S/MIME Working Group J. Schaad Internet Draft Soaring Hawk Consulting Document: draft-ietf-smime-aes-alg-01.txt R. Housley Expires: September 2, 2001 RSA Laboratories March 2001 Use of the Advanced Encryption 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 [1]. 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/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments or suggestions for improvement may be made on the "ietf- smime" mailing list, or directly to the author. Abstract This document specifies how to incorporate the Advanced Encryption Standard (AES) algorithm [AES] and RSAES-OAEP key transport method of key management into the S/MIME Cryptographic Message Syntax [CMS] as additional algorithms. 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 [2]. 1. Overview This document describes the conventions for using the RSAES-OAEP key transport algorithm and Advanced Encryption Standard (AES) content encryption algorithm with the Cryptographic Message Syntax [CMS] enveloped-data, encrypted-data and authenticated-data content types. While this document presents the use of the two algorithms together, that fact does not imply that they cannot be used independently. Schaad, Housley 1 Use of the AES Algorithm in CMS November 2000 They are presented together simply because the initial usage of each will be as a matched pair. When the variant of the RSA key transport algorithm specified in PKCS #1 Version 1.5 [PKCS#1v1.5] is used for key management, it is vulnerable to adaptive chosen ciphertext attacks. This attack is explained in [RSALAB] and [CRYPTO98]. The use of PKCS #1 Version 1.5 key transport in interactive applications is especially vulnerable. Exploitation of this identified vulnerability, revealing 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 providing information on the successes or failures of attempted decryption operations. The attack appears significantly less feasible in store-and-forward environments, such as S/MIME. 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 Bleichenbacher's and other chosen-ciphertext attacks, without changing the way the RSA key transport algorithm is used. 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) when RSA encryption is used to provide confidentiality, such as key transport. This document specifies the use of RSAES-OAEP key transport algorithm in the Cryptographic Message Syntax (CMS) [CMS]. CMS can be used in either a store-and-forward or an interactive request-response environment. 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. CMS values are generated using ASN.1 [X.208-88], using the Basic Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules (DER) [X.509-88]. 2. Enveloped-data Conventions Schaad, Housley 2 Use of the AES Algorithm in CMS November 2000 The CMS enveloped-data content type consists of 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. The AES algorithm is used to encrypt the content. 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. AES can be used with the enveloped-data content type using any of the following key management techniques defined in [CMS] Section 6. 1) Key Transport: The AES CEK is uniquely wrapped for each recipient using the recipient's public RSA key and other values. Section 2.XX provides additional details. RSAES-OEAP is a key transport algorithm and itÆs usage is described here. 2) Key Agreement: The AES CEK is uniquely wrapped for each recipient using a pairwise symmetric key-encryption key (KEK) generated using DH-ES using the a randomly generated private key value for the originator, the recipient's public DH key and other values. Section 2.XX provides additional details. 3) "Previously Distributed" Symmetric KEK: The AES CEK is wrapped using a "previously distributed" symmetric KEK (such as a Mail List Key). The methods by which the symmetric KEK is generated and distributed are beyond the scope of this document. Section 2.XXX provides more details. 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 either 0 or 2. 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 is dependent on the key managment technique used. Section 2.2, 2.3 and 2.4 provide more inforamtion. The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm field MUST specify a symmetric encryption algorithm. Implementations MUST support the encryption of AES keys, but implementations MAY support other algorithms as well. Schaad, Housley 3 Use of the AES Algorithm in CMS November 2000 The EnvelopedData unprotectedAttrs MAY be present. 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 either 0 or 2. If the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the version MUST be 0. If the RecipientIdentifier is subjectKeyIdentifier, 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 content-encryption key, implementations MUST adjust the parity bits for each DES key comprising the Triple-DES key prior to RSAES-OAEP encryption. 2.3 KeyAgreeRecipientInfo Fields This section describes the conventions for using ES-DH and AES with the CMS enveloped-data content type to support key agreement. When key agreement is used, then the RecipientInfo keyAgreeRecipientInfo CHOICE MUST be used. The KeyAgreeRecipient version MUST be 3. The EnvelopedData originatorInfo field must be the originatorKey alternative. The originatoryKey algorithm fields MUST contain the dh-public-number object identifier with absent parameters. The originatorKey publicKey MUST contain the senderÆs ephemeral public key. The EnvelopedData ukm MAY be absent. Schaad, Housley 4 Use of the AES Algorithm in CMS November 2000 The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH algorithm identifier. 2.3.1 ES-DH/AES Key Derivation Generation of the an AES key used in doing AES-KeyWrap is done using the method in [DH] with the following modifications: The Hash function H will be [SHA-256] rather than SHA-1. NOTE: 2 examples to be provided at this location. 2.3.2 AES CEK Wrap Process To be supplied. 