S/MIME Working Group R. Housley Internet Draft RSA Laboratories expires in six months June 2002 Use of the RSAES-OAEP Key Transport Algorithm in CMS 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 Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document describes the use of the RSAES-OAEP key transport method of key management within the Cryptographic Message Syntax. 1 Introduction This draft is being discussed on the "ietf-smime" mailing list. To join the list, send a message to with the single word "subscribe" in the body of the message. Also, there is a Web site for the mailing list at . PKCS #1 Version 1.5 [PKCS#1v1.5] specifies a widely deployed variant of the RSA key transport algorithm. PKCS #1 Version 1.5 key Housley [Page 1] INTERNET DRAFT June 2002 transport is vulnerable to adaptive chosen ciphertext attacks, especially when it is used to for key management in interactive applications. This attack is often referred to as the "Million Message Attack," and it explained in [RSALABS] and [CRYPTO98]. Exploitation of this vulnerability, which reveals the result of a particular RSA decryption, requires access to an oracle which will respond to hundreds of thousands of ciphertexts, which are constructed adaptively in response to previously received replies that provide information on the successes or failures of attempted decryption operations. The attack is significantly less feasible in store-and-forward environments, such as S/MIME. RFC 3218 [MMA] discussed the countermeasures to this attack that are available when PKCS #1 Version 1.5 key transport is used in conjunction with the Cryptographic Message Syntax (CMS) [CMS]. When PKCS #1 Version 1.5 key transport is applied as an intermediate encryption layer within an interactive request-response communications environment, exploitation could be more feasible. However, Secure Sockets Layer (SSL) [SSL] and Transport Layer Security (TLS) [TLS] protocol implementations could include countermeasures that detect and prevent the Million Message Attack and other chosen-ciphertext attacks. These countermeasures are performed within the protocol level. In the interest of long-term security assurance, it is prudent to adopt an improved cryptographic technique rather than embedding countermeasures within protocols. An updated version of PKCS #1 has been published, PKCS #1 Version 2.0 [PKCS#1v2.0]. 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#1v1.5], 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) for RSA key transport. This document specifies the use of RSAES-OAEP key transport algorithm in the CMS. The CMS can be used in either a store-and-forward or an interactive request-response environment. The CMS supports variety of architectures for certificate-based key management, particularly the one defined by the PKIX working group [PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the same RSA public key information. Thus, a certified RSA public key may be used with either RSA key transport technique. The CMS uses ASN.1 [X.208-88], the Basic Encoding Rules (BER) Housley [Page 2] INTERNET DRAFT June 2002 [X.209-88], and the Distinguished Encoding Rules (DER) [X.509-88]. Throughout this document, when the terms MUST, MUST NOT, SHOULD and MAY are used in capital letters, their use conforms to the definitions in [STDWORDS]. These key word definitions help make the intent of standards documents as clear as possible. These key words are used in this document to help implementers achieve interoperability. 2 Enveloped-data Conventions The CMS enveloped-data content type consists of an encrypted content and wrapped content-encryption keys for one or more recipients. The RSAES-OAEP key transport algorithm is used to wrap the content- encryption key for one recipient. Compliant software MUST meet the requirements for constructing an enveloped-data content type stated in [CMS] Section 6, "Enveloped- data Content Type". A content-encryption key MUST be randomly generated for each instance of an enveloped-data content type. The content-encryption key is used to encipher the content. 