Network Working Group J. Schaad
Request for Comments: 4211 Soaring Hawk Consulting
Obsoletes: 2511 September 2005
Category: Standards Track
Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)
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 Certificate Request Message Format (CRMF)
syntax and semantics. This syntax is used to convey a request for a
certificate to a Certification Authority (CA), possibly via a
Registration Authority (RA), for the purposes of X.509 certificate
production. The request will typically include a public key and the
associated registration information. This document does not define a
certificate request protocol.
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RFC 4211 Internet X.509 CRMF September 2005
Table Of Contents
1. Introduction and Terminology ....................................3
2. Overview ........................................................3
2.1. Changes since RFC 2511 .....................................4
3. CertReqMessage Syntax ...........................................4
4. Proof-of-Possession (POP) .......................................5
4.1. Signature Key POP ..........................................7
4.2. Key Encipherment Keys ......................................9
4.2.1. Private Key Info Content Type ......................11
4.2.2. Private Key Structures .............................12
4.2.3. Challenge-Response Guidelines ......................13
4.3. Key Agreement Keys ........................................14
4.4. Use of Password-Based MAC .................................14
5. CertRequest syntax .............................................16
6. Controls Syntax ................................................18
6.1. Registration Token Control ................................18
6.2. Authenticator Control .....................................19
6.3. Publication Information Control ...........................19
6.4. Archive Options Control ...................................21
6.5. OldCert ID Control ........................................23
6.6. Protocol Encryption Key Control ...........................23
7. RegInfo Controls ...............................................23
7.1. utf8Pairs .................................................23
7.2. certReq ...................................................24
8. Object Identifiers .............................................24
9. Security Considerations ........................................25
10. References ....................................................26
10.1. Normative References .....................................26
10.2. Informative References ...................................27
11. Acknowledgements ..............................................28
Appendix A. Use of RegInfo for Name-Value Pairs ..................29
A.1. Defined Names ............................................29
A.2. IssuerName, SubjectName, and Validity Value Encoding .....29
Appendix B. ASN.1 Structures and OIDs ............................32
Appendix C. Why do Proof-of-Possession (POP) .....................38
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1. Introduction and Terminology
This document describes the Certificate Request Message Format
(CRMF). A Certificate Request Message object is used within a
protocol to convey a request for a certificate to a Certification
Authority (CA), possibly via a Registration Authority (RA), for the
purposes of X.509 certificate production. The request will typically
include a public key and the associated registration information.
The certificate request object defined in this document is not a
stand-alone protocol. The information defined in this document is
designed to be used by an externally defined Certificate Request
Protocol (CRP). The referencing protocol is expected to define what
algorithms are used, and what registration information and control
structures are defined. Many of the requirements in this document
refer to the referencing Certificate Request Protocol (CRP).
Certificate requests may be submitted by an RA requesting a
certificate on behalf of a Subject, by a CA requesting a cross-
certificate from another CA, or directly by an End Entity (EE).
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document (in uppercase, as shown) are to be interpreted as
described in RFC 2119 [RFC2119].
2. Overview
Construction of a certification request involves the following steps:
a) A CertRequest object is constructed. This object may include the
public key, all or a portion of the Subject name, other requested
certificate fields, and additional control information related to
the registration process. Depending on the CRP, this information
can be specified by the Subject and potentially modified by an
RA, or specified by the RA based on knowledge of the Subject or
documentation presented by the Subject.
b) If required, a proof-of-possession (of the private key
corresponding to the public key for which a certificate is being
requested) value is calculated.
c) Additional registration information can be combined with the
proof-of-possession value and the CertRequest structure to form a
CertReqMessage. Additional registration information can be added
by both the Subject and an RA.
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d) The CertReqMessage is securely communicated to a CA. Specific
means of secure transport are to be specified by each CRP that
refers to this document.
2.1. Changes since RFC 2511
1. Addition of an introduction section.
2. Addition of the concept of a CRP and language relating to CRPs.
3. In section 6.2, changed regToken to authenticator.
4. Add information describing the contents of the EncryptedValue
structure.
5. Changed name and contents of OID {id-regInfo 1}.
6. Added text detailing what goes into the fields of the different
structures defined in the document.
7. Replaced Appendix A with a reference to [RFC2875]. The only
difference is that the old text specified to use subject alt name
instead of subject name if subject name was empty. This is not
possible for a CA certificate issued using PKIX. It would
however be useful to update RFC 2875 to have this fallback
position.
7. Insert Appendix C describing why POP is necessary and what some
of the different POP attacks are.
8. pop field in the CertReqMsg structure has been renamed to popo to
avoid confusion between POP and pop.
9. The use of the EncryptedValue structure has been deprecated in
favor of the EnvelopedData structure.
10. Add details on how private keys are to be structured when
encrypted.
11. Allow for POP on key agreement algorithms other than DH.
3. CertReqMessage Syntax
A certificate request message is composed of the certificate request,
an optional proof-of-possession field, and an optional registration
information field.
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CertReqMessages ::= SEQUENCE SIZE (1..MAX) OF CertReqMsg
CertReqMsg ::= SEQUENCE {
certReq CertRequest,
popo ProofOfPossession OPTIONAL,
-- content depends upon key type
regInfo SEQUENCE SIZE(1..MAX) of AttributeTypeAndValue OPTIONAL
}
The fields of CertReqMsg have the following meaning:
certReq contains the template of the certificate being requested.
The template is filled in by (or on behalf of) the Subject. Not
all fields within the template need to be specified. Details on
this field are found in section 5.
popo contains the value used to demonstrate that the entity that
will be identified as the Subject of the certificate is actually
in possession of the corresponding private key. This field varies
in structure and content based on the public key algorithm and the
mode (encryption vs. signature) in which the algorithm is used, as
specified in the KeyUsage field of the certificate to be issued.
