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S/MIME Working Group B. Kaliski
Internet Draft RSA Laboratories
Document: draft-ietf-smime-cms-rsa-kem-00.txt May 2003
Category: Standards
Use of the RSA-KEM Key Transport Algorithm in CMS
<draft-ietf-smime-cms-rsa-kem-00.txt>
Status of this Memo
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Abstract
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-
forward) mechanism for transporting keying data to a recipient using
the recipient's RSA public key. This document specifies the
conventions for using the RSA-KEM Key Transport Algorithm with the
Cryptographic Message Syntax (CMS).
Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[STDWORDS].
Kaliski Standards - Exp: November 2003 [Page 1]
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INTERNET DRAFT May 2003 1. Introduction The RSA-KEM Key Transport Algorithm is a one-pass (store-and- forward) mechanism for transporting keying data to a recipient using the recipient's RSA public key. Most previous key transport algorithms based on the RSA public-key cryptosystem (e.g., the popular PKCS #1 v1.5 algorithm [PKCS1]) have the following general form: 1. Format or "pad" the keying data to obtain an integer m. 2. Encrypt the integer m with the recipient's RSA public key: c = m^e mod n 3. Output c as the encrypted keying data. The RSA-KEM Key Transport Algorithm takes a different approach that provides higher security assurance, by encrypting a _random_ integer with the recipient's public key, and using a symmetric key wrapping scheme to encrypt the keying data. It has the following form: 1. Generate a random integer z between 0 and n-1. 2. Encrypt the integer z with the recipient's RSA public key: c = z^e mod n. 3. Derive a key-encrypting key KEK from the integer z. 4. Wrap the keying data using KEK to obtain wrapped keying data KD. 5. Output c and KD as the encrypted keying data. This different approach provides higher security assurance because the input to the underlying RSA operation is random and independent of the message, and the key-encrypting key KEK is derived from it in a strong way. As a result, the algorithm enjoys a "tight" security proof in the random oracle model. It is also architecturally convenient because the public-key operations are separate from the symmetric operations on the keying data. One benefit is that the length of the keying data is bounded only by the symmetric key wrapping scheme, not the size of the RSA modulus. The RSA-KEM Key Transport Algorithm in various forms is being adopted in several draft standards including ANSI X9.44 [ANSI-X9.44] and ISO/IEC 18033-2 [ISO-IEC-18033-2]. It has also been recommended by the NESSIE project [NESSIE]. Although the other standards are still in development, the algorithm is fairly stable across the drafts. For completeness, a specification of the algorithm is given Kaliski Standards - Exp: November 2003 [Page 2]

INTERNET DRAFT May 2003 in Appendix A of this document; ASN.1 syntax is given in Appendix B. NOTE: The term KEM stands for "key encapsulation mechanism" and refers to the first three steps of the process above. The formalization of key transport algorithms (or more generally, asymmetric encryption schemes) in terms of key encapsulation mechanisms is a result of research by Victor Shoup leading to the development of the ISO/IEC 18033-2 standard [SHOUP]. 2. Use in CMS The RSA-KEM Key Transport Algorithm MAY be employed for one or more recipients in the CMS enveloped-data content type (Section 6 of [CMS]), where the keying data processed by the algorithm is the CMS content-encryption key. The RSA-KEM Key Transport Algorithm SHOULD be considered for new CMS-based applications as a replacement for the widely implemented RSA encryption algorithm specified originally in PKCS #1 v1.5 (see [PKCS1] and Section 4.2.1 of [CMSALGS]), which is vulnerable to chosen-ciphertext attacks. The RSAES-OAEP Key Transport Algorithm has also been proposed as a replacement (see [PKCS1] and [CMS- OAEP]). RSA-KEM has the advantage over RSAES-OAEP of a tighter security proof, but the disadvantage of slightly longer encrypted keying data. 2.1 Underlying Components A CMS implementation that supports the RSA-KEM Key Transport Algorithm MUST support at least the following underlying components: * For the key derivation function, KDF2 (see [ANSI-X9.44][IEEE- P1363a]) based on SHA-1 (see [NIST-SHA2]) (this function is also specified as the key derivation function in [ANSI-X9.63]) * For the key wrapping scheme, AES-Wrap-128, i.e., the AES Key Wrap with a 128-bit key encrypting key (see [AES-WRAP]) An implementation SHOULD also support KDF2 based on SHA-256 (see [NIST-SHA2]), and the Triple-DES Key Wrap (see [3DES-WRAP]). It MAY support other underlying components. 2.2 RecipientInfo Conventions When the RSA-KEM Key Transport Algorithm is employed for a recipient, the RecipientInfo alternative for that recipient MUST be KeyTransRecipientInfo. The algorithm-specific fields of the KeyTransRecipientInfo value MUST have the following values: * keyEncryptionAlgorithm.algorithm MUST be id-kts2-basic (see Appendix B) Kaliski Standards - Exp: November 2003 [Page 3]

