RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey,
ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
recipientEncryptedKeys RecipientEncryptedKeys }
OriginatorIdentifierOrKey ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier,
originatorKey [1] OriginatorPublicKey }
OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
kekid KEKIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KEKIdentifier ::= SEQUENCE {
keyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0
keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType }
DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Digest ::= OCTET STRING
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [3] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
RevocationInfoChoices ::= SET OF RevocationInfoChoice
RevocationInfoChoice ::= CHOICE {
crl CertificateList,
other [1] IMPLICIT OtherRevocationInfoFormat }
OtherRevocationInfoFormat ::= SEQUENCE {
otherRevInfoFormat OBJECT IDENTIFIER,
otherRevInfo ANY DEFINED BY otherRevInfoFormat }
CertificateChoices ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat }
AttributeCertificateV2 ::= AttributeCertificate
OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat }
CertificateSet ::= SET OF CertificateChoices
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
UserKeyingMaterial ::= OCTET STRING
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
-- Content Type Object Identifiers
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 2 }
-- The CMS Attributes
MessageDigest ::= OCTET STRING
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
Countersignature ::= SignerInfo
-- Attribute Object Identifiers
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
-- Obsolete Extended Certificate syntax from PKCS#6
ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate }
ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
END -- of CryptographicMessageSyntax2004
12.2. Version 1 Attribute Certificate ASN.1 Module
AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS All
IMPORTS
-- Imports from RFC 3280 [PROFILE], Appendix A.1
AlgorithmIdentifier, Attribute, CertificateSerialNumber,
Extensions, UniqueIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from RFC 3280 [PROFILE], Appendix A.2
GeneralNames
FROM PKIX1Implicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-implicit(19) }
-- Imports from RFC 3281 [ACPROFILE], Appendix B
AttCertValidityPeriod, IssuerSerial
FROM PKIXAttributeCertificate
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) attribute-cert(12) } ;
-- Definition extracted from X.509-1997 [X.509-97], but
-- different type names are used to avoid collisions.
AttributeCertificateV1 ::= SEQUENCE {
acInfo AttributeCertificateInfoV1,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING }
AttributeCertificateInfoV1 ::= SEQUENCE {
version AttCertVersionV1 DEFAULT v1,
subject CHOICE {
baseCertificateID [0] IssuerSerial,
-- associated with a Public Key Certificate
subjectName [1] GeneralNames },
-- associated with a name
issuer GeneralNames,
signature AlgorithmIdentifier,
serialNumber CertificateSerialNumber,
attCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL }
AttCertVersionV1 ::= INTEGER { v1(0) }
END -- of AttributeCertificateVersion1
13. References
13.1. Normative References
[ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281, April
2002.
[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.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
[X.501-88] CCITT. Recommendation X.501: The Directory - Models.
1988.
[X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988.
[X.509-97] ITU-T. Recommendation X.509: The Directory -
Authentication Framework. 1997.
[X.509-00] ITU-T. Recommendation X.509: The Directory -
Authentication Framework. 2000.
13.2. Informative References
[CMS1] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[CMS2] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3369, August 2002.
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[MSAC] Microsoft Development Network (MSDN) Library,
"Authenticode", April 2004 Release.
[MSG] Ramsdell, B., "S/MIME Version 3.1 Message
Specification", RFC 3851, July 2004.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June
1999.
[OLDMSG] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification", RFC
2311, March 1998.
[PKCS#6] RSA Laboratories. PKCS #6: Extended-Certificate Syntax
Standard, Version 1.5. November 1993.
[PKCS#7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[PKCS#9] RSA Laboratories. PKCS #9: Selected Attribute Types,
Version 1.1. November 1993.
[PWRI] Gutmann, P., "Password-based Encryption for CMS", RFC
3211, December 2001.
[RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
December 1994.
14. Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, encrypting data, and authenticating
data.
Implementations must protect the signer’s private key. Compromise of
the signer’s private key permits masquerade.
Implementations must protect the key management private key, the
key-encryption key, and the content-encryption key. Compromise of
the key management private key or the key-encryption key may result
in the disclosure of all contents protected with that key.
Similarly, compromise of the content-encryption key may result in
disclosure of the associated encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable
modification of the authenticated content.
The key management technique employed to distribute message-
authentication keys must itself provide data origin authentication,
otherwise the contents are delivered with integrity from an unknown
source. Neither RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static
Diffie-Hellman [DH-X9.42] provide the necessary data origin
authentication. Static-Static Diffie-Hellman [DH-X9.42] does provide
the necessary data origin authentication when both the originator and
recipient public keys are bound to appropriate identities in X.509
certificates.
When more than two parties share the same message-authentication key,
data origin authentication is not provided. Any party that knows the
message-authentication key can compute a valid MAC, therefore the
contents could originate from any one of the parties.
Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), and
padding. Also, the generation of public/private key pairs relies on
a random numbers. 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 1750 [RANDOM] offers important guidance in
this area.
When using key agreement algorithms or previously distributed
symmetric key-encryption keys, a key-encryption key is used to
encrypt the content-encryption key. If the key-encryption and
content-encryption algorithms are different, the effective security
is determined by the weaker of the two algorithms. If, for example,
content is encrypted with Triple-DES using a 168-bit Triple-DES
content-encryption key, and the content-encryption key is wrapped
with RC2 using a 40-bit RC2 key-encryption key, then at most 40 bits
of protection is provided. A trivial search to determine the value
of the 40-bit RC2 key can recover the Triple-DES key, and then the
Triple-DES key can be used to decrypt the content. Therefore,
implementers must ensure that key-encryption algorithms are as strong
or stronger than content-encryption algorithms.
Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptoanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will be reduced. Therefore, cryptographic
algorithm implementations should be modular, allowing new algorithms
to be readily inserted. That is, implementors should be prepared for
the set of algorithms that must be supported to change over time.
The countersignature unsigned attribute includes a digital signature
that is computed on the content signature value, thus the
countersigning process need not know the original signed content.
This structure permits implementation efficiency advantages; however,
this structure may also permit the countersigning of an inappropriate
signature value. Therefore, implementations that perform
countersignatures should either verify the original signature value
prior to countersigning it (this verification requires processing of
the original content), or implementations should perform
countersigning in a context that ensures that only appropriate
signature values are countersigned.
15. Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
Jim Schaad, Dave Solo, Paul Timmel, and Sean Turner for their efforts
and support.
16. Author’s Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
17. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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