Details on this field are found in section 4.
regInfo field SHOULD contain only supplementary information
relating to the context of the certificate request, where such
information is required to fulfill the request. This information
might include subscriber contact information, billing information,
or other ancillary information useful to fulfillment of the
request.
Information directly related to certificate content SHOULD be
included in the certReq content. However, inclusion of additional
certReq content by RAs can invalidate the popo field (depending on
the details of the POP method used). Therefore, data intended for
certificate content MAY be provided in regInfo.
It is the responsibility of a referencing CRP to define the details
of what can be specified in the regInfo field. This document
describes one method of encoding the information found in this field.
Details on this encoding are found in Appendix A.
4. Proof-of-Possession (POP)
In order to prevent certain attacks (see Appendix C) and to allow a
CA/RA to properly check the validity of the binding between a subject
and a key pair, the PKI management structures specified here make it
possible for a subject to prove that it has possession of (i.e., is
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able to use) the private key corresponding to the public key for
which a certificate is requested. A given CRP is free to choose how
to enforce POP (e.g., out-of-band procedural means versus the CRMF
in-band message) in its certification exchanges. Within a given CRP,
CAs and RAs are free to choose from among the POP methods provided
(i.e., this is a policy issue local to an RA/CA). A CRP SHOULD
define either which POP methods are required, or specify a mechanism
for clients to discover the POP methods supported.
Any CRP referencing this document MUST enforce POP by some means.
There are currently many non-PKIX operational protocols in use
(various electronic mail protocols are one example) that do not
explicitly check the binding between the end entity and the private
key. Until operational protocols that do verify the binding (for
signature, encryption, and key agreement key pairs) exist, and are
ubiquitous, this binding cannot be assumed to have been verified by
the CA/RA. Therefore, one cannot truly know if the binding of the
public key and the identity in the certificate is actually correct.
POP is accomplished in different ways depending on the type of key
for which a certificate is requested. If a key can be used for
multiple purposes (e.g., a signing and decryption RSA key), then any
of the methods MAY be used. Protocol designers need to be aware that
there can be hardware limitations on what POP methods may be usable,
e.g., if the private key is maintained in a hardware token.
This specification allows for cases where POP is validated by the CA,
the RA, or both. Some policies require the CA to verify POP during
certificate issuance, in which case the RA MUST forward the end
entity's CertRequest and ProofOfPossession fields unaltered to the
CA. (In this case, the RA could verify the POP and reject failing
certificate requests rather than forwarding them to the CA.) If the
CA is not required by policy to verify POP, then the RA SHOULD
forward the end entity's request and proof, unaltered, to the CA as
above. If this is not possible (for example because the RA verifies
POP by an out-of-band method), then the RA uses the raVerified
element to attest to the CA that the required proof has been
validated. If the CA/RA uses an out-of-band method to verify POP
(such as physical delivery of CA/RA-generated private keys), then the
ProofOfPossession field is omitted.
ProofOfPossession ::= CHOICE {
raVerified [0] NULL,
signature [1] POPOSigningKey,
keyEncipherment [2] POPOPrivKey,
keyAgreement [3] POPOPrivKey }
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The fields of ProofOfPossession have the following meaning:
raVerified indicates that the RA has performed the POP required on
the certificate request. This field is used by an RA when 1) the
CA is not required to do its own POP verification and 2) the RA
needs to change the contents of the certReq field. CRPs MUST
provide a method for the RA to sign the ProofOfPossession. A
requestor MUST NOT set this field and an RA/CA MUST NOT accept a
ProofOfPossession where the requestor sets this field.
signature is used for performing POP with signature keys. The
details of this field are covered in section 4.1.
keyEncipherment is used for performing POP with key encipherment
encryption based keys (i.e., RSA). The details of this field are
covered in section 4.2.
keyAgreement is used for performing POP with key agreement type
encryption keys (i.e., DH). The details of this field are covered
in section 4.3.
4.1. Signature Key POP
POP for a signature key is accomplished by performing a signature
operation on a piece of data containing the identity for which the
certificate is desired.
There are three cases that need to be looked at when doing a POP for
a signature key:
1. The certificate subject has not yet established an authenticated
identity with a CA/RA, but has a password and identity string
from the CA/RA. In this case, the POPOSigningKeyInput structure
would be filled out using the publicKeyMAC choice for authInfo,
and the password and identity would be used to compute the
publicKeyMAC value. The public key for the certificate being
requested would be placed in both the POPOSigningKeyInput and the
Certificate Template structures. The signature field is computed
over the DER-encoded POPOSigningKeyInput structure.
2. The CA/RA has established an authenticated identity for the
certificate subject, but the requestor is not placing it into the
certificate request. In this case, the POPOSigningKeyInput
structure would be filled out using the sender choice for
authInfo. The public key for the certificate being requested
would be placed in both the POPOSigningKeyInput and the
Certificate Template structures. The signature field is computed
over the DER-encoded POPOSigningKeyInput structure.
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3. The certificate subject places its name in the Certificate
Template structure along with the public key. In this case the
poposkInput field is omitted from the POPOSigningKey structure.
The signature field is computed over the DER-encoded certificate
template structure.
