S/MIME Working Group R. Housley Internet-Draft Vigil Security Updates: 3852 (if approved) April 2007 Cryptographic Message Syntax (CMS) Authenticated-Enveloped-Data Content Type <draft-ietf-smime-cms-auth-enveloped-03.txt> Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 Abstract This document describes an additional content type for the Cryptographic Message Syntax (CMS). The authenticated-enveloped-data content type is intended for use with authenticated encryption modes. All of the various key management techniques that are supported in the CMS enveloped-data content type are also supported by the CMS authenticated-enveloped-data content type. Housley [Page 1]
INTERNET-DRAFT authenticated-enveloped-data April 2007 1. Introduction This document describes an additional content type for the Cryptographic Message Syntax (CMS) [CMS]. The authenticated- enveloped-data content type is intended for use with authenticated encryption modes, where an arbitrary content is both authenticated and encrypted. Also, some associated data, in the form of authenticated attributes can also be authenticated. All of the various key management techniques that are supported in the CMS enveloped-data content type are also supported by the CMS authenticated-enveloped-data content type. The conventions for using the AES-CCM and the AES-GCM authenticated encryption algorithms with the CMS authenticated-enveloped-data content type defined in this document can be found in [AEALGS]. The authenticated-enveloped-data content type, like all of the other CMS content types, employs ASN.1 [X.208-88], and it uses both the Basic Encoding Rules [X.209-88] and the Distinguished Encoding Rules (DER) [X.509-88]. 1.1 Terminology In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as described in [STDWORDS]. 1.2 Version Numbers The major data structure (AuthEnvelopedData) includes a version number as the first item in the data structure. The version number is intended to avoid ASN.1 decode errors. Some implementations do not check the version number prior to attempting a decode, and then if a decode error occurs, the version number is checked as part of the error handling routine. This is a reasonable approach; it places error processing outside of the fast path. This approach is also forgiving when an incorrect version number is used by the sender. Whenever the structure is updated, a higher version number will be assigned. However, to ensure maximum interoperability the higher version number is only used when the new syntax feature is employed. That is, the lowest version number that supports the generated syntax is used. Housley [Page 2]
INTERNET-DRAFT authenticated-enveloped-data April 2007 2. Authenticated-enveloped-data Content Type The authenticated-enveloped-data content type consists of an authenticated and encrypted content of any type and encrypted content-authenticated-encryption keys for one or more recipients. The combination of the authenticated and encrypted content and one encrypted content-authenticated-encryption key for a recipient is a "digital envelope" for that recipient. Any type of content can be enveloped for an arbitrary number of recipients using any of the supported key management techniques for each recipient. In addition, authenticated but not encrypted attributes may be provided by the originator. The typical application of the authenticated-enveloped-data content type will represent one or more recipients' digital envelopes on an encapsulated content. Authenticated-enveloped-data is constructed by the following steps: 1. A content-authenticated-encryption key for a particular content-authenticated-encryption algorithm is generated at random. 2. The content-authenticated-encryption key is encrypted for each recipient. The details of this encryption depend on the key management algorithm used, but four general techniques are supported: key transport: the content-authenticated-encryption key is encrypted in the recipient's public key; key agreement: the recipient's public key and the sender's private key are used to generate a pairwise symmetric key- encryption key, then the content-authenticated-encryption key is encrypted in the pairwise symmetric key-encryption key; symmetric key-encryption keys: the content-authenticated- encryption key is encrypted in a previously distributed symmetric key-encryption key; and passwords: the content-authenticated-encryption key is encrypted in a key-encryption key that is derived from a password or other shared secret value. 3. For each recipient, the encrypted content-authenticated- encryption key and other recipient-specific information are collected into a RecipientInfo value, defined in Section 6.2 of [CMS]. Housley [Page 3]
