S/MIME Working Group R. Housley Internet-Draft Vigil Security Updates: 3852 (if approved) January 2007 The CMS AuthEnvelopedData Content Type <draft-ietf-smime-cms-auth-enveloped-00.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 a "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 AuthEnvelopedData content type is intended for use with authenticated encryption modes. All of the various key management techniques that are supported in the EnvelopedData content type are also supported by the AuthEnvelopedData content type. Housley [Page 1]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 1. Introduction This document describes an additional content type for the Cryptographic Message Syntax (CMS) [CMS]. The AuthEnvelopedData 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 EnvelopedData content type are also supported by the AuthEnvelopedData content type. The AuthEnvelopedData 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 if a decode error occurs, then 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. 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-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 Housley [Page 2]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 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-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, then the content-authenticated-encryption key is encrypted in the pairwise symmetric 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]. 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 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]. Housley [Page 3]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 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. 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. 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]. Housley [Page 4]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 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 AuthenticatedData content type uses the same type to carry authenticated attributes. The AuthAttributes type is defined in Section 9.1 of [CMS]. Useful attribute types are defined in Section 11 of [CMS]. authEncryptedContentInfo is the authenticated and encrypted content. The EnvelopedData 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 AuthenticatedData 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 AuthenticatedData 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 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. The inputs to the authenticated encryption algorithm are the content (the data, which is padded if necessary), the DER-encoded authenticated attributes (the AAD), and the content-authenticated- encryption key. Under control of a content-authenticated-encryption Housley [Page 5]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 key, the authenticated encryption operation maps an arbitrary string of octets (the data) to another 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 encryptedContentInfo encryptedContent 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 and 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 AuthEnvelopedData provides all of the tools needed to avoid misuse of counter mode. When using key transport or key agreement, a fresh key should be generated for each content. However, when using symmetric key-encryption keys or passwords, one cannot assume that a fresh key is generated. Therefore, authenticated encryption algorithms that make use of counter mode must support the use of an unpredictable nonce value in the counter block, and this unpredictable nonce value (sometimes called a "salt") must be carried as 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 Housley [Page 6]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 that the memory and processor resources are available for a precomputation attack, then the theoretical strength of any block 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. 4 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 } Housley [Page 7]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 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 5. 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. Informative References [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. Housley [Page 8]
INTERNET-DRAFT AuthEnvelopedData Content Type January 2007 7. Authors' Address Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley@vigilsec.com Full Copyright Statement Copyright (C) The Internet Society (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 9]