POPOSigningKey ::= SEQUENCE {
poposkInput [0] POPOSigningKeyInput OPTIONAL,
algorithmIdentifier AlgorithmIdentifier,
signature BIT STRING }
The fields of POPOSigningKey have the following meaning:
poposkInput contains the data to be signed, when present. This
field MUST be present when the certificate template does not
contain both the public key value and a subject name value.
algorithmIdentifier identifiers the signature algorithm and an
associated parameters used to produce the POP value.
signature contains the POP value produce. If poposkInput is
present, the signature is computed over the DER-encoded value of
poposkInput. If poposkInput is absent, the signature is computed
over the DER-encoded value of certReq.
POPOSigningKeyInput ::= SEQUENCE {
authInfo CHOICE {
sender [0] GeneralName,
-- used only if an authenticated identity has been
-- established for the sender (e.g., a DN from a
-- previously-issued and currently-valid certificate)
publicKeyMAC PKMACValue },
-- used if no authenticated GeneralName currently exists for
-- the sender; publicKeyMAC contains a password-based MAC
-- on the DER-encoded value of publicKey
publicKey SubjectPublicKeyInfo } -- from CertTemplate
The fields of POPOSigningKeyInput have the following meaning:
sender contains an authenticated identity that has been previously
established for the subject.
publicKeyMAC contains a computed value that uses a shared secret
between the CA/RA and the certificate requestor.
publicKey contains a copy of the public key from the certificate
template. This MUST be exactly the same value as is contained in
the certificate template.
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PKMACValue ::= SEQUENCE {
algId AlgorithmIdentifier,
value BIT STRING }
The fields of PKMACValue have the following meaning:
algId identifies the algorithm used to compute the MAC value. All
implementations MUST support id-PasswordBasedMAC. The details on
this algorithm are presented in section 4.4.
value contains the computed MAC value. The MAC value is computed
over the DER-encoded public key of the certificate subject.
The CA/RA identifies the shared secret to be used by looking at 1)
the general name field in the certificate request or 2) either the
regToken (see section 6.1) or authToken (see section 6.2) controls.
4.2. Key Encipherment Keys
POP for key encipherment keys is accomplished by one of three
different methods. The private key can be provided to the CA/RA, an
encrypted challenge from the CA/RA can be decrypted (direct method),
or the created certificate can be returned encrypted and used as the
challenge response (indirect method).
POPOPrivKey ::= CHOICE {
thisMessage [0] BIT STRING, -- deprecated
subsequentMessage [1] SubsequentMessage,
dhMAC [2] BIT STRING, -- deprecated
agreeMAC [3] PKMACValue,
encryptedKey [4] EnvelopedData }
-- for keyAgreement (only), possession is proven in this message
-- (which contains a MAC (over the DER-encoded value of the
-- certReq parameter in CertReqMsg, which must include both subject
-- and publicKey) based on a key derived from the end entity's
-- private DH key and the CA's public DH key);
-- the dhMAC value MUST be calculated as per the directions given
-- in RFC 2875 for static DH proof-of-possession.
SubsequentMessage ::= INTEGER {
encrCert (0),
challengeResp (1) }
The fields of POPOPrivKey have the following meaning:
thisMessage contains the encrypted private key for which a
certificate is to be issued. The possession of the private key is
proved by providing it to the CA/RA. This field was incorrectly
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typed when the specification was first written. The correct way
to use this field is to create an EncryptedValue structure where
the encrypted content is the private key, the EncryptedValue
structure is then wrapped in the BIT STRING type. This field has
been deprecated in favor of encryptedKey.
subsequentMessage is used to indicate that the POP will be
completed by decrypting a message from the CA/RA and returning a
response. The type of message to be decrypted is indicated by the
value used.
encrCert indicates that the certificate issued is to be
returned in an encrypted form. The requestor is required to
decrypt the certificate and prove success to the CA/RA. The
details of this are provided by the CRP.
challengeResponse indicates that a challenge message is to be
sent from the CA/RA to the requestor. The details of the
challenge message and the response are to be provided by the
CRP.
dhMAC is used for Diffie-Hellman key agreement keys. It contains
a computed MAC that is obtained by using the requestor's private
key and the CA/RA public key. The use of this field is deprecated
in favor of the agreeMAC field. Details are covered in section
4.3.
agreeMAC is used for key agreement keys. It contains a computed
MAC that is obtained by using the requestor's private key and a
matching CA/RA public key. Details are covered in section 4.3.
macAlg contains the algorithm identifying the method used to
compute the MAC value.
macValue contains the computed MAC value.
encryptedKey contains the encrypted private key matching the
public key for which the certificate is to be issued. It also
contains an identification value to indicate it was constructed by
the requestor of the certificate. The enveloped content type MUST
be id-ct-encKeyWithID.
It is expected that protocols that incorporate this specification
will include the confirmation and challenge-response messages
necessary for a complete protocol.
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4.2.1. Private Key Info Content Type
This content type is used for 1) proving possession of private keys
and 2) escrow of private keys (using the archive options control in
section 6.4). This structure is based on the private key info
structure from [PKCS8] but has one deliberate difference. There is a
potential attack on escrow agents if they decrypt the private key but
don't know to whom the encrypted key is supposed to belong. An
attacker could intercept the encrypted private key, build a
certificate request around it and then ask for a recovery operation
on the private key.
This content type and its structure are:
id-ct-encKeyWithID OBJECT IDENTIFIER ::= {id-ct 21}
EncKeyWithID ::= SEQUENCE {
privateKey PrivateKeyInfo,
identifier CHOICE {
string UTF8String,
generalName GeneralName
} OPTIONAL
}
PrivateKeyInfo ::= SEQUENCE {
version INTEGER,
privateKeyAlgorithm AlgorithmIdentifier,
privateKey OCTET STRING,
attributes [0] IMPLICIT Attributes OPTIONAL
}
Attributes ::= SET OF Attribute
The fields of EncKeyWithID are defined as:
privateKey contains the encoded private key. Definitions for
three private key formats are included in this document.