INTERNET-DRAFT authenticated-enveloped-data April 2007 4. Any attributes that are to be authenticated but not encrypted are collected in the authenticated attributes. 5. The attributes collected in step 4 are authenticated and the content is authenticated and encrypted with the content- authenticated-encryption key. If the authenticated encryption algorithm requires either the additional authenticated data (AAD) or the content to be padded to a multiple of some block size, then the padding is added as described in Section 6.3 of [CMS]. 6. Any attributes that are to be provided without authentication or encryption are collected in the unauthenticated attributes. 7. The RecipientInfo values for all the recipients, the authenticated attributes, then unauthenticated attributes, and the authenticated and encrypted content are collected together to form an AuthEnvelopedData value as defined in Section 2.1. A recipient opens the digital envelope by decrypting one of the encrypted content-authenticated-encryption keys, and then using the recovered key to decrypt and verify the integrity of the authenticated and encrypted content as well as verifying the integrity of the authenticated attributes. The recipient MUST verify the integrity of the received content before releasing any information, especially the plaintext of the content. If the integrity verification fails, the receiver MUST destroy all of the plaintext of the content. This section is divided into three parts. The first part describes the AuthEnvelopedData content type, the second part describes the authentication and encryption process, and third part describes the key encryption process. Housley [Page 4]
INTERNET-DRAFT authenticated-enveloped-data April 2007 2.1 AuthEnvelopedData Type The following object identifier identifies the authenticated- enveloped-data content type: id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 23 } The authenticated-enveloped-data content type MUST have ASN.1 type AuthEnvelopedData: AuthEnvelopedData ::= SEQUENCE { version CMSVersion, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, authAttrs [1] IMPLICIT AuthAttributes OPTIONAL, authEncryptedContentInfo EncryptedContentInfo, mac MessageAuthenticationCode, unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL } The fields of type AuthEnvelopedData have the following meanings: version is the syntax version number. It MUST be set to 0. originatorInfo optionally provides information about the originator. It is present only if required by the key management algorithm. It may contain certificates and CRLs, and the OriginatorInfo type is defined in Section 6.1 of [CMS]. recipientInfos is a collection of per-recipient information. There MUST be at least one element in the collection. The RecipientInfo type is defined in Section 6.2 of [CMS]. authAttrs optionally contains the authenticated attributes. The CMS authenticated-data content type uses the same type to carry authenticated attributes. The authAttrs MUST be present if the content type carried in EncryptedContentInfo is not id-data. AuthAttributes MUST be DER encoded, even if the rest of the AuthEnvelopedData structure is BER encoded. The AuthAttributes type is defined in Section 9.1 of [CMS]; however, in this case, the message-digest attribute SHOULD NOT be included. Useful attribute types are defined in Section 11 of [CMS]. Note: Similar to AuthenticatedData, the DER encoded AuthAttributes are carried in the AuthEnvelopedData structure and used in the computation of the MAC. This is different than SignedData, where slightly different encodings of the signed Housley [Page 5]
INTERNET-DRAFT authenticated-enveloped-data April 2007 attributes are used in the SigendData structure and the computation of the digest value. authEncryptedContentInfo is the authenticated and encrypted content. The CMS enveloped-data content type uses the same type to carry the encrypted content. The EncryptedContentInfo type is defined in Section 6.1 of [CMS]. mac is the integrity check value (ICV) or message authentication code (MAC) that is generated by the authenticated encryption algorithm. The CMS authenticated-data content type uses the same type to carry a MAC. In this case, the MAC covers the authenticated attributes and the content directly, and a digest algorithm is not used. The MessageAuthenticationCode type is defined in Section 9.1 of [CMS]. unauthAttrs optionally contains the unauthenticated attributes. The CMS authenticated-data content type uses the same type to carry unauthenticated attributes. The UnauthAttributes type is defined in Section 9.1 of [CMS]. Useful attribute types are defined in Section 11 of [CMS]. 2.2. Authentication and Encryption Process The content-authenticated-encryption key for the desired content- authenticated-encryption algorithm is randomly generated. If the authenticated encryption algorithm requires the content to be padded to a multiple of some block size, then the padding MUST be added as described in Section 6.3 of [CMS]. This padding method is well defined if and only if the number of octets in the block size is less than 256. If optional authenticated attributes are present, then they are DER encoded. The result will be used as the authenticated associated data (AAD) input to the authenticated encryption algorithm. If the authenticated encryption algorithm requires the AAD to be padded to a multiple of some block size, then the padding MUST be added as described in Section 6.3 of [CMS]. This padding method is well defined if and only if number of octets in the block size is less than 256. The inputs to the authenticated encryption algorithm are the content (the data, which is padded if necessary), the DER-encoded authenticated attributes (the AAD, which is padded if necessary), and the content-authenticated-encryption key. Under control of a content-authenticated-encryption key, the authenticated encryption operation maps an arbitrary string of octets (the data) to another Housley [Page 6]
