path: root/doc/rfc/rfc4757.txt

Network Working Group                                      K. Jaganathan
Request for Comments: 4757                                        L. Zhu
Category: Informational                                        J. Brezak
                                                   Microsoft Corporation
                                                           December 2006


    The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

IESG Note

   This document documents the RC4 Kerberos encryption types first
   introduced in Microsoft Windows 2000.  Since then, these encryption
   types have been implemented in a number of Kerberos implementations.
   The IETF Kerberos community supports publishing this specification as
   an informational document in order to describe this widely
   implemented technology.  However, while these encryption types
   provide the operations necessary to implement the base Kerberos
   specification [RFC4120], they do not provide all the required
   operations in the Kerberos cryptography framework [RFC3961].  As a
   result, it is not generally possible to implement potential
   extensions to Kerberos using these encryption types.  The Kerberos
   encryption type negotiation mechanism [RFC4537] provides one approach
   for using such extensions even when a Kerberos infrastructure uses
   long-term RC4 keys.  Because this specification does not implement
   operations required by RFC 3961 and because of security concerns with
   the use of RC4 and MD4 discussed in Section 8, this specification is
   not appropriate for publication on the standards track.













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Abstract

   The Microsoft 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 cryptography (128-bit key
   lengths), and provide exportable (meet United States government
   export restriction requirements) encryption.  This document describes
   the implementation of those encryption types.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Key Generation ..................................................3
   3. Basic Operations ................................................4
   4. Checksum Types ..................................................5
   5. Encryption Types ................................................6
   6. Key Strength Negotiation ........................................8
   7. GSS-API Kerberos V5 Mechanism Type ..............................8
      7.1. Mechanism Specific Changes .................................8
      7.2. GSS-API MIC Semantics ......................................9
      7.3. GSS-API WRAP Semantics ....................................11
   8. Security Considerations ........................................15
   9. IANA Considerations ............................................15
   10. Acknowledgements ..............................................15
   11. References ....................................................16
      11.1. Normative References .....................................16
      11.2. Informative References ...................................16



















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1.  Introduction

   The Microsoft Windows 2000 implementation of Kerberos contains new
   encryption and checksum types for two reasons.  First, for export
   reasons early in the development process, 56-bit DES encryption could
   not be exported, and, second, 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 was
   not available for export when Windows 2000 was released, 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 Microsoft Windows 2000.

   Note that these cryptosystems aren't intended to be complete,
   general-purpose Kerberos encryption or checksum systems as defined in
   [RFC3961]: there is no one-one mapping between the operations in this
   documents and the primitives described in [RFC3961].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
   be interpreted as described in [RFC2119].

2.  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 [RFC4120] and [RFC3961].  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 follows:

      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 an MD4 [RFC1320] hash
   operation is performed on just the UNICODE characters of the password
   (not including the terminating zero octets).



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   For an account with a password of "foo", this String2Key("foo") will
   return:

           0xac, 0x8e, 0x65, 0x7f, 0x83, 0xdf, 0x82, 0xbe,
           0xea, 0x5d, 0x43, 0xbd, 0xaf, 0x78, 0x00, 0xcc

3.  Basic Operations

   The MD5 HMAC function is defined in [RFC2104].  It is used in this
   encryption type for checksum operations.  Refer to [RFC2104] 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 [RFC1321].  In this document, this function is referred to
   as MD5(Data) returning the checksum of the data.

   RC4 is a stream cipher licensed by RSA Data Security.  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.  With each message, the
   message type (T) is used as a component of the keying material.  The
   following table summarizes the different key derivation values used
   in the various operations.  Note that these differ from the key
   derivations used in other Kerberos encryption types.  T = the message
   type, encoded as a little-endian four-byte integer.

      1.  AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
          client key (T=1)
      2.  AS-REP Ticket and TGS-REP Ticket (includes TGS session key or
          application session key), encrypted with the service key (T=2)
      3.  AS-REP encrypted part (includes TGS session key or application
          session key), encrypted with the client key (T=8)
      4.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
          session key (T=4)
      5.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
          authenticator subkey (T=5)
      6.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed
          with the TGS session key (T=6)
      7.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes TGS
          authenticator subkey), encrypted with the TGS session key T=7)
      8.  TGS-REP encrypted part (includes application session key),
          encrypted with the TGS session key (T=8)
      9.  TGS-REP encrypted part (includes application session key),
          encrypted with the TGS authenticator subkey (T=8)




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      10. AP-REQ Authenticator cksum, keyed with the application session
          key (T=10)
      11. AP-REQ Authenticator (includes application authenticator
          subkey), encrypted with the application session key (T=11)
      12. AP-REP encrypted part (includes application session subkey),
          encrypted with the application session key (T=12)
      13. KRB-PRIV encrypted part, encrypted with a key chosen by the
          application.  Also for data encrypted with GSS Wrap (T=13)
      14. KRB-CRED encrypted part, encrypted with a key chosen by the
          application (T=14)
      15. KRB-SAFE cksum, keyed with a key chosen by the application.
          Also for data signed in GSS MIC (T=15)

      Relative to RFC-1964 key uses:

      T = 0 in the generation of sequence number for the MIC token
      T = 0 in the generation of sequence number for the WRAP token
      T = 0 in the generation of encrypted data for the WRAPPED token

   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.  The nonce(n) function returns a pseudo-random number of
   "n" octets.

4.  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 = the Key
      T = the message type, encoded as a little-endian four-byte integer

      CHKSUM(K, T, data)

           Ksign = HMAC(K, "signaturekey")  //includes zero octet at end
           tmp = MD5(concat(T, data))
           CHKSUM = HMAC(Ksign, tmp)








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5.  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 follows:

     T = the message type, encoded as a little-endian four-byte integer.