Specifications for asymmetric algorithms need to include both the
public and private key definitions for consistency.
identifier contains a name that the CA/RA can associate with the
requestor. This will generally be either the DN of a certificate
or a text token passed and known to both the requestor and the
CA/RA. This field MUST be present if the purpose is to prove
possession of the private key. The field SHOULD be present if
archiving a key and the archive agent is expected to decrypt the
key.
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The fields of PrivatekeyInfo are define as:
version MUST be the value 0
privateKeyAlgorithm contains the identifier for the private key
object
privateKey is an octet string whose contents is the private key
and whose format is defined by the value of privateKeyAlgorithm.
attributes is a set of attributes. They are extended information
that is part of the private key information.
4.2.2. Private Key Structures
We are defining the structures here to be used for three algorithms.
4.2.2.1. D-H Private Keys
When creating a PrivateKeyInfo for a D-H key, the following rules
apply:
1. The privateKeyAlgorithm MUST be set to id-dh-private-number.
The parameter for id-dh-private-number is DomainParameters
(imported from [PKIXALG]).
2. The ASN structure for privateKey MUST be
DH-PrivateKey ::= INTEGER
3. The attributes field MUST be omitted.
4.2.2.2. DSA Private Keys
When creating a PrivateKeyInfo for a DSA key, the following rules
apply:
1. The privateKeyAlgorithm MUST be set to id-dsa. The parameters
for id-dsa is Dss-Parms (imported from [PKIXALG]).
2. The ASN structure for privateKey MUST be
DSA-PrivateKey ::= INTEGER
3. The attributes field MUST be omitted.
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4.2.2.3. RSA Private Keys
When creating a PrivateKeyInfo for an RSA key, the following rules
apply:
1. The privateKeyAlgorithm MUST be set to rsaEncryption.
2. The ASN structure for privateKey MUST be RSAPrivateKey (defined
in [PKCS1])
3. The attributes field MUST be omitted.
4.2.3. Challenge-Response Guidelines
The following provides guidelines to enrollment protocol authors
about how an indirect proof-of-possession is expected to work and
about some of the areas where one needs to be careful in crafting the
messages to implement this POP method.
1. The original enrollment request includes a proof of identity of
some type and the public portion of the encryption key. Note
that the proof of identity needs to cover the public portion of
the encryption key to prevent substitution attacks (where the
attacker changes your public key for his public key).
2. The response message from the server includes an encrypted data
value of some type. That value needs to be authenticated in some
fashion as having come from the server. The specification needs
to include the specifics of how this value is returned for the
different key types. For RSA keys, the value can be specified as
being directly encrypted by the RSA public key; this will not
work for a D-H key where you need to specify an indirect
mechanism to encrypt the value.
3. The second request message includes a hash of the decrypted
value. This message MUST NOT be just the hash of the encrypted
value, as one should never "sign" a completely random value. It
is desirable to include information such as the identity string
in the hashing process so that this can be made explicitly. This
returned value MUST be included in a second proof of identity.
It is strongly suggested that transaction identifiers and nonce
values be required when performing indirect POP, as this allows for
1) tying the different messages in the process together and 2)
letting each entity inject some amount of random data into the
process of doing identity proofs.
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4.3. Key Agreement Keys
POP for key agreement keys is accomplished by one of four different
methods. The first three are identical to those presented above for
key encryption keys. The fourth method takes advantage of the fact
that a shared secret is produced and that the value can be used to
MAC information.
When the direct or indirect encryption methods presented above are
used, the CA/RA will need to create an ephemeral key for those cases
where the encryption algorithm parameters do not match between the
CA/RA and the requestor.
The end entity may also MAC the certificate request (using a shared
secret key derived from computation) as a fourth alternative for
demonstrating POP. This option may be used only if the CA/RA already
has a certificate that is known to the end entity and if the Subject
is able to use the CA/RA's parameters.
For the DH key agreement algorithm, all implementations MUST support
the static DH Proof-of-Possession. Details on this algorithm can be
found in section 3 of [RFC2875]. NOTE: If either the subject or
issuer name in the CA certificate is empty, then the alternative name
should be used in its place.
4.4. Use of Password-Based MAC
This MAC algorithm was designed to take a shared secret (a password)
and use it to compute a check value over a piece of information. The
assumption is that, without the password, the correct check value
cannot be computed. The algorithm computes the one-way function
multiple times in order to slow down any dictionary attacks against
the password value.
The algorithm identifier and parameter structure used for Password-
Based MAC is:
id-PasswordBasedMAC OBJECT IDENTIFIER ::=
{ 1 2 840 113533 7 66 13}
PBMParameter ::= SEQUENCE {
salt OCTET STRING,
owf AlgorithmIdentifier,
iterationCount INTEGER,
mac AlgorithmIdentifier
)
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The fields of PEMParameter have the following meaning:
salt contains a randomly generated value used in computing the key
of the MAC process. The salt SHOULD be at least 8 octets (64
bits) long.
owf identifies the algorithm and associated parameters used to
compute the key used in the MAC process. All implementations MUST
support SHA-1.
iterationCount identifies the number of times the hash is applied
during the key computation process. The iterationCount MUST be a
minimum of 100. Many people suggest using values as high as 1000
iterations as the minimum value. The trade off here is between
protection of the password from attacks and the time spent by the
server processing all of the different iterations in deriving
passwords. Hashing is generally considered a cheap operation but
this may not be true with all hash functions in the future.
mac identifies the algorithm and associated parameters of the MAC
function to be used. All implementations MUST support HMAC-SHA1
[HMAC]. All implementations SHOULD support DES-MAC and Triple-
DES-MAC [PKCS11].
