Network Working Group R. Housley
Request for Comments: 3370 RSA Laboratories
Obsoletes: 2630, 3211 August 2002
Category: Standards Track

Cryptographic Message Syntax (CMS) Algorithms

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 (2002). All Rights Reserved.

Abstract

This document describes the conventions for using several
cryptographic algorithms with the Cryptographic Message Syntax (CMS).
The CMS is used to digitally sign, digest, authenticate, or encrypt
arbitrary message contents.

Table of Contents

1 IntrodUCtion ............................................... 2
1.1 Changes Since RFC2630 ..................................... 2
1.2 Terminology ................................................ 2
2 Message Digest Algorithms .................................. 3
2.1 SHA-1 ...................................................... 3
2.2 MD5 ........................................................ 3
3 Signature Algorithms ....................................... 4
3.1 DSA ........................................................ 4
3.2 RSA ........................................................ 5
4 Key Management Algorithms .................................. 6
4.1 Key Agreement Algorithms ................................... 6
4.1.1 X9.42 Ephemeral-Static Diffie-Hellman ...................... 7
4.1.2 X9.42 Static-Static Diffie-Hellman ......................... 8
4.2 Key Transport Algorithms ................................... 9
4.2.1 RSA (PKCS #1 v1.5) ......................................... 10
4.3 Symmetric Key-Encryption Key Algorithms .................... 10
4.3.1 Triple-DES Key Wrap ........................................ 11
4.3.2 RC2 Key Wrap ............................................... 12
4.4 Key Derivation Algorithms .................................. 12

4.4.1 PBKDF2 ..................................................... 13
5 Content Encryption Algorithms .............................. 13
5.1 Triple-DES CBC ............................................. 14
5.2 RC2 CBC .................................................... 14
6 Message Authentication Code (MAC) Algorithms ............... 15
6.1 HMAC with SHA-1 ............................................ 15
7 ASN.1 Module ............................................... 16
8 References ................................................. 18
9 Security Considerations .................................... 20
10 Acknowledgments ............................................ 22
11 Author's Address ........................................... 23
12 Full Copyright Statement ................................... 24

1 Introduction

The Cryptographic Message Syntax (CMS) [CMS] is used to digitally
sign, digest, authenticate, or encrypt arbitrary message contents.
This companion specification describes the use of common
cryptographic algorithms with the CMS. Implementations of the CMS
may support these algorithms; implementations of the CMS may also
support other algorithms as well. However, if an implementation
chooses to support one of the algorithms discussed in this document,
then the implementation MUST do so as described in this document.

The CMS values are generated using ASN.1 [X.208-88], using BER-
encoding [X.209-88]. Algorithm identifiers (which include ASN.1
object identifiers) identify cryptographic algorithms, and some
algorithms require additional parameters. When needed, parameters
are specified with an ASN.1 structure. The algorithm identifier for
each algorithm is specified, and when needed, the parameter structure
is specified. The fields in the CMS employed by each algorithm are
identified.

1.1 Changes Since RFC2630

This document obsoletes section 12 of RFC2630 [OLDCMS]. RFC3369
[CMS] obsoletes the rest of RFC2630. Separation of the protocol and
algorithm specifications allows each one to be updated without
impacting the other. However, the conventions for using additional
algorithms with the CMS are likely to be specified in separate
documents.

1.2 Terminology

In this document, the key Words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, and MAY are to be interpreted as described
in [STDWORDS].

2 Message Digest Algorithms

This section specifies the conventions employed by CMS
implementations that support SHA-1 or MD5.

Digest algorithm identifiers are located in the SignedData
digestAlgorithms field, the SignerInfo digestAlgorithm field, the
DigestedData digestAlgorithm field, and the AuthenticatedData
digestAlgorithm field.

Digest values are located in the DigestedData digest field and the
Message Digest authenticated attribute. In addition, digest values
are input to signature algorithms.

