug Whiting and Niels Ferguson worked with me to develop CCM mode.
We developed CCM mode as part of the IEEE 802.11i security effort.
One of the most attractive aspects of CCM mode is that it is
unencumbered by patents. I acknowledge the companies that supported
the development of an unencumbered authenticated encryption mode (in
alphabetical order):
Hifn
Intersil
MacFergus
RSA Security
Also, I thank Tero Kivinen for his comprehensive review of this
document.
13. References
This section provides normative and informative references.
13.1. Normative References
[AES] NIST, FIPS PUB 197, "Advanced Encryption Standard (AES),"
November 2001.
[DOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[CCM] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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13.2. Informative References
[ARCH] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[ROAD] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[JUMBO] Borman, D., Deering, S., and R. Hinden, "IPv6
Jumbograms", RFC 2675, August 1999.
Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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RFC, FYI, BCP