CAT working group M. Swift Internet Draft J. Brezak Document: draft-brezak-win2k-krb-rc4-hmac-01.txt Microsoft Category: Informational October 1999 The Windows 2000 RC4-HMAC Kerberos encryption type 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. 1. Abstract The Windows 2000 implementation of Kerberos introduces a new encryption type based on the RC4 encryption algorithm and using an MD5 HMAC for checksum. This is offered as an alternative to using the existing DES based encryption types. The RC4-HMAC encryption types are used to ease upgrade of existing Windows NT environments, provide strong crypto (128-bit key lengths), and provide exportable (meet United States government export restriction requirements) encryption. The Windows 2000 implementation of Kerberos contains new encryption and checksum types for two reasons: for export reasons early in the development process, 56 bit DES encryption could not be exported, and because upon upgrade from Windows NT 4.0 to Windows 2000, accounts will not have the appropriate DES keying material to do the standard DES encryption. Furthermore, 3DES is not available for export, and there was a desire to use a single flavor of encryption in the product for both US and international products. As a result, there are two new encryption types and one new checksum type introduced in Windows 2000. 2. Conventions used in this document Swift Category - Informational 1 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 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]. 3. Key Generation On upgrade from existing Windows NT domains, the user accounts would not have a DES based key available to enable the use of DES base encryption types specified in RFC 1510. The key used for RC4-HMAC is the same as the existing Windows NT key (NT Password Hash) for compatibility reasons. Once the account password is changed, the DES based keys are created and maintained. Once the DES keys are available DES based encryption types can be used with Kerberos. The RC4-HMAC String to key function is defined as follow: String2Key(password) K = MD4(UNICODE(password)) The RC4-HMAC keys are generated by using the Windows UNICODE version of the password. Each Windows UNICODE character is encoded in little-endian format of 2 octets each. Then performing an MD4 [6] hash operation on just the UNICODE characters of the password (not including the terminating zero octets). 4. Basic Operations The MD5 HMAC function is defined in [3]. It is used in this encryption type for checksum operations. Refer to [3] for details on its operation. In this document this function is referred to as HMAC(Key, Data) returning the checksum using the specified key on the data. The basic MD5 hash operation is used in this encryption type and defined in [7]. In this document this function is referred to as MD5(Data) returning the checksum of the data. The basic RC4 encryption operation is used in this encryption type and defined in [8]. In this document the function is referred to as RC4(Key, Data) returning the encrypted data using the specified key on the data. These encryption types use key derivation as defined in [9] (RFC- 1510BIS) in Section titled "Key Derivation". With each message, the message type (T) is used as a component of the keying material. All strings in this document are ASCII unless otherwise specified. The lengths of ASCII encoded character strings include the trailing terminator character (0). The concat(a,b,c,...) function will return the logical concatenation (left to right) of the values of the arguments. Swift Category - Informational 2 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 The nonce(n) function returns a pseudo-random number of "n" octets. 5. Checksum Types There is one checksum type used in this encryption type. The Kerberos constant for this type is: #define KERB_CHECKSUM_HMAC_MD5 (-138) The function is defined as follows: K - is the Key T - the message type, encoded as a little-endian four byte integer CHKSUM(K, T, data) Ksign = HMAC(K, "signature key") //includes zero octet at end tmp = MD5(concat(T, data)) CHKSUM = HMAC(Ksign, tmp) 6. Encryption Types There are two encryption types used in these encryption types. The Kerberos constants for these types are: #define KERB_ETYPE_RC4_HMAC 23 #define KERB_ETYPE_RC4_HMAC_EXP 24 The basic encryption function is defined as follow: T = the message type, encoded as a little-endian four byte integer. ENCRYPT(K, T, data) if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP) L = concat("fortybits", T) //includes zero octet at //end of string constant Else L = T Ksign = HMAC(K,L) Confounder = nonce(8) // get an 8 octet nonce for a confounder Checksum = HMAC(Ksign, concat(Confounder, data)) Ke = Ksign if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP) memset(&Ke[7], 0x0ab, 9) Ke2 = HMAC(Ke, Checksum) data = RC4(Ke2, data) The header field on the encrypted data