S/MIME Working Group R. Housley Internet Draft RSA Laboratories expires in six months August 2001 Cryptographic Message Syntax (CMS) Algorithms <draft-ietf-smime-cmsalg-03.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 several cryptographic algorithms for use with the Cryptographic Message Syntax (CMS) [CMS]. CMS is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. This document obsoletes section 12 of RFC 2630. [CMS] obsoletes the rest of RFC 2630. 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. 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 August 2001 Table of Contents Status of this Memo .............................................. 1 Abstract ......................................................... 1 Table of Contents ................................................ 2 1 Introduction ................................................. 3 2 Message Digest Algorithms .................................... 3 2.1 SHA-1 ................................................. 3 2.2 MD5 ................................................... 4 3 Signature Algorithms ......................................... 4 3.1 DSA ................................................... 4 3.2 RSA ................................................... 5 4 Key Management Algorithms .................................... 6 4.1 Key Agreement Algorithms .............................. 7 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 ................................. 11 4.4 Key Derivation Algorithms ............................. 12 4.4.1 PBKDF2 ....................................... 13 5 Content Encryption Algorithms ................................ 13 5.1 Triple-DES CBC ........................................ 13 5.2 RC2 CBC ............................................... 14 6 Message Authentication Code (MAC) Algorithms ................. 14 6.1 HMAC with SHA-1 ....................................... 14 7 Triple-DES and RC2 Key Wrap Algorithms ....................... 15 7.1 Key Checksum .......................................... 15 7.2 Triple-DES Key Wrap ................................... 16 7.3 Triple-DES Key Unwrap ................................. 16 7.4 RC2 Key Wrap .......................................... 17 7.5 RC2 Key Unwrap ........................................ 17 Appendix A: ASN.1 Module ........................................ 18 References ....................................................... 21 Security Considerations .......................................... 22 Acknowledgments .................................................. 25 Author's Address ................................................. 25 Full Copyright Statement ......................................... 26 Housley [Page 2]

INTERNET DRAFT August 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 common cryptographic algorithms. CMS implementations MAY support these algorithms; CMS implementations MAY support other algorithms as well. 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. In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, and MAY are to be interpreted as described by Scott Bradner 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 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 Housley [Page 3]

INTERNET DRAFT August 2001 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 SHOULD accept SHA-1 AlgorithmIdentifiers with absent parameters. Implementations SHOULD generate SHA-1 AlgorithmIdentifiers with absent 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 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. Housley [Page 4]

INTERNET DRAFT August 2001 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, 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, 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 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. Housley [Page 5]

INTERNET DRAFT August 2001 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, 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 MD5 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 either the sha1WithRSAEncryption algorithm identifier or the md5WithRSAEncryption algorithm identifier is 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. Housley [Page 6]

INTERNET DRAFT August 2001 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 section 7. 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 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: Housley [Page 7]

INTERNET DRAFT August 2001 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 be 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 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. 4.1.2 X9.42 Static-Static Diffie-Hellman Static-Static Diffie-Hellman key agreement is defined in RFC 2631 [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 Housley [Page 8]

INTERNET DRAFT August 2001 subjectKeyIdentifier alternative. In both cases, the recipient's certificate contains the sender's static public key, and the certificate subject public key information field MUST contain 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 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 discussed in section 7. 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. Housley [Page 9]

INTERNET DRAFT August 2001 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 RFC 2313 [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 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 RSA encryption scheme is also defined in RFC 2437 [NEWPKCS#1]. Within RFC 2437, 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 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- 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 RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and AuthenticatedData RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm fields. Housley [Page 10]

INTERNET DRAFT August 2001 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 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 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. 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 168-bit Triple-DES key-encryption key. Similarly, a 40-bit RC2 content-encryption key MAY be wrapped with Housley [Page 11]

INTERNET DRAFT August 2001 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 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. 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 Housley [Page 12]

INTERNET DRAFT August 2001 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 specified in RFC 2898 [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 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 13]

INTERNET DRAFT August 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 This section specifies the conventions employed by CMS implementations that support the HMAC with SHA-1 message authentication code (MAC). Housley [Page 14]

INTERNET DRAFT August 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 } 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 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. CMS implementations that support HMAC with SHA-1 MUST handle both an AlgorithmIdentifier parameters field which contains a NULL and an AlgorithmIdentifier with an absent parameters. 7 Triple-DES and RC2 Key Wrap Algorithms This section specifies algorithms for wrapping content-encryption keys with Triple-DES and RC2 key-encryption keys. Encryption of a Triple-DES content-encryption key with a Triple-DES key-encryption key uses the algorithm specified in sections 7.2 and 7.3. Encryption of a RC2 content-encryption key with a RC2 key-encryption key uses the algorithm specified in sections 7.4 and 7.5. Both of these algorithms rely on the key checksum algorithm specified in section 7.1. 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 Housley [Page 15]

INTERNET DRAFT August 2001 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, a Two-key Triple-DES key MUST NOT be used to wrap a Three-key Triple- DES key. 7.1 Key Checksum The CMS Key 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 7.1, 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. Housley [Page 16]

INTERNET DRAFT August 2001 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. 7.3 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 7.1. 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 7.1, call the result ICV. Housley [Page 17]

INTERNET DRAFT August 2001 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 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 7.1. 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 18]

INTERNET DRAFT August 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 -- Directory Authentication Framework (X.509-2000) AlgorithmIdentifier FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1) authenticationFramework(7) 4 } ; -- 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 } Housley [Page 19]

INTERNET DRAFT August 2001 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 } Housley [Page 20]

INTERNET DRAFT August 2001 -- 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 Housley [Page 21]

INTERNET DRAFT August 2001 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. 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. PKCS#5 Kaliski, B. PKCS #5: Password-Based Cryptography Specification, Version 2.0. RFC 2898. September 2000. PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet X.509 Public Key Infrastructure: Certificate and CRL Profile. RFC <TBD>. <Date>. {draft-ietf-pkix-new-part1-*.txt} RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness Recommendations for Security. RFC 1750. December 1994. Housley [Page 22]

INTERNET DRAFT August 2001 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 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 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 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 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 [PROFILE]. Housley [Page 23]

INTERNET DRAFT August 2001 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), 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. 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- 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 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 Housley [Page 24]

INTERNET DRAFT August 2001 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 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 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 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. 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. Housley [Page 25]

INTERNET DRAFT August 2001 Author Address Russell Housley RSA Laboratories 918 Spring Knoll Drive Herndon, VA 20170 USA rhousley@rsasecurity.com Housley [Page 26]

INTERNET DRAFT August 2001 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 itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process shall be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Housley [Page 27]