S/MIME Working Group R. Housley
Internet Draft RSA Laboratories
expires in six months April 2001
Cryptographic Message Syntax (CMS) Algorithms
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
This document describes cryptographic algorithms for use with the
Cryptographic Message Syntax (CMS) [CMS]. CMS is used to digitally
sign, digest, authenticate, or encrypt arbitrary messages.
Once approved, this draft will obsolete section 12 of RFC 2630. The
companion document (draft-ietf-smime-rfc2630bis-00.txt) will obsolete
the rest of RFC 2630. Separation of the protocol and algorithm
specifications allows the IETF to select different mandatory to
implement algorithms in the future without reissuing the protocol
document.
This draft is being discussed on the "ietf-smime" mailing list. To
join the list, send a message to with
the single word "subscribe" in the body of the message. Also, there
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Table of Contents
Status of this Memo .............................................. 1
Abstract ......................................................... 1
Table of Contents ................................................ 3
1 Introduction ................................................. 5
2 Message Digest Algorithms .................................... 35
2.1 SHA-1 ................................................. 35
2.2 MD5 ................................................... 35
3 Signature Algorithms ......................................... 36
3.1 DSA ................................................... 36
3.2 RSA ................................................... 36
4 Key Management Algorithms .................................... 36
4.1 Key Agreement Algorithms .............................. 36
4.1.1 X9.42 Ephemeral-Static Diffie-Hellman ........ 37
4.2 Key Transport Algorithms .............................. 38
4.2.1 RSA .......................................... 39
4.3 Symmetric Key-Encryption Key Algorithms ............... 39
4.3.1 Triple-DES Key Wrap .......................... 40
4.3.2 RC2 Key Wrap ................................. 41
5 Content Encryption Algorithms ................................ 41
5.1 Triple-DES CBC ........................................ 42
5.2 RC2 CBC ............................................... 42
6 Message Authentication Code (MAC) Algorithms ................. 42
6.1 HMAC with SHA-1 ....................................... 43
7 Triple-DES and RC2 Key Wrap Algorithms ....................... 43
7.1 Key Checksum .......................................... 44
7.2 Triple-DES Key Wrap ................................... 44
7.3 Triple-DES Key Unwrap ................................. 44
7.4 RC2 Key Wrap .......................................... 45
7.5 RC2 Key Unwrap ........................................ 46
Appendix A: ASN.1 Module ........................................ 47
References ....................................................... 55
Security Considerations .......................................... 56
Acknowledgments .................................................. 58
Author's Address ................................................. 59
Full Copyright Statement ......................................... 60
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1 Introduction
The Cryptographic Message Syntax (CMS) [CMS] is used to digitally
sign, digest, authenticate, or encrypt arbitrary messages. This
companion specification lists the algorithms that MUST be supported
by CMS implementations. It also lists algorithms that SHOULD be
supported by CMS implementations. Of course, CMS implementations MAY
support other algorithms as well.
Table 1 summarizes the algorithms that CMS implantations MUST support
and SHOULD support.
The CMS values are generated using ASN.1 [X.208-88], using BER-
encoding [X.209-88]. Algorithm are be identified by algorithm
identifiers (ASN.1 object identifiers), and some algorithms require
additional parameters. When needed, parameters are specified with an
ASN.1 structure. The algorithm identifiers for each algorithm are
specified, and, when needed, the parameter structure is specified.
The fields in the CMS employed by each algorithm are identified.
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described by Scott Bradner in [STDWORDS].
Table 1. CMS Implantation Algorithm Requirements
Algorithm Type MUST implement SHOULD implement
-----------------------------------------------------------------
Message Digest SHA-1 MD5
Signature DSA and RSA (*) --
Key Management
Key Agreement -- X9.42 E-S D-H
Key Transport RSA --
Symmetric KEK Wrap Triple-DES Key Wrap RC2 Key Wrap
Content Encryption Triple-DES CBC RC2 CBC
Message Authentication HMAC with SHA-1 --
(*) Note: CMS implementations MUST be able to verify signatures
with both DSA and RSA, and they MUST be able to
generate signatures with at least one of them.
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2 Message Digest Algorithms
CMS implementations MUST support SHA-1. CMS implementations SHOULD
support 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
digest values are located in the Message Digest authenticated
attribute. In addition, digest values are input to signature
algorithms.
2.1 SHA-1
The SHA-1 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 }
The AlgorithmIdentifier parameters field is OPTIONAL. If present,
the parameters field MUST contain an ASN.1 NULL. Implementations
SHOULD accept SHA-1 AlgorithmIdentifiers with absent parameters as
well as NULL parameters. Implementations SHOULD generate SHA-1
AlgorithmIdentifiers with NULL parameters.