2.1 SHA-1

The SHA-1 message digest algorithm is defined in FIPS Pub 180-1
[SHA1]. The algorithm identifier for SHA-1 is:

sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }

There are two possible encodings for the SHA-1 AlgorithmIdentifier
parameters field. The two alternatives arise from the fact that when
the 1988 syntax for AlgorithmIdentifier was translated into the 1997
syntax, the OPTIONAL associated with the AlgorithmIdentifier
parameters got lost. Later the OPTIONAL was recovered via a defect
report, but by then many people thought that algorithm parameters
were mandatory. Because of this history some implementations encode
parameters as a NULL element and others omit them entirely. The
correct encoding is to omit the parameters field; however,
implementations MUST also handle a SHA-1 AlgorithmIdentifier
parameters field which contains a NULL.

The AlgorithmIdentifier parameters field is OPTIONAL. If present,
the parameters field MUST contain a NULL. Implementations MUST
accept SHA-1 AlgorithmIdentifiers with absent parameters.
Implementations MUST accept SHA-1 AlgorithmIdentifiers with NULL
parameters. Implementations SHOULD generate SHA-1
AlgorithmIdentifiers with absent parameters.

2.2 MD5

The MD5 digest algorithm is defined in RFC1321 [MD5]. The algorithm
identifier for MD5 is:

md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }

The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain NULL. Implementations MAY accept the
MD5 AlgorithmIdentifiers with absent parameters as well as NULL
parameters.

3 Signature Algorithms

This section specifies the conventions employed by CMS
implementations that support DSA or RSA (PKCS #1 v1.5).

Signature algorithm identifiers are located in the SignerInfo
signatureAlgorithm field of SignedData. Also, signature algorithm
identifiers are located in the SignerInfo signatureAlgorithm field of
countersignature attributes.

Signature values are located in the SignerInfo signature field of
SignedData. Also, signature values are located in the SignerInfo
signature field of countersignature attributes.

3.1 DSA

The DSA signature algorithm is defined in FIPS Pub 186 [DSS]. DSA
MUST be used with the SHA-1 message digest algorithm.

The algorithm identifier for DSA subject public keys in certificates
is:

id-dsa OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 1 }

DSA signature validation requires three parameters, commonly called
p, q, and g. When the id-dsa algorithm identifier is used, the
AlgorithmIdentifier parameters field is optional. If present, the
AlgorithmIdentifier parameters field MUST contain the three DSA
parameter values encoded using the Dss-Parms type. If absent, the
subject DSA public key uses the same DSA parameters as the
certificate issuer.

Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }

When the id-dsa algorithm identifier is used, the DSA public key,
commonly called Y, MUST be encoded as an INTEGER. The output of this
encoding is carried in the certificate subject public key.

Dss-Pub-Key ::= INTEGER -- Y

The algorithm identifier for DSA with SHA-1 signature values is:

id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 3 }

When the id-dsa-with-sha1 algorithm identifier is used, the
AlgorithmIdentifier parameters field MUST be absent.

When signing, the DSA algorithm generates two values, commonly called
r and s. To transfer these two values as one signature, they MUST be
encoded using the Dss-Sig-Value type:

Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }

3.2 RSA

The RSA (PKCS #1 v1.5) signature algorithm is defined in RFC2437
[NEWPKCS#1]. RFC2437 specifies the use of the RSA signature
algorithm with the SHA-1 and MD5 message digest algorithms.

The algorithm identifier for RSA subject public keys in certificates
is:

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

When the rsaEncryption algorithm identifier is used, the
AlgorithmIdentifier parameters field MUST contain NULL.

When the rsaEncryption algorithm identifier is used, the RSA public
key, which is composed of a modulus and a public eXPonent, MUST be
encoded using the RSAPublicKey type. The output of this encoding is
carried in the certificate subject public key.

RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER } -- e

CMS implementations that include the RSA (PKCS #1 v1.5) signature
algorithm MUST also implement the SHA-1 message digest algorithm.
Such implementations SHOULD also support the MD5 message digest
algorithm.

The rsaEncryption algorithm identifier is used to identify RSA (PKCS
#1 v1.5) signature values regardless of the message digest algorithm
employed. CMS implementations that include the RSA (PKCS #1 v1.5)
signature algorithm MUST support the rsaEncryption signature value
algorithm identifier, and CMS implementations MAY support RSA (PKCS
#1 v1.5) signature value algorithm identifiers that specify both the
RSA (PKCS #1 v1.5) signature algorithm and the message digest
algorithm.

