Internet Draft Editor: Peter Gutmann draft-ietf-smime-password-00.txt University of Auckland June 15, 1999 Expires December 1999 Password-based Encryption for S/MIME 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/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract The Cryptographic Message Syntax data format doesn't currently contain any provisions for password-based data encryption. This document provides a method of encrypting data using user-supplied passwords (and, by extension, any form of variable-length keying material which isn't necessarily an algorithm-specific fixed-format key). 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 . 1. Introduction This document describes a password-based content encryption mechanism for S/MIME. This is implemented as a new RecipientInfo type and is an extension to the RecipientInfo types currently defined in CMS [CMS]. The format of the messages are described in ASN.1:1994 [ASN1]. The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.1 Password-based Content Encryption CMS currently defined three recipient information types for public-key key wrapping (KeyTransRecipientInfo), conventional key wrapping (KEKRecipientInfo), and key agreement (KeyAgreeRecipientInfo). The recipient information described here adds a fourth type, PasswordRecipientInfo, which provides for password-based key wrapping. 1.2 RecipientInfo Types The new recipient information type is an extension to the RecipientInfo type defined in section 6.2 of CMS, extending the types to: RecipientInfo ::= CHOICE { ktri KeyTransRecipientInfo, kari [1] KeyAgreeRecipientInfo, kekri [2] KEKRecipientInfo, pwri [3] PasswordRecipientinfo -- New RecipientInfo type } Although the recipient information generation process is described in terms of a password-based operation (since this will be its most common use), the transformation employed is a general-purpose key derivation one which allows any type of keying material to be converted into a key specific to a particular content-encryption algorithm. 1.2.1 PasswordRecipientInfo Type Recipient information using a user-supplied password is represented in the type PasswordRecipientInfo. Each instance of PasswordRecipientInfo will transfer the content-encryption key (CEK) to one or more recipients who have the previously agreed-upon password. PasswordRecipientInfo ::= SEQUENCE { version CMSVersion, -- Always set to 0 keyDerivationAlgorithm KeyDerivationAlgorithmIdentifier, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, encryptedKey EncryptedKey } The fields of type PasswordRecipientInfo have the following meanings: version is the syntax version number. It shall always be 0. keyDerivationAlgorithm identifies the key-derivation algorithm, and any associated parameters, used to derive the key-encryption key (KEK) from the user-supplied password. keyEncryptionAlgorithm identifies the content-encryption algorithm, and any associated parameters, used to encrypt the CEK with the password-derived KEK. encryptedKey is the result of encrypting the content-encryption key with the password-derived KEK. 1.2.2 Rationale Password-based key wrapping is a two-stage process, a first stage in which the user-supplied password is converted into a KEK, and a second stage in which the KEK is used to encrypt a CEK. These two stages are identified by the two algorithm identifiers. Although the PKCS #5 standard goes one step further to wrap these up into a single algorithm identifier, this design is particular to that standard and may not be applicable for other password-based key wrapping standards. For this reason the two steps are specified separately. 2 Supported Algorithms This section lists the algorithms that must be implemented. Additional algorithms that should be implemented are also included. 2.1 Key Derivation Algorithms These algorithms are used to convert the password into a KEK. The key derivation algorithms are: KeyDerivationAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= { { SYNTAX PBKDF2-params IDENTIFIED BY id-PBKDF2 }, ... } CMS implementations must include PBKDF2 [PKCS5v2]. 2.2 Key Encryption Algorithms These algorithms are used to encrypt the content (the key) using the derived KEK. The content encryption algorithms are: KeyEncryptionAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= PBES2-Encs CMS implementations must include Triple-DES in CBC mode, should include RC2 in CBC mode, and may include other algorithms such as CAST-128, RC5, IDEA, Skipjack, and encryption modes as required. CMS implementations should not include any KSG ciphers such as RC4 or a block cipher in OFB mode, and should not include a block cipher in ECB mode. The use of RC2 has special requirements, see section 2.4 for details. 2.3 Symmetric Key Encryption Algorithms The key wrap algorithm is used to wrap the CEK with the KEK. There is no requirement that the content-encryption algorithm match the KEK algorithm, although care should be taken to ensure that, if different algorithms are used, they offer an equivalent level of security (for example wrapping a Triple-DES key with an RC2/40 key leads to a severe impedance mismatch in encryption strength). The key wrap algorithm specified below is independent of the content-encryption or wrapping algorithms, relying only on the use of a block cipher to perform the wrapping. 2.3.1 Key Wrap The key wrap algorithm encrypts a CEK with a KEK in a manner which ensures that every bit of plaintext effects every bit of ciphertext. This makes it equivalent in function to the package transform [PACKAGE] without requiring additional mechanisms or resources such as hash functions or cryptographically strong random numbers. The key wrap algorithm is as follows: 1. Pad the key out to a multiple of the KEK cipher block size using random data so that the total data size is at least two KEK cipher blocks long. The padding data does not have to be cryptographically strong, although unpredictability helps. 2. Encrypt the padded key using the KEK. 3. Without resetting the IV (that is, using the last ciphertext block as the IV), encrypt the encrypted padded key a second time. The resulting double-encrypted data is the EncryptedKey. 2.3.2 Key Unwrap 1. Using the n-1'th ciphertext block as the IV, decrypt the n'th ciphertext block. 