2.2 MD5
The MD5 digest algorithm is defined in RFC 1321 [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
CMS implementations MUST support both DSA and RSA. CMS
implementations MUST be able to verify signatures with both DSA and
RSA. CMS implementations MUST be able to generate signatures with
either DSA or RSA. CMS implementations MAY be able to generate
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signatures with both DSA and RSA.
Signature algorithm identifiers are located in the SignerInfo
signatureAlgorithm field. Also, signature algorithm identifiers are
located in the SignerInfo signatureAlgorithm field of
countersignature attributes.
Signature values are located in the SignerInfo signature field.
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 is
always used with the SHA-1 message digest algorithm. The algorithm
identifier for DSA is:
id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 3 }
The AlgorithmIdentifier parameters field MUST NOT be present.
3.2 RSA
The RSA signature algorithm is defined in RFC 2437 [NEWPKCS#1]. RFC
2437 specifies the use of the RSA signature algorithm with the SHA-1
and MD5 message digest algorithms. The algorithm identifier for RSA
is:
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
CMS implementations MUST support RSA with SHA-1. CMS implementations
SHOULD support RSA with MD5.
4 Key Management Algorithms
CMS accommodates three general key management techniques: key
agreement, key transport, and previously distributed symmetric key-
encryption keys.
4.1 Key Agreement Algorithms
CMS implementations SHOULD support key agreement using X9.42
Ephemeral-Static Diffie-Hellman (X9.42 E-S D-H).
Any symmetric encryption algorithm that a CMS implementation includes
as a content-encryption algorithm MUST also be included as a key-
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encryption algorithm. CMS implementations SHOULD include key
agreement of Triple-DES pairwise key-encryption keys. CMS
implementations SHOULD include key agreement of RC2 pairwise key-
encryption keys. CMS implementations MUST include Triple-DES
wrapping of Triple-DES content-encryption keys and RC2 wrapping of
RC2 content-encryption keys. The key wrap algorithms for Triple-DES
and RC2 are described in section 7.
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 168-bit Triple-DES key-encryption key.
Similarly, a 40-bit RC2 content-encryption key MAY be wrapped with
128-bit RC2 key-encryption key.
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 RFC 2631
[DH-X9.42]. When using Ephemeral-Static Diffie-Hellman, the
EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
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.
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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 absent. 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 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 Ephemeral-Static Diffie-
Hellman key agreement algorithm. Triple-DES and RC2 key wrap
algorithms are discussed in section 7. 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 KeyWrapAlgortihm.
4.2 Key Transport Algorithms
CMS implementations MUST support key transport using RSA. RSA
implementations MUST support key transport of Triple-DES content-
encryption keys. RSA implementations SHOULD support key transport of
RC2 content-encryption keys.
Key transport algorithm identifiers are located in the EnvelopedData
RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KeyTransRecipientInfo
keyEncryptionAlgorithm fields.
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Key transport encrypted content-encryption keys are located in the
EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey
field. Key transport encrypted message-authentication keys are
located in the AuthenticatedData RecipientInfos KeyTransRecipientInfo
encryptedKey field.
4.2.1 RSA
The RSA key transport algorithm is the RSA encryption scheme defined
in RFC 2313 [PKCS#1], block type is 02, where the message to be
encrypted is the content-encryption key. The algorithm identifier
for RSA 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 encryption, as defined in RFC 2313 [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 RFC [MMA].
Note that the same encryption scheme is also defined in RFC 2437
[NEWPKCS#1]. Within RFC 2437, this scheme is called RSAES-
PKCS1-v1_5.
4.3 Symmetric Key-Encryption Key Algorithms
CMS implementations MUST support symmetric key-encryption key
management. CMS implementations MUST include Triple-DES key-
encryption keys wrapping Triple-DES content-encryption keys. CMS
implementations SHOULD include RC2 key-encryption keys wrapping RC2
content-encryption keys. 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-encryption algorithms. For example, a 40-bit
RC2 content-encryption key MAY be wrapped with 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
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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. In conjunction with key agreement algorithms, CMS
implementations MUST include encryption of content-encryption keys
with the pairwise key-encryption key generated using a key agreement
algorithm. 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. 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
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 7.2. The corresponding key unwrap algorithm is specified in
section 7.3.
Out-of-band distribution of the Triple-DES key-encryption key used to
encrypt the Triple-DES content-encryption key is beyond of the scope
of this document.
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4.3.2 RC2 Key Wrap
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 7.4. The
corresponding key unwrap algorithm is specified in section 7.5.
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.
5 Content Encryption Algorithms
CMS implementations MUST support Three-Key Triple-DES in CBC mode.
MS implementations SHOULD support Two-Key Triple-DES in CBC mode.