The algorithm identifier for RSA (PKCS #1 v1.5) with SHA-1 signature
values is:

sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }

The algorithm identifier for RSA (PKCS #1 v1.5) with MD5 signature
values is:

md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }

When the rsaEncryption, sha1WithRSAEncryption, or
md5WithRSAEncryption signature value algorithm identifiers are used,
the AlgorithmIdentifier parameters field MUST be NULL.

When signing, the RSA algorithm generates a single value, and that
value is used directly as the signature value.

4 Key Management Algorithms

CMS accommodates the following general key management techniques: key
agreement, key transport, previously distributed symmetric key-
encryption keys, and passwords.

4.1 Key Agreement Algorithms

This section specifies the conventions employed by CMS
implementations that support key agreement using X9.42 Ephemeral-
Static Diffie-Hellman (X9.42 E-S D-H) and X9.42 Static-Static
Diffie-Hellman (X9.42 S-S D-H).

When a key agreement algorithm is used, a key-encryption algorithm is
also needed. Therefore, when key agreement is supported, a key-
encryption algorithm MUST be provided for each content-encryption
algorithm. The key wrap algorithms for Triple-DES and RC2 are
described in RFC3217 [WRAP].

For key agreement of RC2 key-encryption keys, 128 bits MUST be
generated as input to the key expansion process used to compute the
RC2 effective key [RC2].

Key agreement algorithm identifiers are located in the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
keyEncryptionAlgorithm fields.

Key wrap algorithm identifiers are located in the KeyWrapAlgorithm
parameters within the EnvelopedData RecipientInfos
KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.

Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
encryptedKey field. Wrapped message-authentication keys are located
in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
RecipientEncryptedKeys encryptedKey field.

4.1.1 X9.42 Ephemeral-Static Diffie-Hellman

Ephemeral-Static Diffie-Hellman key agreement is defined in RFC2631
[DH-X9.42]. When using Ephemeral-Static Diffie-Hellman, the
EnvelopedData RecipientInfos KeyAgreeRecipientInfo field is used as
follows:

version MUST be 3.

originator MUST be the originatorKey alternative. The
originatorKey algorithm field MUST contain the dh-public-number
object identifier with absent parameters. The originatorKey
publicKey field MUST contain the sender's ephemeral public key.
The dh-public-number object identifier is:

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }

ukm may be present or absent. CMS implementations MUST support
ukm being absent, and CMS implementations SHOULD support ukm being
present. When present, the ukm is used to ensure that a different
key-encryption key is generated when the ephemeral private key
might be used more than once.

keyEncryptionAlgorithm MUST be the id-alg-ESDH algorithm
identifier. The algorithm identifier parameter field for id-alg-
ESDH is KeyWrapAlgorithm, and this parameter MUST be present. The
KeyWrapAlgorithm denotes the symmetric encryption algorithm used
to encrypt the content-encryption key with the pairwise key-
encryption key generated using the X9.42 Ephemeral-Static Diffie-
Hellman key agreement algorithm. Triple-DES and RC2 key wrap
algorithms are described in RFC3217 [WRAP]. The id-alg-ESDH
algorithm identifier and parameter syntax is:

id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
alg(3) 5 }

KeyWrapAlgorithm ::= AlgorithmIdentifier

recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier MUST contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey MUST contain the
content-encryption key encrypted with the X9.42 Ephemeral-Static
Diffie-Hellman generated pairwise key-encryption key using the
algorithm specified by the KeyWrapAlgorithm.

