S/MIME Working Group R. Housley Internet Draft RSA Laboratories expires in six months April 2001 Cryptographic Message Syntax (CMS) Algorithms <draft-ietf-smime-cmsalg-00.txt> Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 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 <ietf-smime-request@imc.org> with the single word "subscribe" in the body of the message. Also, there is a Web site for the mailing list at <http://www.imc.org/ietf- smime/>. Housley [Page 1]

INTERNET DRAFT April 2001 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 Housley [Page 2]

INTERNET DRAFT April 2001 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. Housley [Page 3]

INTERNET DRAFT April 2001 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 Housley [Page 4]

INTERNET DRAFT April 2001 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- Housley [Page 5]

INTERNET DRAFT April 2001 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. Housley [Page 6]

INTERNET DRAFT April 2001 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. Housley [Page 7]

INTERNET DRAFT April 2001 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 <TBD> [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 Housley [Page 8]

INTERNET DRAFT April 2001 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. Housley [Page 9]

INTERNET DRAFT April 2001 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. Housley [Page 10]

INTERNET DRAFT April 2001 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. Housley [Page 11]

INTERNET DRAFT April 2001 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, Housley [Page 12]

INTERNET DRAFT April 2001 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. Housley [Page 13]

INTERNET DRAFT April 2001 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 Housley [Page 14]

INTERNET DRAFT April 2001 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. Housley [Page 15]

INTERNET DRAFT April 2001 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 } Housley [Page 16]

INTERNET DRAFT April 2001 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 <TBD>. <Date>. {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. Housley [Page 17]

INTERNET DRAFT April 2001 MMA Rescorla, E. Preventing the Million Message Attack on CMS. RFC <TBD>. <Date>. {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 Housley [Page 18]

INTERNET DRAFT April 2001 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- Housley [Page 19]

INTERNET DRAFT April 2001 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. Housley [Page 20]

INTERNET DRAFT April 2001 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 <TBD> [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 Housley [Page 21]

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INTERNET DRAFT April 2001
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