CMS implementations SHOULD support RC2 in CBC mode.
Content encryption algorithms 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.
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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 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 RFC 2268 [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
CMS implementations that support authenticatedData MUST support HMAC
with SHA-1.
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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 RFC 2104 [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 }
The AlgorithmIdentifier parameters field must be absent.
7 Triple-DES and RC2 Key Wrap Algorithms
CMS implementations MUST include encryption of a Triple-DES content-
encryption key with a Triple-DES key-encryption key using the
algorithm specified in Sections 7.2 and 7.3. CMS implementations
SHOULD include encryption of a RC2 content-encryption key with a RC2
key-encryption key using the algorithm specified in Sections 7.4 and
7.5. Triple-DES and RC2 content-encryption keys are encrypted in
Cipher Block Chaining (CBC) mode [MODES].
Key Transport algorithms allow for the content-encryption key to be
directly encrypted; however, key agreement and symmetric key-
encryption key algorithms encrypt the content-encryption key with a
second symmetric encryption algorithm. This section describes how
the Triple-DES or RC2 content-encryption key is formatted and
encrypted.
Key agreement algorithms generate a pairwise key-encryption key, and
a key wrap algorithm is used to encrypt the content-encryption key
with the pairwise key-encryption key. Similarly, a key wrap
algorithm is used to encrypt the content-encryption key in a
previously distributed key-encryption key.
The key-encryption key is generated by the key agreement algorithm or
distributed out of band. 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].
The same algorithm identifier is used for both Two-key Triple-DES and
Three-key Triple-DES. When the length of the content-encryption key
to be wrapped is a Two-key Triple-DES key, a third key with the same
value as the first key is created. Thus, all Triple-DES content-
encryption keys are wrapped like Three-key Triple-DES keys. However,
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a Two-key Triple-DES key MUST NOT be used to wrap a Three-key Triple-
DES key.
7.1 Key Checksum
The CMS Checksum Algorithm is used to provide a content-encryption
key integrity check value. The algorithm is:
1. Compute a 20 octet SHA-1 [SHA1] message digest on the
content-encryption key.
2. Use the most significant (first) eight octets of the message
digest value as the checksum value.
7.2 Triple-DES Key Wrap
The Triple-DES key wrap algorithm encrypts a Triple-DES content-
encryption key with a Triple-DES key-encryption key. The Triple-DES
key wrap algorithm is:
1. Set odd parity for each of the DES key octets comprising
the content-encryption key, call the result CEK.
2. Compute an 8 octet key checksum value on CEK as described above
in Section 12.6.1, call the result ICV.
3. Let CEKICV = CEK || ICV.
4. Generate 8 octets at random, call the result IV.
5. Encrypt CEKICV in CBC mode using the key-encryption key. Use
the random value generated in the previous step as the
initialization vector (IV). Call the ciphertext TEMP1.
6. Let TEMP2 = IV || TEMP1.
7. Reverse the order of the octets in TEMP2. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP3.
8. Encrypt TEMP3 in CBC mode using the key-encryption key. Use
an initialization vector (IV) of 0x4adda22c79e82105.
The ciphertext is 40 octets long.
Note: When the same content-encryption key is wrapped in different
key-encryption keys, a fresh initialization vector (IV) must be
generated for each invocation of the key wrap algorithm.
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7.3 Triple-DES Key Unwrap
The Triple-DES key unwrap algorithm decrypts a Triple-DES content-
encryption key using a Triple-DES key-encryption key. The Triple-DES
key unwrap algorithm is:
1. If the wrapped content-encryption key is not 40 octets, then
error.
2. Decrypt the wrapped content-encryption key in CBC mode using
the key-encryption key. Use an initialization vector (IV)
of 0x4adda22c79e82105. Call the output TEMP3.
3. Reverse the order of the octets in TEMP3. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP2.
4. Decompose the TEMP2 into IV and TEMP1. IV is the most
significant (first) 8 octets, and TEMP1 is the least significant
(last) 32 octets.
5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use
the IV value from the previous step as the initialization vector.
Call the ciphertext CEKICV.
6. Decompose the CEKICV into CEK and ICV. CEK is the most significant
(first) 24 octets, and ICV is the least significant (last) 8 octets.
7. Compute an 8 octet key checksum value on CEK as described above
in Section 12.6.1. If the computed key checksum value does not
match the decrypted key checksum value, ICV, then error.
8. Check for odd parity each of the DES key octets comprising CEK.
If parity is incorrect, then there is an error.
9. Use CEK as the content-encryption key.
7.4 RC2 Key Wrap
The RC2 key wrap algorithm encrypts a RC2 content-encryption key with
a RC2 key-encryption key. The RC2 key wrap algorithm is:
1. Let the content-encryption key be called CEK, and let the length
of the content-encryption key in octets be called LENGTH. LENGTH
is a single octet.