4.1.2 X9.42 Static-Static Diffie-Hellman

Static-Static Diffie-Hellman key agreement is defined in RFC2631
[DH-X9.42]. When using Static-Static Diffie-Hellman, the
EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
used as follows:

version MUST be 3.

originator MUST be either the issuerAndSerialNumber or
subjectKeyIdentifier alternative. In both cases, the originator's
certificate contains the sender's static public key. RFC3279
[CERTALGS] specifies the AlgorithmIdentifier parameters syntax and
values that are included in the originator's certificate. The
originator's certificate subject public key information field MUST
contain the dh-public-number object identifier:

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }

ukm MUST be present. The ukm ensures that a different key-
encryption key is generated for each message between the same
sender and recipient.

keyEncryptionAlgorithm MUST be the id-alg-SSDH algorithm
identifier. The algorithm identifier parameter field for id-alg-
SSDH is KeyWrapAlgorihtm, and this parameter MUST be present. The
KeyWrapAlgorithm denotes the symmetric encryption algorithm used
to encrypt the content-encryption key with the pairwise key-
encryption key generated using the X9.42 Static-Static Diffie-
Hellman key agreement algorithm. Triple-DES and RC2 key wrap
algorithms are described in RFC3217 [WRAP]. The id-alg-SSDH
algorithm identifier and parameter syntax is:

id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
alg(3) 10 }

KeyWrapAlgorithm ::= AlgorithmIdentifier

recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier MUST contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey MUST contain the content-
encryption key encrypted with the X9.42 Static-Static Diffie-
Hellman generated pairwise key-encryption key using the algorithm
specified by the KeyWrapAlgortihm.

4.2 Key Transport Algorithms

This section specifies the conventions employed by CMS
implementations that support key transport using RSA (PKCS #1 v1.5).

Key transport algorithm identifiers are located in the EnvelopedData
RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm field.

Key transport encrypted content-encryption keys are located in the
EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey
field.

4.2.1 RSA (PKCS #1 v1.5)

The RSA key transport algorithm is the RSA encryption scheme defined
in RFC2313 [PKCS#1], block type is 02, where the message to be
encrypted is the content-encryption key. The algorithm identifier
for RSA (PKCS #1 v1.5) is:

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain NULL.

When using a Triple-DES content-encryption key, CMS implementations
MUST adjust the parity bits for each DES key comprising the Triple-
DES key prior to RSA encryption.

The use of RSA (PKCS #1 v1.5) encryption, as defined in RFC2313
[PKCS#1], to provide confidentiality has a known vulnerability. The
vulnerability is primarily relevant to usage in interactive
applications rather than to store-and-forward environments. Further
information and proposed countermeasures are discussed in the
Security Considerations section of this document and RFC3218 [MMA].

Note that the same RSA encryption scheme is also defined in RFC2437
[NEWPKCS#1]. Within RFC2437, this RSA encryption scheme is called
RSAES-PKCS1-v1_5.

4.3 Symmetric Key-Encryption Key Algorithms

This section specifies the conventions employed by CMS
implementations that support symmetric key-encryption key management
using Triple-DES or RC2 key-encryption keys. When RC2 is supported,
RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
be used with the RC2ParameterVersion parameter set to 58. A CMS
implementation MAY support mixed key-encryption and content-
encryptionalgorithms. For example, a 40-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key or
with a 128-bit RC2 key-encryption key.

Key wrap algorithm identifiers are located in the EnvelopedData
RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KEKRecipientInfo
keyEncryptionAlgorithm fields.

Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfos KEKRecipientInfo encryptedKey field. Wrapped
message-authentication keys are located in the AuthenticatedData
RecipientInfos KEKRecipientInfo encryptedKey field.

The output of a key agreement algorithm is a key-encryption key, and
this key-encryption key is used to encrypt the content-encryption
key. To support key agreement, key wrap algorithm identifiers are
located in the KeyWrapAlgorithm parameter of the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
keyEncryptionAlgorithm fields. However, only key agreement
algorithms that inherently provide authentication ought to be used
with AuthenticatedData. Wrapped content-encryption keys are located
in the EnvelopedData RecipientInfos KeyAgreeRecipientInfo
RecipientEncryptedKeys encryptedKey field, wrapped message-
authentication keys are located in the AuthenticatedData
RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
encryptedKey field.

4.3.1 Triple-DES Key Wrap

A CMS implementation MAY support mixed key-encryption and content-
encryption algorithms. For example, a 128-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key.

Triple-DES key encryption has the algorithm identifier:

id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

The AlgorithmIdentifier parameter field MUST be NULL.