2.4 KEKRecipientInfo Fields This section describes the conventions for using AES with the CMS enveloped-data content type to support "previously distributed" symmetric KEKs. When a "previously distributed" symmetric KEK is used to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE MUST be used. The methods used to generate and distribute the symmetric KEK are beyond the scope of this document. The KEKRecipientInfo fields MUST be populated as specified in [CMS] Section 6.2.3, "KEKRecipientInfo Type". The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be the id-NIST-AES-KEY- WRAP OID indicating that the AES wrap function is used to wrap the AES CEK. The KEKRecipientInfo keyEncryptionAlgorithm parameters field MUST be absent. The KEKRecipientInfo encryptedKey field MUST include the AES CEK wrapped using the "previously distributed" symmetric KEK as input to the AES wrap function. To Be Decided û Do we have multiple sizes of key wrap algorithms. 3. Encrypted-data Conventions To be supplied. 4. Authenticated-data Conventions To be supplied. 5. Algorithm Identifiers and Parameters 5.1 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 ::= Schaad, Housley 5 Use of the AES Algorithm in CMS November 2000 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: 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, OCTET STRING SIZE (0) } id-sha1 OBJECT IDENTIFIER ::= iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 } id-mgf1 OBJECT IDENTIFIER ::= iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 8 } id-pSpecified OBJECT IDENTIFIER ::= iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(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 Schaad, Housley 6 Use of the AES Algorithm in CMS November 2000 MUST omit the maskGenFunc field when MFG1 with 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. 5.2 AES Algorithm Identifiers and Parameters AES is added to the set of symmetric content encryption algorithms in CMS. The AES content-encryption algorithm in Cipher Block Chaining (CBC) mode for the three different key sizes are identified by the OID: id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 } id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 } The AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain a AES-IV associated with this OID contains the initialization vector IV: AES-IV ::= OCTET STRING (SIZE(16)) 6 SMIMECapabilities Attribute Conventions An S/MIME client SHOULD announce the set of cryptographic functions it supports by using the S/MIME capabilities attribute. This attribute provides a partial list of OIDs of cryptographic functions and MUST be signed by the client. The algorithm OIDs SHOULD be logically separated in functional categories and MUST be ordered with respect to their preference. RFC 2633, 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. 6.1 RSAES-OEAP SMIMECapability Attribute Schaad, Housley 7 Use of the AES Algorithm in CMS November 2000 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: 30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00 6.2 AES S/MIME Capability Attributes If an S/MIME client is required to support symmetric encryption with AES, the capabilities attribute MUST contain the AES OID specified above in the category of symmetric algorithms. The parameter associated with this OID MUST is AESSMimeCapability. AESSMimeCapabilty ::= NULL The encodings for the mandatory key sizes are: Key Size Capability 128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00 196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00 256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00 When a sending agent creates an encrypted message, it has to decide which type of encryption algorithm to use. In general the decision process involves information obtained from the capabilities lists included in messages received from the recipient, as well as other information such as private agreements, user preferences, legal restrictions, and so on. If users require AES for symmetric encryption, the S/MIME clients on both the sending and receiving side MUST support it, and it MUST be set in the user preferences. 7. Security Considerations Note on mix of OEAP and v1.5 RSA encryption from RFC 2437 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. Schaad, Housley 8 Use of the AES Algorithm in CMS November 2000 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. 8. Open Issues - Key wrap algorithm is undetermined. - Mandatory key sizes for Key Wrap - Mandatory key sizes for AES algorithm - Supplying any patent and licensing statements. - References to each algorithm that would be acceptable to the RFC editor. - Does the oid for key derivation need to be changed since we are using SHA-256 not SHA-1? References CMS Housley, R. Cryptographic Message Syntax. RFC 2630. June 1999. 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. DH E. Rescorla, ôDiffie-Hellman Key Agreement Methodö, RFC 2631, June 1999. MUSTSHOULD Bradner, S. Key Words for Use in RFCs to Indicate Requirement Levels. BCP 14, RFC 2119. March 1997. PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5. RFC 2313. March 1998. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. RFC 2437. October 1998. PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet Schaad, Housley 9 Use of the AES Algorithm in CMS November 2000 X.509 Public Key Infrastructure: Certificate and CRL Profile. RFC 2459. January 1999. 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. [Available at http://www.rsasecurity.com/rsalabs/bulletins] SHA1 National Institute of Standards and Technology. FIPS Pub 180-1: Secure Hash Standard. 17 April 1995. SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol, Version 3.0. Netscape Communications. November 1996. [Available at http://draft-freier-ssl-version3-02.txt] TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0. RFC 2246. January 1999. 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. Acknowledgements This document is the result of contributions from many professionals. We appreciate the hard work of all members of the IETF S/MIME Working Group. We wish to extend a special thanks to Burt Kaliski. Author's Addresses Jim Schaad Soaring Hawk Consulting Email: jimsch@exmsft.com Russell Housley RSA Laboratories 918 Spring Knoll Drive Herndon, VA 20170 USA Schaad, Housley 10 Use of the AES Algorithm in CMS November 2000 rhousley@rsasecurity.com 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 Schaad, Housley 11
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