INTERNET DRAFT May 2003 * keyEncryptionAlgorithm.parameters MUST be a value of type KTS2-Parms (see Appendix B) * encryptedKey MUST be the encrypted keying data output by the algorithm (see Appendix A) 2.3 Certificate Conventions A recipient who employs the RSA-KEM Key Transport Algorithm MAY identify the public key in a certificate by the same AlgorithmIdentifier as for the PKCS #1 v1.5 algorithm, i.e., using the rsaEncryption object identifier [PKCS1]. If the recipient wishes only to employ the RSA-KEM Key Transport Algorithm with a given public key, the recipient MUST identify the public key in the certificate using the id-kts2-basic object identifier (see Appendix B) where the KTS2-Params value indicates the underlying components with which the algorithm is to be employed. [[matching rules to be added]] 2.4 SMIMECapabilities Attribute Conventions [[to be added]] 3. Security Considerations The security of the RSA-KEM Key Transport Algorithm described in this document has been shown to be tightly related to the difficulty of either solving the RSA problem or breaking the underlying symmetric key wrapping scheme, if the underlying key derivation function is modeled as a random oracle [SHOUP]. While in practice a random-oracle result does not provide an actual security proof for any particular key derivation function, the result does provide assurance that the general construction is reasonable; a key derivation function would need to be particularly weak to lead to an attack that is not possible in the random oracle model. The RSA key size and the underlying components should be selected consistent with the desired symmetric security level for an application. Several security levels have been identified in [NIST- GUIDELINES]. For brevity, the first three levels are mentioned here: * 80-bit security. The RSA key size SHOULD be at least 1024 bits, the hash function underlying KDF2 SHOULD be SHA-1 or above, and the symmetric key-wrapping scheme SHOULD be AES Key Wrap or Triple-DES Key Wrap. * 112-bit security. The RSA key size SHOULD be at least 2048 bits, the hash function underlying KDF2 SHOULD be SHA-224 or above, and the symmetric key-wrapping scheme SHOULD be AES Key Wrap or Triple-DES Key Wrap. Kaliski Standards - Exp: November 2003 [Page 4]

INTERNET DRAFT May 2003 * 128-bit security. The RSA key size SHOULD be at least 3072 bits, the hash function underlying KDF2 SHOULD be SHA-256 or above, and the symmetric key-wrapping scheme SHOULD be AES Key Wrap. Note that the AES Key Wrap MAY be used at all three of these levels; the use of AES does not require a 128-bit security level for other components. The security of the algorithm also depends on the strength of the random number generator, which SHOULD have a comparable security level. For further discussion on random number generation, please see [RANDOM]. Implementations SHOULD NOT reveal information about intermediate values or calculations, whether by timing or other "side channels", or otherwise an opponent may be able to determine information about the keying data and/or the recipient's private key. Although not all intermediate information may be useful to an opponent, it is preferable to conceal as much information as is it practical to, unless analysis specifically indicates that the information would not be useful. Parties MAY wish to formalize the assurance that one another's implementations are correct through implementation validation, e.g. NIST's Cryptographic Module Validation Program (CMVP). 4. References 4.1 Normative References 3DES-WRAP Housley, R. Triple-DES and RC2 Key Wrapping. RFC 3217. December 2001. AES-WRAP Schaad, J. and R. Housley. Advanced Encryption Standard (AES) Key Wrap Algorithm. RFC 3394. September 2002. ANSI-X9.63 American National Standard X9.63-2002: Public Key Cryptography for the Financial Services Industry: Key Agreement and Key Transport Using Elliptic Curve Cryptography. CMS Housley, R. Cryptographic Message Syntax. RFC 3369. August 2002. CMSALGS Housley, R. Cryptographic Message Syntax (CMS) Algorithms. RFC 3370. August 2002. NIST-SHA2 National Institute of Standards and Technology (NIST). FIPS 180-2: Secure Hash Standard. August 2002. Kaliski Standards - Exp: November 2003 [Page 5]