The key wrap algorithm used to encrypt a Triple-DES content-
encryption key with a Triple-DES key-encryption key is specified in
section 3.1 of RFC3217 [WRAP]. The corresponding key unwrap
algorithm is specified in section 3.2 of RFC3217 [WRAP].

Out-of-band distribution of the Triple-DES key-encryption key used to
encrypt the Triple-DES content-encryption key is beyond the scope of
this document.

4.3.2 RC2 Key Wrap

A CMS implementation MAY support mixed key-encryption and content-
encryption algorithms. For example, a 128-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key.
Similarly, a 40-bit RC2 content-encryption key MAY be wrapped with a
128-bit RC2 key-encryption key.

RC2 key encryption has the algorithm identifier:

id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

The AlgorithmIdentifier parameter field MUST be RC2wrapParameter:

RC2wrapParameter ::= RC2ParameterVersion

RC2ParameterVersion ::= INTEGER

The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the RC2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0),
because the one octet (A0) encoding represents a negative number.

RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
be used with the RC2ParameterVersion parameter set to 58.

The key wrap algorithm used to encrypt a RC2 content-encryption key
with a RC2 key-encryption key is specified in section 4.1 of RFC3217
[WRAP]. The corresponding key unwrap algorithm is specified 4.2 of
RFC3217 [WRAP].

Out-of-band distribution of the RC2 key-encryption key used to
encrypt the RC2 content-encryption key is beyond of the scope of this
document.

4.4 Key Derivation Algorithms

This section specifies the conventions employed by CMS
implementations that support password-based key management using
PBKDF2.

Key derivation algorithms are used to convert a password into a key-
encryption key as part of the password-based key management
technique.

Key derivation algorithm identifiers are located in the EnvelopedData
RecipientInfos PasswordRecipientInfo keyDerivationAlgorithm and
AuthenticatedData RecipientInfos PasswordRecipientInfo
keyDerivationAlgorithm fields.

The key-encryption key that is derived from the password is used to
encrypt the content-encryption key.

The content-encryption keys encrypted with password-derived key-
encryption keys are located in the EnvelopedData RecipientInfos
PasswordRecipientInfo encryptedKey field. The message-authentication
keys encrypted with password-derived key-encryption keys are located
in the AuthenticatedData RecipientInfos PasswordRecipientInfo
encryptedKey field.

4.4.1 PBKDF2

The PBKDF2 key derivation algorithm is specified in RFC2898
[PKCS#5]. The KeyDerivationAlgorithmIdentifer identifies the key-
derivation algorithm, and any associated parameters used to derive
the key-encryption key from the user-supplied password. The
algorithm identifier for the PBKDF2 key derivation algorithm is:

id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-5(5) 12 }

The AlgorithmIdentifier parameter field MUST be PBKDF2-params:

PBKDF2-params ::= SEQUENCE {
salt CHOICE {
specified OCTET STRING,
otherSource AlgorithmIdentifier },
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm hMAC-SHA1, parameters NULL } }

Within the PBKDF2-params, the salt MUST use the specified OCTET
STRING.

5 Content Encryption Algorithms

This section specifies the conventions employed by CMS
implementations that support content encryption using Three-Key
Triple-DES in CBC mode, Two-Key Triple-DES in CBC mode, or RC2 in CBC
mode.

Content encryption algorithm identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.

Content encryption algorithms are used to encipher the content
located in the EnvelopedData EncryptedContentInfo encryptedContent
field and the EncryptedData EncryptedContentInfo encryptedContent
field.

5.1 Triple-DES CBC

The Triple-DES algorithm is described in ANSI X9.52 [3DES]. The
Triple-DES is composed from three sequential DES [DES] operations:
encrypt, decrypt, and encrypt. Three-Key Triple-DES uses a different
key for each DES operation. Two-Key Triple-DES uses one key for the
two encrypt operations and a different key for the decrypt operation.
The same algorithm identifiers are used for Three-Key Triple-DES and
Two-Key Triple-DES. The algorithm identifier for Triple-DES in
Cipher Block Chaining (CBC) mode is:

des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

The AlgorithmIdentifier parameters field MUST be present, and the
parameters field must contain a CBCParameter:

CBCParameter ::= IV

IV ::= OCTET STRING -- exactly 8 octets

5.2 RC2 CBC

The RC2 algorithm is described in RFC2268 [RC2]. The algorithm
identifier for RC2 in CBC mode is:

rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }

The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain a RC2CBCParameter:

RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets

The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the rc2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0), since
the one octet (A0) encoding represents a negative number.

