CAT working group                                              M. Swift 
Internet Draft                                                J. Brezak 
Document: draft-brezak-win2k-krb-rc4-hmac-00.txt              Microsoft 
Category: Informational                                 September, 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 desired 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 
    

  
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   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 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(Key, Data) returning the checksum using the specified key on 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. 
    
   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. 
  
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   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(Ksign, concat(T, data)) 
        CHKSUM = HMAC(K, 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 = "fiftysixbits" //includes zero octet at end 
        Else 
                L = ""  // one octet of zero 
        Ksign = HMAC(K, concat(L, T)) 
        Confounder = nonce(8) // get an 8 octet nonce for a confounder 
        Checksum = HMAC(Ksign, concat(Confounder, data)) 
        Ke = Ksign 
        if (L == "fiftysixbits") 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; 
    

  
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   The character constant "fiftysixbits" evolved from the time when a 
   56-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. 
    
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. 
    


  
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   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: 
    
   GSS-ENCRYPT(K, T, data) 
        IV = SND_SEQ 
        K = XOR(K, 0xf0f0f0f0f0f0f0f0f0f0f0f0f0f0f0) 
        if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP) 
                L = "fortybits" //includes zero octet at end 
        else 
                L = "" // one octet of zero 
        Ksign = HMAC(K, concat(L, T)) 
        Ke = Ksign 
        if (L == "fortybits") 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. 
    
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 
 
   5  Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC-1964, 
      June 1996 
 
 
  
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   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 
    
    



















  
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11. Full Copyright Statement 
 
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