INTERNET DRAFT May 2003 STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate Requirement Levels. RFC 2119. March 1997. 4.2 Informative References ANSI-X9.44 ANSI X9F1 Working Group. ANSI X9.44: Public Key Cryptography for the Financial Services Industry - - Key Establishment Using Integer Factorization Cryptography. Draft D4.1, April 1, 2003. CMS-OAEP Housley, R. Use of the RSAES-OAEP Key Transport Algorithm in CMS. Internet Draft <draft-ietf- smime-cms-rsaes-oaep-07.txt>. December 2002. IEEE-P1363a IEEE P1363 Working Group. IEEE P1363a: Standard Specifications for Public Key Cryptography: Additional Techniques. Draft D12, May 12, 2003. Available via http://grouper.ieee.org/groups/1363. ISO-IEC-18033-2 ISO/IEC 18033-2: Information technology -- Security techniques -- Encryption algorithms -- Part 2: Asymmetric Ciphers. Committee Draft, December 18, 2002. NESSIE NESSIE Consortium. Portfolio of Recommended Cryptographic Primitives. February 27, 2003. Available via http://www.cryptonessie.org/. NIST-GUIDELINES National Institute of Standards and Technology. Special Publication 800-57: Recommendation for Key Management. Part 1: General Guideline. Draft, January 2003. Available via http://csrc.nist.gov/CryptoToolkit/tkkeymgmt.html. NIST-SCHEMES National Institute of Standards and Technology. Special Publication 800-56: Recommendation on Key Establishment Schemes. Draft 2.0, January 2003. Available via http://csrc.nist.gov/CryptoToolkit/tkkeymgmt.html. PKCS1 Jonsson, J. and B. Kaliski. PKCS #1: RSA Cryptography Specifications Version 2.1. RFC 3447. February 2003. RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness Recommendations for Security. RFC 1750. December 1994. SHOUP Shoup, V. A Proposal for an ISO Standard for Public Key Encryption. Version 2.1, December 20, 2001. Available via http://www.shoup.net/papers/. Kaliski Standards - Exp: November 2003 [Page 6]

INTERNET DRAFT May 2003 5. IANA Considerations Within the CMS, algorithms are identified by object identifiers (OIDs). All of the OIDs used in this document were assigned in Public-Key Cryptography Standards (PKCS) documents, Accredited Standards Committee (ASC) X9 documents, or by the National Institute of Standards and Technology (NIST). No further action by the IANA is necessary for this document or any anticipated updates. 6. Acknowledgments This document is one part of a strategy to align algorithm standards produced by ASC X9, ISO/IEC JTC1 SC27, NIST, and the IETF. I would like to thank the members of the ANSI X9F1 working group for their contributions to drafts of ANSI X9.44 which led to this specification. My thanks as well to Russ Housley as well for his guidance and encouragement. 7. Author Address Burt Kaliski RSA Laboratories 174 Middlesex Turnpike Bedford, MA 01730 USA bkaliski@rsasecurity.com Appendix A. RSA-KEM Key Transport Algorithm The RSA-KEM Key Transport Algorithm is a one-pass (store-and- forward) mechanism for transporting keying data to a recipient using the recipient's RSA public key. With this type of algorithm, a sender encrypts the keying data using the recipient's public key to obtain encrypted keying data. The recipient decrypts the encrypted keying data using the recipient's private key to recover the keying data. A.1 Underlying Components The algorithm has the following underlying components: * KDF, a key derivation function, which derives keying data of a specified length from a shared secret value * Wrap, a symmetric key wrapping scheme, which encrypts keying data using a key-encrypting key In the following, kekLen denotes the length in bytes of the key- encrypting key for the underlying symmetric key-wrapping scheme. In this scheme, the length of the keying data to be transported MUST be among the lengths supported by the underlying symmetric key Kaliski Standards - Exp: November 2003 [Page 7]