6 Message Authentication Code Algorithms

This section specifies the conventions employed by CMS
implementations that support the HMAC with SHA-1 message
authentication code (MAC).

MAC algorithm identifiers are located in the AuthenticatedData
macAlgorithm field.

MAC values are located in the AuthenticatedData mac field.

6.1 HMAC with SHA-1

The HMAC with SHA-1 algorithm is described in RFC2104 [HMAC]. The
algorithm identifier for HMAC with SHA-1 is:

hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) 8 1 2 }

There are two possible encodings for the HMAC with SHA-1
AlgorithmIdentifier parameters field. The two alternatives arise
from the fact that when the 1988 syntax for the AlgorithmIdentifier
type was translated into the 1997 syntax, the OPTIONAL associated
with the AlgorithmIdentifier parameters got lost. Later the OPTIONAL
was recovered via a defect report, but by then many people thought
that algorithm parameters were mandatory. Because of this history
some implementations may encode parameters as a NULL while others
omit them entirely.

The AlgorithmIdentifier parameters field is OPTIONAL. If present,
the parameters field MUST contain a NULL. Implementations MUST
accept HMAC with SHA-1 AlgorithmIdentifiers with absent parameters.
Implementations MUST accept HMAC with SHA-1 AlgorithmIdentifiers with
NULL parameters. Implementations SHOULD generate HMAC with SHA-1
AlgorithmIdentifiers with absent parameters.

7 ASN.1 Module

CryptographicMessageSyntaxAlgorithms
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cmsalg-2001(16) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

-- EXPORTS All
-- The types and values defined in this module are exported for use
-- in the other ASN.1 modules. Other applications may use them for
-- their own purposes.

IMPORTS
-- Imports from RFC3280 [PROFILE], Appendix A.1
AlgorithmIdentifier
FROM PKIX1Explicit88 { iso(1)
identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) mod(0)
pkix1-explicit(18) } ;

-- Algorithm Identifiers

sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }

md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }

id-dsa OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
x9-57(10040) x9cm(4) 1 }

id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57(10040) x9cm(4) 3 }

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }

sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }

id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 10 }

id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }

hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-5(5) 12 }

-- Public Key Types

Dss-Pub-Key ::= INTEGER -- Y

RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER } -- e

DHPublicKey ::= INTEGER -- y = g^x mod p

-- Signature Value Types

Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }

-- Algorithm Identifier Parameter Types

Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }

DHDomainParameters ::= SEQUENCE {
p INTEGER, -- odd prime, p=jq +1
g INTEGER, -- generator, g
q INTEGER, -- factor of p-1
j INTEGER OPTIONAL, -- subgroup factor
validationParms ValidationParms OPTIONAL }

ValidationParms ::= SEQUENCE {
seed BIT STRING,
pgenCounter INTEGER }

KeyWrapAlgorithm ::= AlgorithmIdentifier

RC2wrapParameter ::= RC2ParameterVersion

RC2ParameterVersion ::= INTEGER

CBCParameter ::= IV

IV ::= OCTET STRING -- exactly 8 octets

RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets

PBKDF2-params ::= SEQUENCE {
salt CHOICE {
specified OCTET STRING,
otherSource AlgorithmIdentifier },
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm hMAC-SHA1, parameters NULL } }

END -- of CryptographicMessageSyntaxAlgorithms

8 References

[3DES] American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation.
1998.

[CERTALGS] Bassham, L., Housley, R. and W. Polk, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC3279, April 2002.

[CMS] Housley, R., "Cryptographic Message Syntax", RFC3269,
August 2002.

[DES] American National Standards Institute. ANSI X3.106,
"American National Standard for Information Systems -
Data Link Encryption". 1983.

[DH-X9.42] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.