2.1 EnvelopedData Fields The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be populated as follows: The EnvelopedData version MUST be 0, 2, or 3. The EnvelopedData originatorInfo field is not used for the RSAES-OAEP key transport algorithm. However, this field MAY be present to support recipients using other key management algorithms. The EnvelopedData recipientInfos CHOICE MUST be KeyTransRecipientInfo. See section 2.2 for further discussion of KeyTransRecipientInfo. The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm field MUST be a symmetric encryption algorithm identifier. The EnvelopedData unprotectedAttrs MAY be present. Housley [Page 3] INTERNET DRAFT June 2002 2.2 KeyTransRecipientInfo Fields The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax. The fields of the EnvelopedData syntax must be populated as follows: The KeyTransRecipientInfo version MUST be 0 or 2. If the RecipientIdentifier uses the issuerAndSerialNumber alternative, then the version MUST be 0. If the RecipientIdentifier uses the subjectKeyIdentifier alternative, then the version MUST be 2. The KeyTransRecipientInfo RecipientIdentifier provides two alternatives for specifying the recipient's certificate, and thereby the recipient's public key. The recipient's certificate MUST contain a RSA public key. The content-encryption key is encrypted with the recipient's RSA public key. The issuerAndSerialNumber alternative identifies the recipient's certificate by the issuer's distinguished name and the certificate serial number; the subjectKeyIdentifier identifies the recipient's certificate by the X.509 subjectKeyIdentifier extension value. The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the RSAES-OAEP algorithm, and its associated parameters, was used to encrypt the content-encryption key for the recipient. The key- encryption process is described in [PKCS#1v2.0]. See section 3 of this document for the algorithm identifier and the parameter syntax. The KeyTransRecipientInfo encryptedKey is the result of encrypting the content-encryption key in the recipient's RSA public key using the RSAES-OAEP algorithm. When using a Triple-DES [3DES] content- encryption key, implementations MUST adjust the parity bits to ensure odd parity for each octet of each DES key comprising the Triple-DES key prior to RSAES-OAEP encryption. 3 RSAES-OAEP Algorithm Identifiers and Parameters The RSAES-OAEP key transport algorithm is the RSA encryption scheme defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted is the content-encryption key. The algorithm identifier for RSAES- OAEP is: id-RSAES-OAEP OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7 } The AlgorithmIdentifier parameters field must be present, and the parameters field must contain RSAES-OAEP-params. RSAES-OAEP-params have the following syntax: Housley [Page 4] INTERNET DRAFT June 2002 RSAES-OAEP-params ::= SEQUENCE { hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier } sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL } mgf1SHA1Identifier AlgorithmIdentifier ::= { id-mgf1, sha1Identifier } pSpecifiedEmptyIdentifier AlgorithmIdentifier ::= { id-pSpecified, nullOctetString } nullOctetString OCTET STRING (SIZE (0)) ::= { ''H } id-sha1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 } pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } The fields of type RSAES-OAEP-params have the following meanings: hashFunc identifies the one-way hash function. Implementations MUST support SHA-1 [SHA1]. The SHA-1 algorithm identifier is comprised of the id-sha1 object identifier and a parameter of NULL. Implementations that perform encryption MUST omit the hashFunc field when SHA-1 is used, indicating that the default algorithm was used. Implementations that perform decryption MUST recognize both the id-sha1 object identifier and an absent hashFunc field as an indication that SHA-1 was used. maskGenFunc identifies the mask generation function. Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a one-way hash function, and it is identified in the parameter field of the MFG1 algorithm identifier. Implementations MUST support SHA-1 [SHA1]. The MFG1 algorithm identifier is comprised of the id-mgf1 object identifier and a parameter that contains the algorithm identifier of the one-way hash function employed with MFG1. The SHA-1 algorithm identifier is comprised of the id-sha1 object identifier and a parameter of NULL. Implementations that perform encryption MUST omit the maskGenFunc field when MFG1 with Housley [Page 5] INTERNET DRAFT June 2002 SHA-1 is used, indicating that the default algorithm was used. Implementations that perform decryption MUST recognize both the id-mgf1 and id-sha1 object identifiers as well as an absent maskGenFunc field as an indication that MFG1 with SHA-1 was used. pSourceFunc identifies the source (and possibly the value) of the encoding parameters, commonly called P. Implementations MUST represent P by an algorithm identifier, id-pSpecified, indicating that P is explicitly provided as an OCTET STRING in the parameters. The default value for P is an empty string. In this case, pHash in EME-OAEP contains the hash of a zero length string. Implementations MUST support a zero length P value. Implementations that perform encryption MUST omit the pSourceFunc field when a zero length P value is used, indicating that the default value was