in KDC messages is: typedef struct _RC4_MDx_HEADER { UCHAR Checksum[16]; UCHAR Confounder[8]; } RC4_MDx_HEADER, *PRC4_MDx_HEADER; Swift Category - Informational 3 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 The character constant "fortybits" evolved from the time when a 40- bit key length was all that was exportable from the United States. It is now used to recognize that the key length is of "exportable" length. In this description, the key size is actually 56-bits. 7. Key Strength Negotiation A Kerberos client and server can negotiate over key length if they are using mutual authentication. If the client is unable to perform full strength encryption, it may propose a key in the "subkey" field of the authenticator, using a weaker encryption type. The server must then either return the same key or suggest its own key in the subkey field of the AP reply message. The key used to encrypt data is derived from the key returned by the server. If the client is able to perform strong encryption but the server is not, it may propose a subkey in the AP reply without first being sent a subkey in the authenticator. 8. GSSAPI Kerberos V5 Mechanism Type 8.1 Mechanism Specific Changes The GSSAPI per-message tokens also require new checksum and encryption types. The GSS-API per-message tokens must be changed to support these new encryption types (See [5] Section 1.2.2). The sealing algorithm identifier (SEAL_ALG) for an RC4 based encryption is: Byte 4..5 SEAL_ALG 0x10 0x00 - RC4 The signing algorithm identifier (SGN_ALG) for MD5 HMAC is: Byte 2..3 SGN ALG 0x11 0x00 - HMAC The only support quality of protection is: #define GSS_KRB5_INTEG_C_QOP_DEFAULT 0x0 In addition, when using an RC4 based encryption type, the sequence number is sent in big-endian rather than little-endian order. 8.2 GSSAPI Checksum Type The GSSAPI checksum type and algorithm is defined in Section 5. Only the first 8 octets of the checksum are used. The resulting checksum is stored in the SGN_CKSUM field (See [5] Section 1.2) for GSS_GetMIC() and GSS_Wrap(conf_flag=FALSE). 8.3 GSSAPI Encryption Types There are two encryption types for GSSAPI message tokens, one that is 128 bits in strength, and one that is 56 bits in strength as defined in Section 6. Swift Category - Informational 4 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 All padding is rounded up to 1 byte. One byte is needed to say that there is 1 byte of padding. The DES based mechanism type uses 8 byte padding. See [5] Section 1.2.2.3. The encryption mechanism used for GSS based messages is as follow: T = the message type, encoded as a little-endian four byte integer. GSS-ENCRYPT(K, T, data) IV = SND_SEQ K = XOR(K, 0xf0f0f0f0f0f0f0f0f0f0f0f0f0f0f0) if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP) L = concat("fortybits", T) //includes zero octet at end else L = T Ksign = HMAC(K, L) Ke = Ksign if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP) memset(&Ke[7], 0x0ab, 9) Ke2 = HMAC(Ke, IV) Data = RC4(Ke2, data) SND_SEQ = RC4(Ke, seq#) The sequence number (SND_SEQ) and IV are used as defined in [5] Section 1.2.2. The character constant "fortybits" evolved from the time when a 40- bit key length was all that was exportable from the United States. It is now used to recognize that the key length is of "exportable" length. In this description, the key size is actually 56-bits. 8. Security Considerations Care must be taken in implementing this encryption type because it uses a stream cipher. If a different IV isnĘt used in each direction when using a session key, the encryption is weak. By using the sequence number as an IV, this is avoided. 9. References 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 3 Krawczyk, H., Bellare, M., Canetti, R.,"HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997 4 Kohl, J., Neuman, C., "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993 Swift Category - Informational 5 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 5 Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC-1964, June 1996 6 R. Rivest, "The MD4 Message-Digest Algorithm", RFC-1320, April 1992 7 R. Rivest, "The MD5 Message-Digest Algorithm", RFC-1321, April 1992 8 RC4 is a proprietary encryption algorithm available under license from RSA Data Security Inc. For licensing information, contact: RSA Data Security, Inc. 100 Marine Parkway Redwood City, CA 94065-1031 9 Neuman, C., Kohl, J., Ts'o, T., "The Kerberos Network Authentication Service (V5)", draft-ietf-cat-kerberos-revisions- 04.txt, June 25, 1999 10. Author's Addresses Mike Swift Microsoft One Microsoft Way Redmond, Washington Email: mikesw@microsoft.com John Brezak Microsoft One Microsoft Way Redmond, Washington Email: jbrezak@microsoft.com Swift Category - Informational 6 Windows 2000 RC4-HMAC Kerberos E-Type October 1999 11. Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. 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