[DSS] National Institute of Standards and Technology. FIPS Pub
186: Digital Signature Standard. 19 May 1994.

[HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
Authentication", RFC2104, February 1997.

[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,
April 1992.

[MMA] Rescorla, E., "Preventing the Million Message Attack on
CMS", RFC3218, January 2002.

[MODES] National Institute of Standards and Technology. FIPS Pub
81: DES Modes of Operation. 2 December 1980.

[NEWPKCS#1] Kaliski, B. and J. Staddon, "PKCS #1: RSA Encryption,
Version 2.0, RFC2437, October 1998.

[OLDCMS] Housley, R., "Cryptographic Message Syntax", RFC2630,
June 1999.

[PKCS#1] Kaliski, B, "PKCS #1: RSA Encryption, Version 2.0", RFC
2437, October, 1998.

[PKCS#5] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification", RFC2898, September 2000.

[PROFILE] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC3280,
April 2002.

[RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security, RFC1750, December 1994.

[RC2] Rivest, R., "A Description of the RC2 (r) Encryption
Algorithm", RFC2268, March 1998.

[SHA1] National Institute of Standards and Technology. FIPS Pub
180-1: Secure Hash Standard. 17 April 1995.

[STDWORDS] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[WRAP] Housley, R., "Triple-DES and RC2 Key Wrapping", RFC3217,
December 2001.

[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.

9 Security Considerations

The CMS provides a method for digitally signing data, digesting data,
encrypting data, and authenticating data. This document identifies
the conventions for using several cryptographic algorithms for use
with the CMS.

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 authentication, otherwise the
content is 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
[PROFILE].

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
content could originate from any one of the parties.

Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), one-time
values (such as the k value when generating a DSA signature), 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 such values 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. RFC1750 [RANDOM] offers important guidance in
this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
PRNG technique.

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 168-bit Triple-DES and the Triple-DES
content-encryption key is wrapped with a 40-bit RC2 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 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.

RFC3217 [WRAP] specifies key wrap algorithms used to encrypt a
Triple-DES content-encryption key with a Triple-DES key-encryption
key [3DES] or to encrypt a RC2 content-encryption key with a RC2
key-encryption key [RC2]. The key wrap algorithms makes use of CBC
mode [MODES]. These key wrap algorithms have been reviewed for use
with Triple-DES and RC2. They have not been reviewed for use with
other cryptographic modes or other encryption algorithms. Therefore,
if a CMS implementation wishes to support ciphers in addition to
Triple-DES or RC2, then additional key wrap algorithms need to be
defined to support the additional ciphers.

Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will reduce. Therefore, cryptographic

algorithm implementations should be modular allowing new algorithms
to be readily inserted. That is, implementers should be prepared to
regularly update the set of algorithms in their implementations.

Users of the CMS, particularly those employing the CMS to support
interactive applications, should be aware that RSA (PKCS #1 v1.5), as
specified in RFC2313 [PKCS#1], is vulnerable to adaptive chosen
ciphertext attacks when applied for encryption purposes.
Exploitation of this identified vulnerability, revealing the result
of a particular RSA decryption, requires Access to an Oracle which
will respond to a large number of ciphertexts (based on currently
available results, hundreds of thousands or more), which are
constructed adaptively in response to previously-received replies
providing information on the successes or failures of attempted
decryption operations. As a result, the attack appears significantly
less feasible to perpetrate for store-and-forward S/MIME environments
than for directly interactive protocols. Where the CMS constructs
are applied as an intermediate encryption layer within an interactive
request-response communications environment, exploitation could be
more feasible.

An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[NEWPKCS#1]. This updated document supersedes RFC2313. PKCS #1
Version 2.0 preserves support for the encryption padding format
defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality. Designers of protocols and
systems employing CMS for interactive environments should either
consider usage of OAEP, or should ensure that information which could
reveal the success or failure of attempted PKCS #1 Version 1.5
decryption operations is not provided. Support for OAEP will likely
be added to a future version of the CMS algorithm specification.

See RFC3218 [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.

10 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, and Dave Solo for their efforts and support.

11 Author Address

Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
EMail: rhousley@rsasecurity.com

12. Full Copyright Statement

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