INTERNET-DRAFT authenticated-enveloped-data April 2007 string of octets (the ciphertext) and it computes an authentication tag over the AAD and the data. The encrypted data is included in the AuthEnvelopedData authEncryptedContentInfo encryptedContent as an OCTET STRING, and the authentication tag is included in the AuthEnvelopedData mac. 2.3. Key Encryption Process The input to the key encryption process -- the value supplied to the recipient's key-encryption algorithm -- is just the "value" of the content-authenticated-encryption key. Any of the aforementioned key management techniques can be used for each recipient of the same encrypted content. 3. Security Considerations This specification defines an additional CMS content type. The security considerations provided in [CMS] apply to this content type as well. Many authenticated encryption algorithms make use of a block cipher in counter mode to provide encryption. When used properly, counter mode provides strong confidentiality. Bellare, Desai, Jokipii, Rogaway show in [BDJR] that the privacy guarantees provided by counter mode are at least as strong as those for CBC mode when using the same block cipher. Unfortunately, it is easy to misuse counter mode. If counter block values are ever used for more that one encryption operation with the same key, then the same key stream will be used to encrypt both plaintexts, and the confidentiality guarantees are voided. Fortunately, the CMS authenticated-enveloped-data content type provides all of the tools needed to avoid misuse of counter mode. All of the existing key management techniques permit a fresh content- encryption key to be generated for each content. In addition, existing authenticated encryption algorithms that make use of counter mode support the use of an unpredictable nonce value in the counter block. This unpredictable nonce value (sometimes called a "salt") should be carried in an algorithm identifier parameter. There are fairly generic precomputation attacks against all block cipher modes that allow a meet-in-the-middle attack against the key. These attacks require the creation and searching of huge tables of ciphertext associated with known plaintext and known keys. Assuming that the memory and processor resources are available for a precomputation attack, then the theoretical strength of any block Housley [Page 7]
INTERNET-DRAFT authenticated-enveloped-data April 2007 cipher mode is limited to 2^(n/2) bits, where n is the number of bits in the key. The use of long keys is the best countermeasure to precomputation attacks. Use of an unpredictable nonce value in the counter block significantly increases the size of the table that the attacker must compute to mount a successful precomputation attack. Implementations must randomly generate content-authenticated- encryption keys, padding, and unpredictable nonce values. Also, the generation of public/private key pairs relies on a random numbers. The use of inadequate pseudo-random number generators (PRNGs) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities, rather than brute force searching the whole key space. The generation of quality random numbers is difficult. RFC 4086 [RANDOM] offers important guidance in this area. If the message-digest attribute is included in the AuthAttributes, then attribute value will contain the unencrypted one-way hash value of the plaintext of the content. Disclosure of this hash value enables content tracking, and it can be used to determine if the plaintext matches one or more candidate contents. For these reasons, the AuthAttributes SHOULD NOT contain the message-digest attribute. 4. IANA Considerations None. {{{ RFC Editor: Please remove this section prior to publication. }}} Housley [Page 8]
INTERNET-DRAFT authenticated-enveloped-data April 2007 5. ASN.1 Module CMS-AuthEnvelopedData-2007 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-authEnvelopedData(31) } 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 -- Imports from RFC 3852 [CMS], Section 12.1 AuthAttributes, CMSVersion, EncryptedContentInfo, MessageAuthenticationCode, OriginatorInfo, RecipientInfos, UnauthAttributes FROM CryptographicMessageSyntax2004 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) } ; id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 23 } AuthEnvelopedData ::= SEQUENCE { version CMSVersion, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, authAttrs [1] IMPLICIT AuthAttributes OPTIONAL, authEncryptedContentInfo EncryptedContentInfo, mac MessageAuthenticationCode, unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL } END -- of CMS-AuthEnvelopedData-2007 Housley [Page 9]
INTERNET-DRAFT authenticated-enveloped-data April 2007 6. References 6.1. Normative References [CMS] Housley, R., "Cryptographic Message Syntax", RFC 3852, July 2004. [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. 6.2. Informative References [AEALGS] Housley, R., "Using AES-CCM and AES-GCM Authenticated Encryption in the Cryptographic Message Syntax (CMS)", work in progress. draft-ietf-smime-cms-aes-ccm-and-gcm. [BDJR] Bellare, M., Desai, A., Jokipii, E., and P. Rogaway, "A Concrete Security Treatment of Symmetric Encryption: Analysis of the DES Modes of Operation", Proceedings 38th Annual Symposium on Foundations of Computer Science, 1997. [RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Recommendations for Security", RFC 4086, June 2005. 7. Authors' Address Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley@vigilsec.com Housley [Page 10]
INTERNET-DRAFT authenticated-enveloped-data April 2007 Copyright and IPR Statements Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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. 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 must 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 are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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 11]