The following is pseudo-code for the algorithm:
Inputs:
pw - an octet string containing the user's password
data - an octet string containing the value to be MAC-ed
Iter - iteration count
Output:
MAC - an octet string containing the resultant MAC value
1. Generate a random salt value S
2. Append the salt to the pw. K = pw || salt.
3. Hash the value of K. K = HASH(K)
4. If Iter is greater than zero. Iter = Iter - 1. Goto step 3.
5. Compute an HMAC as documented in [HMAC].
MAC = HASH( K XOR opad, HASH( K XOR ipad, data) )
Where opad and ipad are defined in [HMAC].
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5. CertRequest syntax
The CertRequest syntax consists of a request identifier, a template
of certificate content, and an optional sequence of control
information.
CertRequest ::= SEQUENCE {
certReqId INTEGER, -- ID for matching request and reply
certTemplate CertTemplate, --Selected fields of cert to be issued
controls Controls OPTIONAL } -- Attributes affecting issuance
CertTemplate ::= SEQUENCE {
version [0] Version OPTIONAL,
serialNumber [1] INTEGER OPTIONAL,
signingAlg [2] AlgorithmIdentifier OPTIONAL,
issuer [3] Name OPTIONAL,
validity [4] OptionalValidity OPTIONAL,
subject [5] Name OPTIONAL,
publicKey [6] SubjectPublicKeyInfo OPTIONAL,
issuerUID [7] UniqueIdentifier OPTIONAL,
subjectUID [8] UniqueIdentifier OPTIONAL,
extensions [9] Extensions OPTIONAL }
OptionalValidity ::= SEQUENCE {
notBefore [0] Time OPTIONAL,
notAfter [1] Time OPTIONAL } --at least one must be present
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
The fields of CertRequest have the following meaning:
certReqId contains an integer value that is used by the
certificate requestor to associate a specific certificate request
with a certificate response.
certTemplate contains a template of an X.509 certificate. The
requestor fills in those fields for which specific values are
desired. Details on the fields are given below.
controls contains attributes that are not part of the certificate,
but control the context in which the certificate is to be issued.
Details on the controls defined in this document can be found in
section 6. Other documents may define other controls. CRPs are
responsible for specifying which controls are required.
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The fields of CertTemplate have the following meaning:
version MUST be 2 if supplied. It SHOULD be omitted.
serialNumber MUST be omitted. This field is assigned by the CA
during certificate creation.
signingAlg MUST be omitted. This field is assigned by the CA
during certificate creation.
issuer is normally omitted. It would be filled in with the CA
that the requestor desires to issue the certificate in situations
where an RA is servicing more than one CA.
validity is normally omitted. It can be used to request that
certificates either start at some point in the future or expire at
some specific time. A case where this field would commonly be
used is when a cross certificate is issued for a CA. In this case
the validity of an existing certificate would be placed in this
field so that the new certificate would have the same validity
period as the existing certificate. If validity is not omitted,
then at least one of the sub-fields MUST be specified. The sub-
fields are as follows:
notBefore contains the requested start time of the certificate.
The time follows the same rules as the notBefore time in
[PROFILE].
notAfter contains the requested expiration time of the
certificate. The time follows the same rules as the notAfter
time in [PROFILE].
subject is filled in with the suggested name for the requestor.
This would normally be filled in by a name that has been
previously issued to the requestor by the CA.
publicKey contains the public key for which the certificate is
being created. This field MUST be filled in if the requestor
generates its own key. The field is omitted if the key is
generated by the RA/CA.
issuerUID MUST be omitted. This field has been deprecated in
[PROFILE].
subjectUID MUST be omitted. This field has been deprecated in
[PROFILE].
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extensions contains extensions that the requestor wants to have
placed in the certificate. These extensions would generally deal
with things such as setting the key usage to keyEncipherment.
With the exception of the publicKey field, the CA/RA is permitted to
alter any requested field. The returned certificate needs to be
checked by the requestor to see if the fields have been set in an
acceptable manner. CA/RA SHOULD use the template fields if possible.
There are cases where all fields of the template can be omitted. If
the key generation is being done at the CA/RA and the identity proof
is placed in a different location (such as the id-regCtrl-regToken
below), then there are no fields that need to be specified by the
certificate requestor.
6. Controls Syntax
The generator of a CertRequest may include one or more control values
pertaining to the processing of the request.
Controls ::= SEQUENCE SIZE(1..MAX) OF AttributeTypeAndValue
The following controls are defined by this document: regToken
(section 6.1); authenticator (section 6.2); pkiPublicationInfo
(section 6.3); pkiArchiveOptions (section 6.4); oldCertID (section
6.5); protocolEncrKey (section 6.6). Each CRP MUST define the set of
controls supported by that protocol. Additional controls may be
defined by additional RFCs or by the CRP protocol itself.
6.1. Registration Token Control
A regToken control contains one-time information (either based on a
secret value or other shared information) intended to be used by the
CA to verify the identity of the subject prior to issuing a
certificate. Upon receipt of a certification request containing a
value for regToken, the receiving CA verifies the information in
order to confirm the identity claimed in the certification request.
The value for regToken may be generated by the CA and provided out of
band to the subscriber, or may otherwise be available to both the CA
and the subscriber. The security of any out-of-band exchange should
be commensurate with the risk that the CA will tolerate with regard
to accepting an intercepted value from someone other than the
intended subscriber. The regToken value is not encrypted on return,
if the data is considered to be sensitive, it needs to be shrouded by
the requestor.