INTERNET DRAFT May 2003 wrapping scheme. (The AES Key Wrap, for instance, requires the length of the keying data to be a multiple of 8 bytes, and at least 16 bytes.) Usage and formatting of the keying data (e.g., parity adjustment for Triple-DES keys) is outside the scope of this algorithm. With some key derivation functions, it is possible to include other information besides the shared secret value in the input to the function. Also, with some symmetric key wrapping schemes, it is possible to associate a label with the keying data. Such uses are outside the scope of this document, as they are not directly supported by CMS. A.2 Sender's Operations Let (n,e) be the recipient's RSA public key (see [PKCS1] for details) and let K be the keying data to be transported. Let nLen denote the length in bytes of the modulus n, i.e., the least integer such that 2^{8*nLen} > n. The sender performs the following operations: 1. Generate a random integer z between 0 and n-1 (see Note), and convert z to a byte string Z of length nLen, most significant byte first: z = RandomInteger (0, n-1) Z = IntegerToString (z, nLen) 2. Encrypt the random integer z using the recipient's public key (n,e) and convert the resulting integer c to a ciphertext C, a byte string of length nLen: c = z^e mod n C = IntegerToString (c, nLen) 3. Derive a key-encrypting key KEK of length kekLen bytes from the byte string Z using the underlying key derivation function: KEK = KDF (Z, kekLen) 4. Wrap the keying data K using the underlying key wrapping scheme with the key-encrypting key KEK to obtain wrapped keying data WK: WK = Wrap (KEK, K) 5. Concatenate the ciphertext C and the wrapped keying data WK to obtain the encrypted keying data EK: EK = C || WK Kaliski Standards - Exp: November 2003 [Page 8]

INTERNET DRAFT May 2003 6. Output the encrypted keying data EK. NOTE: The random integer z MUST be generated independently at random for different encryption operations, whether for the same or different recipients. A.3 Recipient's Operations Let (n,d) be the recipient's RSA private key (see [PKCS1]; other private key formats are allowed) and let EK be the encrypted keying data. Let nLen denote the length in bytes of the modulus n. The recipient performs the following operations: 1. Separate the encrypted keying data EK into a ciphertext C of length nLen bytes and wrapped keying data WK: C || WK = EK If the length of the encrypted keying data is less than nLen bytes, output "decryption error" and stop. 2. Convert the ciphertext C to an integer c, most significant byte first. Decrypt the integer c using the recipient's private key (n,d) to recover an integer z (see Note): c = StringToInteger (C) z = c^d mod n If the integer c is not between 0 and n-1, output "decryption error" and stop. 3. Convert the integer z to a byte string Z of length nLen, most significant byte first (see Note): Z = IntegerToString (z, nLen) 4. Derive a key-encrypting key KEK of length kekLen bytes from the byte string Z using the underlying key derivation function (see Note): KEK = KDF (Z, kekLen) 5. Unwrap the wrapped keying data WK using the underlying key wrapping scheme with the key-encrypting key KEK to recover the keying data K: K = Unwrap (KEK, WK) Kaliski Standards - Exp: November 2003 [Page 9]