2. Using the decrypted n'th ciphertext block as the IV, decrypt the 1st ... n-1'th ciphertext blocks. This strips the outer layer of encryption. 3. Decrypt the inner layer of encryption using the KEK. The size of the key in the padded data is determined by the algorithm specified in the ContentEncryptionAlgorithmIdentifier. 2.3.3 Example Given a content-encryption algorithm of Skipjack and a KEK algorithm of Triple-DES, the wrap steps are as follows: 1. Pad the 80-bit (10-byte) Skipjack CEK to 16 bytes (two triple-DES blocks) using 6 bytes of random data. 2. Using the IV given in the KeyEncryptionAlgorithmIdentifer, encrypted the padded Skipjack key. 3. Without resetting the IV, encrypt the encrypted padded key a second time. The unwrap steps are as follows: 1. Using the first 8 bytes of the double-encrypted key as the IV, decrypt the second 8 bytes. 2. Without resetting the IV, decrypt the first 8 bytes. 3. Decrypt the inner layer of encryption using the the IV given in the KeyEncryptionAlgorithmIdentifer to recover the padded Skipjack key. 2.3.4 Rationale for the Double Wrapping If many CEK's are encrypted in a standard way with the same KEK and the KEK has a 64-bit block size then after about 2^32 encryptions there is a high probability of a collision between different blocks of encrypted CEK's. If an opponent manages to obtain a CEK, they may be able to solve for other CEK's. The double-encryption wrapping process, which makes every bit of ciphertext dependent on every bit of the CEK, eliminates this collision problem. Since the IV is applied to the inner layer of encryption, even wrapping the same CEK with the same KEK will result in a completely different wrapped key each time. 2.4 Special Handling for RC2 Keys For a variety of historical, political, and software-peculiarity reasons which are beyond the scope of this document, the handling of keys for the RC2 algorithm [RC2] by different implementations is somewhat arbitrary. In particular, the choice of actual vs effective key bits used in the algorithm is often unclear. The standard RC2 AlgorithmIdentifier only allows the effective key bits to be specified, leaving the actual key bits to be communicated via out-of-band means, which in some cases means hardcoding them into applications. Solving this problem requires two things, a precise definition of how keys represented with the standard RC2 AlgorithmIdentifier are handled, and a new RC2 AlgorithmIdentifier which allows keys currently in use by different applications to be handled. 2.4.1 Handling of RC2 with RFC 2268 AlgorithmIdentifier RFC 2268 defines the following AlgorithmIdentifier for RC2: rc2CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) US(840) rsadsi(113549) encryptionAlgorithm(3) 2} RC2-CBCParameter ::= CHOICE { iv IV, params SEQUENCE { version INTEGER, iv OCTET STRING } } where the version field encodes the effective key size in a complex manner specified in the RFC. Where this algorithm identifier is used, the actual key size shall be 128 bits, and the effective key size is given by the version field. When RC2 is to be used, implementations should use this AlgorithmIdentifier and parameters, and when this AlgorithmIdentifier is used the actual key size must not be a value other than 128 bits (to use a different size, see section 2.4.2). 2.4.2 Handling of RC2 with Other Key Sizes If the use of an actual key size of other than 128 bits is required, implementations must use the following AlgorithmIdentifier: rc2CBC OBJECT IDENTIFIER ::= {1 3 6 1 4 1 3029 666 13} (provisional) RC2-CBCParameter ::= SEQUENCE { actualKeySize INTEGER, -- Actual key size in bits effectiveKeySize INTEGER, -- Effective key size in bits iv OCTET STRING } This allows arbitrary actual and effective key sizes to be specified for compatibility with existing usage. Although implementations should not use this alternative (using instead the one in section 2.4.1) experience has shown that implementors will continue to use oddball RC2 parameters anyway, so new implementations should be prepared to encounter and handle actual and effective key sizes ranging from 40 up to around 200 bits. 2.4.3 Rationale The reason for providing for the handling of oddball key sizes is compatibility with existing applications, for example a mailing-list exploder or mail gateway may take an RSA-wrapped CEK generated by a current application and repackage it with a KEK, so we need a mechanism for handling strange key lengths in a manner which is compatible with existing usage. The alternative RC2 AlgorithmIdentifier, although not recommended, provides a means of ensuring this compatibility. 3. Security Considerations The security of this recipient information type rests on the security of the underlying mechanisms employed, for which further information can be found in CMS and PKCS5v2. Author Address Peter Gutmann University of Auckland Private Bag 92019 Auckland, New Zealand pgut001@cs.auckland.ac.nz References ASN1 Recommendation X.680: Specification of Abstract Syntax Notation One (ASN.1), 1994. CMS Cryptographic Message Syntax, draft-ietf-smime-cms-11.txt, Russ Housley, April 1999. PKCS5v2 PKCS #5 v2.0: Password-Based Cryptography Standard, RSA Laboratories, 25 March 1999. RFC2119 Key Words for Use in RFC's to Indicate Requirement Levels, S.Bradner, March 1997. RFC2268 A Description of the RC2(r) Encryption Algorithm, R.Rivest, March 1998. PACKAGE All-or-Nothing Encryption and the Package Transform, R.Rivest, Proceedings of Fast Software Encryption '97, Haifa, Israel, January 1997. Appendix A: ASN.1 Module PasswordRecipientInfo { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) pwri(n+1) } DEFINITIONS IMPLICIT TAGS ::= BEGIN IMPORTS FROM PKCS5 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-5(5) } PBKDF2-params, PBES2-Encs; PasswordRecipientInfo ::= SEQUENCE { version CMSVersion, -- Always set to 0 keyDerivationAlgorithm KeyDerivationAlgorithmIdentifier, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, encryptedKey EncryptedKey } KeyDerivationAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= { { SYNTAX PBKDF2-params IDENTIFIED BY id-PBKDF2 }, ... } KeyEncryptionAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= PBES2-Encs END Full Copyright Statement Copyright (C) The Internet Society 1999. All Rights Reserved. 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