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The regToken control is used only for initialization of an end entity
into the PKI, whereas the authenticator control (see section 7.2) can
be used for the initial as well as subsequent certification requests.
In some instances of use the value for regToken could be a text
string or a numeric quantity such as a random number. In the latter
case, the value is encoded as a text string representation of the
binary quantity. The encoding of regToken SHALL be UTF8String.
id-regCtrl-regToken OBJECT IDENTIFIER ::= { id-regCtrl 1 }
Without prior agreement between the subscriber and CA agents, this
value would be a textual shared secret of some type. If a computed
value based on that shared secret is to be used instead, it is
suggested that the CRP define a new registration control for that
specific computation.
6.2. Authenticator Control
An authenticator control contains information used on an ongoing
basis to establish a non-cryptographic check of identity in
communication with the CA. Examples include: mother's maiden name,
last four digits of social security number, or other knowledge-based
information shared with the subscriber's CA; a hash of such
information; or other information produced for this purpose. The
value for an authenticator control may be generated by the subscriber
or by the CA.
In some instances of use, the value for authenticator could be a text
string or a numeric quantity such as a random number. The value in
the latter case is encoded as a text string representation of the
binary quantity. The encoding of authenticator SHALL be UTF8String.
id-regCtrl-authenticator OBJECT IDENTIFIER ::= { id-regCtrl 2 }
When deciding whether to use an authenticator or a regToken, use the
following guidelines. If the value is a one-time usage value, then
regToken would be used. If the value has a long-term usage, then the
authenticator control would be used.
6.3. Publication Information Control
The pkiPublicationInfo control enables subscribers to influence the
CA/RA's publication of the certificate. This control is considered
advisory and can be ignored by CAs/RAs. It is defined by the
following OID and syntax:
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id-regCtrl-pkiPublicationInfo OBJECT IDENTIFIER ::= { id-regCtrl 3 }
PKIPublicationInfo ::= SEQUENCE {
action INTEGER {
dontPublish (0),
pleasePublish (1) },
pubInfos SEQUENCE SIZE (1..MAX) OF SinglePubInfo OPTIONAL }
SinglePubInfo ::= SEQUENCE {
pubMethod INTEGER {
dontCare (0),
x500 (1),
web (2),
ldap (3) },
pubLocation GeneralName OPTIONAL }
The fields of PKIPublicationInfo have the following meaning:
action indicates whether or not the requestor wishes the CA/RA to
publish the certificate. The values and their means are:
dontPublish indicates that the requester wishes the CA/RA not
to publish the certificate (this may indicate that the
requester intends to publish the certificate him/herself). If
dontPublish is used, the pubInfos field MUST be omitted.
pleasePublish indicates that the requestor wishes the CA/RA to
publish the certificate.
pubInfos holds the locations where the requestor desires the CA/RA
to publish the certificate. This field is omitted if the
dontPublish choice is selected. If the requestor wants to specify
some locations for the certificate to be published, and to allow
the CA/RA to publish in other locations, it would specify multiple
values of the SinglePubInfo structure, one of which would be
dontCare.
The fields of SinglePubInfo have the following meaning:
pubMethod indicates the address type for the location at which the
requestor desires the certificate to be placed by the CA/RA.
dontCare indicates that the CA/RA can publish the certificate
in whatever locations it chooses. If dontCare is used, the
pubInfos field MUST be omitted.
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x500 indicates that the requestor wishes for the CA/RA to
publish the certificate in a specific location. The location
is indicated in the x500 field of pubLocation.
ldap indicates that the requestor wishes for the CA/RA to
publish the certificate in a specific location. The location
is indicated in the ldap field of pubLocation.
web indicates that the requestor wishes for the CA/RA to
publish the certificate in a specific location. The location
is indicated in the http field of pubLocation.
pubLocation contains the address at which the certificate is to be
placed. The choice in the general name field is dictated by the
pubMethod selection in this structure.
Publication locations can be supplied in any order. All locations
are to be processed by the CA for purposes of publication.
6.4. Archive Options Control
The pkiArchiveOptions control enables subscribers to supply
information needed to establish an archive of the private key
corresponding to the public key of the certification request. It is
defined by the following OID and syntax:
id-regCtrl-pkiArchiveOptions OBJECT IDENTIFIER ::= { id-regCtrl 4 }
PKIArchiveOptions ::= CHOICE {
encryptedPrivKey [0] EncryptedKey,
-- the actual value of the private key
keyGenParameters [1] KeyGenParameters,
-- parameters which allow the private key to be re-generated
archiveRemGenPrivKey [2] BOOLEAN }
-- set to TRUE if sender wishes receiver to archive the private
-- key of a key pair that the receiver generates in response to
-- this request; set to FALSE if no archival is desired.
EncryptedKey ::= CHOICE {
encryptedValue EncryptedValue, -- deprecated
envelopedData [0] EnvelopedData }
-- The encrypted private key MUST be placed in the envelopedData
-- encryptedContentInfo encryptedContent OCTET STRING.
EncryptedValue ::= SEQUENCE {
intendedAlg [0] AlgorithmIdentifier OPTIONAL,
-- the intended algorithm for which the value will be used
symmAlg [1] AlgorithmIdentifier OPTIONAL,
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-- the symmetric algorithm used to encrypt the value
encSymmKey [2] BIT STRING OPTIONAL,
-- the (encrypted) symmetric key used to encrypt the value
keyAlg [3] AlgorithmIdentifier OPTIONAL,
-- algorithm used to encrypt the symmetric key
valueHint [4] OCTET STRING OPTIONAL,
-- a brief description or identifier of the encValue content
-- (may be meaningful only to the sending entity, and used only
-- if EncryptedValue might be re-examined by the sending entity
-- in the future)
encValue BIT STRING }
-- The use of the EncryptedValue field has been deprecated in favor
-- of the EnvelopedData structure.