INTERNET DRAFT May 2003 If the unwrapping operation outputs an error, output "decryption error" and stop. 6. Output the keying data K. NOTE: Implementations SHOULD NOT reveal information about the integer z and the string Z, nor about the calculation of the exponentiation in Step 2, the conversion in Step 3, or the key derivation in Step 4, whether by timing or other "side channels". The observable behavior of the implementation SHOULD be the same at these steps for all ciphertexts C that are in range. (For example, IntegerToString conversion should take the same amount of time regardless of the actual value of the integer z.) The integer z, the string Z and other intemediate results MUST be securely deleted when they are no longer needed. Appendix B. ASN.1 Syntax The ASN.1 syntax for identifying the RSA-KEM Key Transport Algorithm is a special case of the syntax for Key Transport Scheme 2 (KTS2) in the draft ANSI X9.44 [ANSI-X9.44]. The syntax for the scheme is given in Section B.1. The syntax for selected underlying components including those mentioned above is given in B.2. The following object identifier prefixes are used in the definitions below: x9-44 OID ::= { iso(1) identified-organization(3) tc68(133) country(16) x9(840) x9Standards(9) x9-44(44) } pkcs-1 OID ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } nistAlgorithm OID ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101) csor(3) nistAlgorithm(4) } The material in this Appendix is based on a draft standard and is SUBJECT TO CHANGE as that standard is developed. B.1 RSA-KEM Key Transport Algorithm The object identifier for the RSA-KEM Key Transport Algorithm is the same as for the basic KTS2 scheme in the draft ANSI X9.44, id-kts2- basic, which is defined in the draft as id-kts2-basic OID ::= { x9-44 schemes(2) kts2-basic(7) } Kaliski Standards - Exp: November 2003 [Page 10]

INTERNET DRAFT May 2003 The associated parameters for id-kts2-basic have type KTS2-Parms: KTS2-Parms ::= SEQUENCE { kas [0] KTS2-KeyAgreementScheme, kws [1] KTS2-SymmetricKeyWrappingScheme, labelMethod [2] KTS2-LabelMethod } The fields of type KTS2-Parms have the following meanings: * kas identifies the underlying key agreement scheme. For the RSA-KEM Key Transport Algorithm, the scheme is the basic Key Agreement Scheme 1 (KAS1) from the draft ANSI X9.44. The object identifier for the basic KAS1 is id-kas1-basic, which is defined in the draft ANSI X9.44 as id-kas1-basic OID ::= { x9-44 schemes(2) kas1-basic(1) } The associated parameters for id-kas1-basic have type KAS1- Parms: KAS1-Parms ::= SEQUENCE { sves [0] KAS1-SecretValueEncapsulationScheme, kdf [1] KAS1-KeyDerivationFunction, otherInfoMethod [2] KAS1-OtherInfoMethod } The fields of type KAS1-Parms have the following meanings: * sves identifies the underlying secret-value encapsulation mechanism. (In the draft ANSI X9.44, the term "Secret Value Encapsulation Scheme" refers to the first _two_ steps of the RSA-KEM Key Transport Algorithm, which are separated from the key derivation function for architectural reasons.) For the RSA-KEM Key Transport Algorithm, the mechanism is RSASVES1 from the draft ANSI X9.44. The object identifier for RSASVES1 is id-rsasves1, which is defined in the draft ANSI X9.44 as id-rsasves1 OID ::= { x9-44 components(1) rsasves1(2) } This object identifier has no associated parameters. * kdf identifies the underlying key derivation function. For alignment with the draft ANSI X9.44, it MUST be KDF2. However, other key derivation functions MAY be used with CMS. Please see B.2.1 for the syntax for KDF2. Kaliski Standards - Exp: November 2003 [Page 11]

INTERNET DRAFT May 2003 KAS1-KeyDerivationFunction ::= AlgorithmIdentifier * otherInfoMethod specifies the method for formatting other information to be included in the input to the key derivation function. For this version of the document, the method MUST be the "specified other information" method. KAS1-OtherInfoMethod ::= AlgorithmIdentifier The object identifier for the "specified other information" method is id-specifiedOtherInfo: id-specifiedOtherInfo OID ::= [[to be defined]] The associated parameters for id-specifiedOtherInfo have type SpecifiedOtherInfo: SpecifiedOtherInfo ::= OCTET STRING SIZE((0..MAX)) For this version of the document, the value of the other information MUST be the empty string. * kws identifies the underlying symmetric key-wrapping scheme. For alignment with the draft ANSI X9.44, it MUST be an X9- approved symmetric key-wrapping scheme. (See Note.) However, other schemes MAY be used with CMS. Please see B.2.2 for the syntax for the AES and Triple-DES Key Wraps. KTS2-SymmetricKeyWrappingScheme ::= AlgorithmIdentifier * labelMethod specifies the method for formatting a label to be associated with the keying data. For this version of the document, the method MUST be the "specified label" method. KTS2-LabelMethod ::= AlgorithmIdentifier The object identifier for the "specified label" method is id- specifiedLabel, which is defined in the draft ANSI X9.44 as id-specifiedLabel OID ::= { pkcs-1 specifiedLabel(9) } The associated parameters for id-specifiedLabel have type SpecifiedLabel: SpecifiedLabel ::= OCTET STRING SIZE((0..MAX)) For this version of the document, the value of the label MUST be the empty string. NOTE: As of this writing, the AES Key Wrap and the Triple-DES Key Wrap are in the process of being approved by X9. Kaliski Standards - Exp: November 2003 [Page 12]