2. Let LCEK = LENGTH || CEK.
3. Let LCEKPAD = LCEK || PAD. If the length of LCEK is a multiple
of 8, the PAD has a length of zero. If the length of LCEK is
not a multiple of 8, then PAD contains the fewest number of
random octets to make the length of LCEKPAD a multiple of 8.
4. Compute an 8 octet key checksum value on LCEKPAD as described
above in Section 12.6.1, call the result ICV.
5. Let LCEKPADICV = LCEKPAD || ICV.
6. Generate 8 octets at random, call the result IV.
7. Encrypt LCEKPADICV in CBC mode using the key-encryption key.
Use the random value generated in the previous step as the
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initialization vector (IV). Call the ciphertext TEMP1.
8. Let TEMP2 = IV || TEMP1.
9. Reverse the order of the octets in TEMP2. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP3.
10. Encrypt TEMP3 in CBC mode using the key-encryption key. Use
an initialization vector (IV) of 0x4adda22c79e82105.
Note: When the same content-encryption key is wrapped in different
key-encryption keys, a fresh initialization vector (IV) must be
generated for each invocation of the key wrap algorithm.
7.5 RC2 Key Unwrap
The RC2 key unwrap algorithm decrypts a RC2 content-encryption key
using a RC2 key-encryption key. The RC2 key unwrap algorithm is:
1. If the wrapped content-encryption key is not a multiple of 8
octets, then error.
2. Decrypt the wrapped content-encryption key in CBC mode using
the key-encryption key. Use an initialization vector (IV)
of 0x4adda22c79e82105. Call the output TEMP3.
3. Reverse the order of the octets in TEMP3. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP2.
4. Decompose the TEMP2 into IV and TEMP1. IV is the most
significant (first) 8 octets, and TEMP1 is the remaining octets.
5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use
the IV value from the previous step as the initialization vector.
Call the plaintext LCEKPADICV.
6. Decompose the LCEKPADICV into LCEKPAD, and ICV. ICV is the
least significant (last) octet 8 octets. LCEKPAD is the
remaining octets.
7. Compute an 8 octet key checksum value on LCEKPAD as described
above in Section 12.6.1. If the computed key checksum value
does not match the decrypted key checksum value, ICV, then error.
8. Decompose the LCEKPAD into LENGTH, CEK, and PAD. LENGTH is the
most significant (first) octet. CEK is the following LENGTH
octets. PAD is the remaining octets, if any.
9. If the length of PAD is more than 7 octets, then error.
10. Use CEK as the content-encryption key.
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Appendix A: 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 None
-- 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-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 }
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-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 }
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hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
-- Algorithm Parameters
KeyWrapAlgorithm ::= AlgorithmIdentifier
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
CBCParameter ::= IV
IV ::= OCTET STRING -- exactly 8 octets
RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets
END -- of CryptographicMessageSyntaxAlgorithms
References
3DES American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation. 1998.
CMS Housley, R. Cryptographic Message Syntax. RFC . .
{draft-ietf-smime-rfc2630bis-*.txt}
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.
RFC 2104. February 1997.
MD5 Rivest, R. The MD5 Message-Digest Algorithm. RFC 1321.
April 1992.
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MMA Rescorla, E. Preventing the Million Message Attack on CMS.
RFC . . {draft-ietf-smime-pkcs1-*.txt}
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. RFC 2437. October 1998.
PKCS#1 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994.
RC2 Rivest, R. A Description of the RC2 (r) Encryption Algorithm.
RFC 2268. 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. RFC2119. 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.
Security Considerations
The CMS provides a method for digitally signing data, digesting data,
encrypting data, and authenticating data. This document identifies
the cryptographic algorithms for use with 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 messages 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
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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 message 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.
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
message 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, 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,
a message 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.
Section 7 specifies key wrap algorithms used to encrypt a Triple-DES
[3DES] content-encryption key with a Triple-DES key-encryption key or
to encrypt a RC2 [RC2] content-encryption key with a RC2 key-
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encryption key. The key wrap algorithms make 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 cryptoanalysis 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 for
the set of mandatory to implement algorithms to change over time.
The countersignature unauthenticated 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.
Users of CMS, particularly those employing CMS to support interactive
applications, should be aware that PKCS #1 Version 1.5 as specified
in RFC 2313 [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 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 new document supersedes RFC 2313. 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.
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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 RFC [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.
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.
Author Address
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
rhousley@rsasecurity.com
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. In addition, the
ASN.1 module presented in Appendix A may be used in whole or in part
without inclusion of the copyright notice. However, this document
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itself may not be modified in any way, such as by removing the
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