--
-- When EncryptedValue is used to carry a private key (as opposed to
-- a certificate), implementations MUST support the encValue field
-- containing an encrypted PrivateKeyInfo as defined in [PKCS11],
-- section 12.11. If encValue contains some other format/encoding
-- for the private key, the first octet of valueHint MAY be used
-- to indicate the format/encoding (but note that the possible values
-- of this octet are not specified at this time). In all cases, the
-- intendedAlg field MUST be used to indicate at least the OID of
-- the intended algorithm of the private key, unless this information
-- is known a priori to both sender and receiver by some other means.
KeyGenParameters ::= OCTET STRING
The fields of PKIArchiveOptions have the following meaning:
encryptedPrivKey contains an encrypted version of the private key.
keyGenParameters contains the information needed by the requestor
to regenerate the private key. As an example, for many RSA
implementations one could send the first random number(s) tested
for primality. The structure to go here is not defined by this
document. CRPs that define content for this structure MUST define
not only the content that is to go here, but also how that data is
shrouded from unauthorized access.
archiveRemGenPrivKey indicates that the requestor desires that the
key generated by the CA/RA on the requestor's behalf be archived.
The fields of EncryptedKey have the following meaning:
encryptedValue is longer used. This field has been deprecated
along with the EncryptedValue structure.
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envelopedData contains the encrypted value of the private key.
CPRs that use this structure MUST define the entity or entities
for whom the data is to be encrypted (the EE, escrow agents, CAs)
and how that key or set of keys is to be determined. Details on
constructing an EnvelopedData structure are found in [CMS]. The
encrypted content MUST be an id-ct-encKeyWithID. The identifier
can be omitted unless this structure is also being used to do
proof-of-possession.
6.5. OldCert ID Control
If present, the OldCertID control specifies the certificate to be
updated by the current certification request. The OID and syntax is:
id-regCtrl-oldCertID OBJECT IDENTIFIER ::= { id-regCtrl 5 }
CertId ::= SEQUENCE {
issuer GeneralName,
serialNumber INTEGER
}
6.6. Protocol Encryption Key Control
If present, the protocolEncrKey control specifies a key that the CA
is to use in encrypting a response to CertReqMessages. The OID for
this control is id-regCtrl-protocolEncrKey. The parameter structure
for this field is SubjectPublicKeyInfo. (This structure is defined
in [PROFILE].)
id-regCtrl-protocolEncrKey OBJECT IDENTIFIER ::= { id-regCtrl 6 }
This control is used when a CA has information to send to the
subscriber that needs to be encrypted. Such information includes a
private key generated by the CA for use by the subscriber.
7. RegInfo Controls
This section documents the controls that are to be placed in the
regInfo field of the CertReqMsg structure.
7.1. utf8Pairs
This control is used to convey text-based information from the
Subject to an RA to a CA issuing a certificate. The OID for this
structure is id-regInfo-utf8Paris and has a type of UTF8String.
id-regInfo-utf8Pairs OBJECT IDENTIFIER ::= { id-regInfo 1 }
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The name is terminated by the question mark character ('?'). The
value is terminated by the percent character '%'. Name value pairs
can be repeated. Thus the syntax is:
Name?Value%[Name?Value%]*
The %xx mechanism of [RFC1738] is used to encode '?' (%3f) and '%'
(%25) if they are not being used for their reserved purpose. Names
MUST NOT start with a numeric character.
This control can appear multiple times in the regInfo structure.
Resolution of conflicts of information is a matter of local policy on
the RA/CA.
Appendix A contains a set of common names and data formats
corresponding to fields that commonly appear in certificates and
directories.
7.2. certReq
This control is designed to deal with the problem where an RA needs
to modify the certificate template proposed by a Subject, but the
Subject used the certificate template as part of its POP calculation.
In this case, the RA can place a new certificate template in the
regInfo sequence.
This control has the OID id-regInfo-certReq and the structure
CertRequest. There can only be one instance of this attribute in the
regInfo sequence. If this control exists in the regInfo structure,
then the certificate template in the request is ignored. The RA MUST
copy all data from the core template to this attribute.
id-regInfo-certReq OBJECT IDENTIFIER ::= { id-regInfo 2 }
8. Object Identifiers
The OID id-pkix has the value
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
-- arc for Internet X.509 PKI protocols and their components
id-pkip OBJECT IDENTIFIER :: { id-pkix pkip(5) }
-- arc for Registration Controls in CRMF
id-regCtrl OBJECT IDENTIFIER ::= { id-pkip regCtrl(1) }
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RFC 4211 Internet X.509 CRMF September 2005
-- arc for Registration Info in CRMF
id-regInfo OBJECT IDENTIFIER ::= { id-pkip id-regInfo(2) }
9. Security Considerations
Enrollment protocols, by their very nature, involve large amounts of
private information. This can include private keys, identity
numbers, credit card numbers, and the like. The security of any CRP
is based on the security mechanisms of the protocol and/or process
used to communicate between CAs, RAs and EEs. All protocols must
provide for masking, either via encryption or off-line processing, of
all subscriber-sensitive information.
Many enrollment protocols provide for the initial establishment of
identity between the CA/RA and the EE by the use of a token.
Generally this token is delivered using an out-of-band delivery
method (such as the governmental mail system). The security of any
out-of-band exchange needs to be commensurate with the risk that the
CA/RA will tolerate with regard to interception of the token by a
third party.