INTERNET DRAFT May 2003 DISCUSSION TOPIC: In NIST's key establishment schemes recommendation [NIST-SCHEMES], the parties' names are included in the "other information" for key derivation. Should they be included here as well? B.2 Selected Underlying Components B.2.1 Key Derivation Functions The object identifier for KDF2 (see [ANSI-X9.44]) is id-kdf2 OID ::= { x9-44 components(1) kdf2(1) } The associated parameters identify the underlying hash function. For alignment with the draft ANSI X9.44, the hash function MUST be an X9-approved hash function. (See Note.) However, other hash functions MAY be used with CMS. KDF2-Parms ::= AlgorithmIdentifier The object identifier for SHA-1 is id-sha1 OID ::= { iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) sha1(26) } The object identifiers for SHA-256, SHA-384 and SHA-512 are id-sha256 OID ::= { nistAlgorithm hashAlgs(2) sha256(1) } id-sha384 OID ::= { nistAlgorithm hashAlgs(2) sha384(2) } id-sha512 OID ::= { nistAlgorithm hashAlgs(2) sha512(3) } There has been some confusion over whether the various SHA object identifiers have a NULL parameter, or no associated parameters. As also discussed in [PKCS1], implementations SHOULD generate algorithm identifiers without parameters, and MUST accept algorithm identifiers either without parameters, or with NULL parameters. NOTE: As of this writing, only SHA-1 is an X9-approved hash function; SHA-224 and above are in the process of being approved. The object identifier for SHA-224 has not yet been assigned. B.2.2 Symmetric Key Wrapping Schemes The object identifier for the AES Key Wrap depends on the size of the key encrypting key. There are three object identifiers (see [AES-WRAP]): id-aes128-Wrap OID ::= { nistAlgorithm aes(1) aes128-Wrap(5) } id-aes192-Wrap OID ::= { nistAlgorithm aes(1) aes192-Wrap(25) } id-aes256-Wrap OID ::= { nistAlgorithm aes(1) aes256-Wrap(45) } Kaliski Standards - Exp: November 2003 [Page 13]

INTERNET DRAFT May 2003 These object identifiers have no associated parameters. The object identifier for the Triple-DES Key Wrap (see [3DES-WRAP]) is id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 } This object identifier has a NULL parameter. B.3 Example As an example, if the key derivation function is KDF2 based on SHA-1 and the symmetric key wrapping scheme is the AES Key Wrap with a 128-bit KEK, the AlgorithmIdentifier for the RSA-KEM Key Transport Algorithm will have the following value: SEQUENCE { id-kts2-basic, -- basic KTS2 SEQUENCE { -- KTS2-Parms [0] SEQUENCE { -- key agreement scheme id-kas1-basic, -- basic KAS1 SEQUENCE { -- KAS1-Parms [0] SEQUENCE { -- secret value encapsulation scheme id-rsasves1 -- RSASVES1; no parameters }, [1] SEQUENCE { -- key derivation function id-kdf2, -- KDF2 SEQUENCE { -- KDF2-Parms id-sha1 -- no parameters (preferred) } }, [2] SEQUENCE { -- other information method id-specifiedOtherInfo, -- specified other info. ''H -- empty string } } }, [1] SEQUENCE { -- symmetric key wrapping scheme id-aes128-Wrap -- AES-128 Wrap; no parameters }, [2] SEQUENCE { -- label method id-specifiedLabel, -- specified label ''H -- empty string } } } Kaliski Standards - Exp: November 2003 [Page 14]

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INTERNET DRAFT May 2003
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