used. Implementations that perform decryption MUST recognize both the id-pSpecified object identifier and an absent pSourceFunc field as an indication that a zero length P value was used. 4 SMIMECapabilities Attribute Conventions RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be used to specify a partial list of algorithms that the software announcing the SMIMECapabilities can support. When constructing a signedData object, compliant software MAY include the SMIMECapabilities signed attribute announcing that it supports the RSAES-OAEP algorithm. The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the id-RSAES-OAEP object identifier in the capabilityID field and MUST include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters field. rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::= { id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier } } When all of the default settings are selected, the SMIMECapability SEQUENCE representing RSAES-OAEP MUST be DER-encoded as the following hexadecimal string: 30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00 Housley [Page 6] INTERNET DRAFT June 2002 5 References This section provides normative and informative references. 5.1 Normative 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 . . MSG Ramsdell, B. S/MIME Version 3 Message Specification. RFC 2633. June 1999. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. RFC 2437. October 1998. PROFILE Housley, R., W. Polk, W. Ford, and D. Solo. Internet X.509 Public Key Infrastructure: Certificate and Certificate Revocation List (CRL) Profile. RFC 3280. April 2002. SHA1 National Institute of Standards and Technology. FIPS Pub 180-1: Secure Hash Standard. 17 April 1995. STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate Requirement Levels. BCP 14, RFC 2119. 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. X.509-88 CCITT. Recommendation X.509: The Directory - Authentication Framework. 1988. 5.2 Informative References CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against Protocols Based on the RSA Encryption Standard PKCS #1," in H. Krawczyk (editor), Advances in Cryptology - CRYPTO '98 Proceedings, Lecture Notes in Computer Science 1462 (1998), Springer-Verlag, pp. 1-12. MMA Rescorla, E. Preventing the Million Message Attack on Housley [Page 7] INTERNET DRAFT June 2002 Cryptographic Message Syntax. RFC 3218. January 2002. PKCS#1v1.5 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. RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon. Recent Results on PKCS #1: RSA Encryption Standard. RSA Laboratories' Bulletin No. 7, June 26, 1998. [http://www.rsasecurity.com/rsalabs/bulletins] SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol, Version 3.0. Netscape Communications. November 1996. [http://wp.netscape.com/eng/ssl3/draft302.txt] TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0. RFC 2246. January 1999. 6 Security Considerations Implementations must protect the RSA private key and the content- encryption key. Compromise of the RSA private key may result in the disclosure of all messages protected with that key. 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. Compromise of the message-authentication key may result in undetectable modification of the authenticated content. The generation of RSA 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. 7 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. Further, I extend a special thanks to Burt Kaliski. Housley [Page 8] INTERNET DRAFT June 2002 8 Author Address Russell Housley RSA Laboratories 918 Spring Knoll Drive Herndon, VA 20170 USA rhousley@rsasecurity.com Appendix A ASN.1 Module CMS-RSAES-OAEP {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-rsaes-oaep(20) } DEFINITIONS IMPLICIT TAGS ::= BEGIN -- EXPORTS ALL -- IMPORTS -- From PKIX Certificate and CRL Profile AlgorithmIdentifier FROM PKIXExplicit88 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit(18) }; pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 } RSAES-OAEP-params ::= SEQUENCE { hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier } sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL } mgf1SHA1Identifier AlgorithmIdentifier ::= { id-mgf1, sha1Identifier } pSpecifiedEmptyIdentifier AlgorithmIdentifier ::= { id-pSpecified, nullOctetString } nullOctetString OCTET STRING (SIZE (0)) ::= { ''H } Housley [Page 9] INTERNET DRAFT June 2002 id-sha1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::= { id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier } } END Housley [Page 10] INTERNET DRAFT June 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). 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. 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