Implementation must implement Proof-of-Possession (POP) values during
certificate enrollment processes. A good POP algorithm needs to
provide proof of two things: 1) that the key is tied to a specific
user and 2) that the user has use of the key in question. Failure to
implement POP allows people to create certificates where the public
key and the name values do not correctly bind. This allows for
impersonation on signature keys and interception of encrypted
messages.
Implementations must use high entropy random number generators in
producing private keys. Implementations must randomly generate
content-encryption keys, message-authentication keys, initialization
vectors (IVs), salt, and padding. The use of inadequate pseudo-
random number generators (PRNGs) to generate cryptographic keys can
result in little or no security. An attacker may find it much easier
to reproduce the PRNG environment that produced the keys, searching
the resulting small set of possibilities, rather than brute force
searching the whole key space. The generation of quality random
numbers is difficult. RFC 4086 [RANDOM] offers important guidance in
this area and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
PRNG technique.
Implementations must protect private keys. The compromise of a
signer's private key permits third parties to masquerade as the
signer. The compromise of a decryption private key allows for
interception of messages by a third party.
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One feature of the certificate message request syntax is for the key
generation to be performed remotely from the creation of the
certificate request. This feature should never be used for
generation of signing keys. If signing keys are generated for the
user, then an element of repudiation comes into play. The user can
claim that an item was signed by the entity that generated the key as
well as any entity that might have seen the key value during transfer
from the generator the to EE. Care must be taken to protect
encryption keys by the remote key generator to protect against
interception of the keys by a third party. This means that the
encryption algorithms used need to be secure, and a content
encryption key or a key encryption key must be used to mask the
private key during transport back to the user. CRP protocols must
never assume that a signature key generated by the user can be used
to decrypt the package in which an encryption private key is
transported.
This document describes a method by which key escrow may be done.
There are several issues that need to be taken into account when
doing key escrow. First, the client must be able to correctly
identify the entity to which a key is to be escrowed or the CRP must
provide a method by which the client can discover this information.
A CRP cannot assume that the key escrow agent and the CA are the same
entity and thus have the same names. Second, the algorithms used to
mask the private key or other key generation information during
transport to the escrow agent need to be commensurate with the value
of the data being protected by the key. Third, the escrow agent
needs to provide sufficient safeguards that an escrowed key is
returned only to entities that should be able to obtain the private
key. Generally, this should be restricted to the entity that
escrowed the data. Fourth, the escrow data base needs to be stored
in a secure manner. One common method for doing this is to re-
encrypt the data to keys that only the escrow agent has access to.
In this case, one may need to escrow the escrow agent key as well.
Access to either the escrow agent or the archived key would amount to
access to all private keys that have been escrowed with that agent.
10. References
10.1. Normative References
[PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
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[PKCS11] RSA Laboratories, The Public-Key Cryptography Standards -
"PKCS #11 v2.11: Cryptographic Token Interface Standard",
RSA Security Inc., June 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PROFILE] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[PKIXALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[RFC2875] Prafullchandra, H. and J. Schaad, "Diffie-Hellman
Proof-of-Possession Algorithms", RFC 2875, July 2000.
10.2. Informative References
[DSS] National Institute of Standards and Technology, FIPS Pub
186: Digital Signature Standard, May 1994.
[PKCS8] RSA Laboratories, "PKCS #8: Private-Key Information Syntax
Standard", PKCS #8 v1.2, November 1993.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
HMAC-SHA-1", RFC 2202, September 1997.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
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11. Acknowledgements
The working group would like to thank Michael Myers, Carlisle Adams,
Dave Solo, and David Kemp, who authored the original version of this
document.
The working group also gratefully acknowledges the contributions of
Barbara Fox, Warwick Ford, Russ Housley, and John Pawling, whose
review and comments significantly clarified and improved the utility
of this specification. The members of the ca-talk mailing list also
provided significant input with respect to interoperability testing.
The text of Appendix C (Why do POP) was taken from an e-mail message
by Al Arsenault and was originally part of the PKIX Roadmap document.
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Appendix A. Use of RegInfo for Name-Value Pairs
The "value" field of the id-regInfo-utf8Pairs string (with "tag"
field equal to 12 and appropriate "length" field) will contain a
series of UTF-8 name/value pairs.
This Appendix lists some common examples of such pairs for the
purpose of promoting interoperability among independent
implementations of this specification. It is recognized that this
list is not exhaustive and will grow with time and implementation
experience.
A.1. Defined Names
The following table defines a recommended set of named elements. The
value in the column "Name Value" is the exact text string that will
appear in the regInfo.
Name Value
----------
version -- version of this variation of regInfo use
corp_company -- company affiliation of subscriber
org_unit -- organizational unit
mail_firstName -- personal name component
mail_middleName -- personal name component
mail_lastName -- personal name component
mail_email -- subscriber's email address
jobTitle -- job title of subscriber
employeeID -- employee identification number or string
mailStop -- mail stop
issuerName -- name of CA
subjectName -- name of Subject
validity -- validity interval
For example:
version?1%corp_company?Example, Inc.%org_unit?Engineering%
mail_firstName?John%mail_lastName?Smith%jobTitle?Team Leader%
mail_email?john@example.com%
A.2. IssuerName, SubjectName, and Validity Value Encoding
When they appear in id-regInfo-utf8Pairs syntax as named elements,
the encoding of values for issuerName, subjectName, and validity
SHALL use the following syntax. The characters [] indicate an
optional field, ::= and | have their usual BNF meanings, and all
other symbols (except spaces, which are insignificant) outside non-
terminal names are terminals. Alphabetics are case-sensitive.
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issuerName ::=
subjectName ::=
::= | :
::= validity ? []-[]
::=
RFC, FYI, BCP