           OCTET L40[14] = "fortybits";

      The header field on the encrypted data in KDC messages is:

           typedef struct _RC4_MDx_HEADER {
               OCTET Checksum[16];
               OCTET Confounder[8];
           } RC4_MDx_HEADER, *PRC4_MDx_HEADER;


           ENCRYPT (K, export, T, data)
           {
               struct EDATA {
                   struct HEADER {
                           OCTET Checksum[16];
                           OCTET Confounder[8];
                   } Header;
                   OCTET Data[0];
               } edata;

               if (export){
                   *((DWORD *)(L40+10)) = T;
                   K1 = HMAC(K, L40); // where the length of L40 in
                                      // octets is 14
               }
               else
               {
                   K1 = HMAC(K, &T); // where the length of T in octets
                                     // is 4
               }
               memcpy (K2, K1, 16);
               if (export) memset (K1+7, 0xAB, 9);

               nonce (edata.Confounder, 8);
               memcpy (edata.Data, data);




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               edata.Checksum = HMAC (K2, edata);
               K3 = HMAC (K1, edata.Checksum);

               RC4 (K3, edata.Confounder);
               RC4 (K3, data.Data);
           }

           DECRYPT (K, export, T, edata)
           {
               // edata looks like
               struct EDATA {
                   struct HEADER {
                           OCTET Checksum[16];
                           OCTET Confounder[8];
                   } Header;
                   OCTET Data[0];
               } edata;

               if (export){
                   *((DWORD *)(L40+10)) = T;
                   HMAC (K, L40, 14, K1);
               }
               else
               {
                   HMAC (K, &T, 4, K1);
               }
               memcpy (K2, K1, 16);
               if (export) memset (K1+7, 0xAB, 9);

               K3 = HMAC (K1, edata.Checksum);
               RC4 (K3, edata.Confounder);
               RC4 (K3, edata.Data);


               // verify generated and received checksums
             checksum = HMAC (K2, concat(edata.Confounder, edata.Data));
               if (checksum != edata.Checksum)
                   printf("CHECKSUM ERROR  !!!!!!\n");
           }

   The KDC message is encrypted using the ENCRYPT function not including
   the Checksum in the RC4_MDx_HEADER.

   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.




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   The pseudo-random operation [RFC3961] for both enctypes above is
   defined as follows:

           pseudo-random(K, S) = HMAC-SHA1(K, S)

   where K is the protocol key and S is the input octet string.
   HMAC-SHA1 is defined in [RFC2104] and the output of HMAC-SHA1 is the
   20-octet digest.

6.  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.

7.  GSS-API Kerberos V5 Mechanism Type

7.1.   Mechanism Specific Changes

   The Generic Security Service Application Program Interface (GSS-API)
   per-message tokens also require new checksum and encryption types.
   The GSS-API per-message tokens are adapted to support these new
   encryption types.  See [RFC1964] Section 1.2.2.

   The only support quality of protection is:

         #define GSS_KRB5_INTEG_C_QOP_DEFAULT    0x0

   When using this RC4-based encryption type, the sequence number is
   always sent in big-endian rather than little-endian order.

   The Windows 2000 implementation also defines new GSS-API flags in the
   initial token passed when initializing a security context.  These
   flags are passed in the checksum field of the authenticator.  See
   [RFC1964] Section 1.1.1.

   GSS_C_DCE_STYLE - This flag was added for use with Microsoft's
   implementation of Distributed Computing Environment Remote Procedure
   Call (DCE RPC), which initially expected three legs of
   authentication.  Setting this flag causes an extra AP reply to be
   sent from the client back to the server after receiving the server's



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   AP reply.  In addition, the context negotiation tokens do not have
   GSS-API per-message tokens -- they are raw AP messages that do not
   include object identifiers.

           #define GSS_C_DCE_STYLE                 0x1000

   GSS_C_IDENTIFY_FLAG - This flag allows the client to indicate to the
   server that it should only allow the server application to identify
   the client by name and ID, but not to impersonate the client.

           #define GSS_C_IDENTIFY_FLAG             0x2000

   GSS_C_EXTENDED_ERROR_FLAG - Setting this flag indicates that the
   client wants to be informed of extended error information.  In
   particular, Windows 2000 status codes may be returned in the data
   field of a Kerberos error message.  This allows the client to
   understand a server failure more precisely.  In addition, the server
   may return errors to the client that are normally handled at the
   application layer in the server, in order to let the client try to
   recover.  After receiving an error message, the client may attempt to
   resubmit an AP request.

           #define GSS_C_EXTENDED_ERROR_FLAG       0x4000

   These flags are only used if a client is aware of these conventions
   when using the Security Support Provider Interface (SSPI) on the
   Windows platform; they are not generally used by default.

   When NetBIOS addresses are used in the GSS-API, they are identified
   by the GSS_C_AF_NETBIOS value.  This value is defined as:

           #define GSS_C_AF_NETBIOS                0x14

   NetBios addresses are 16-octet addresses typically composed of 1 to
   15 characters, trailing blank (ASCII char 20) filled, with a 16th
   octet of 0x0.

7.2.   GSS-API MIC Semantics

   The GSS-API checksum type and algorithm are 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 [RFC1964] Section 1.2
   for GSS_GetMIC() and GSS_Wrap(conf_flag=FALSE).








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   The GSS_GetMIC token has the following format:

        Byte no         Name        Description
        0..1           TOK_ID     Identification field.
                                  Tokens emitted by GSS_GetMIC() contain
                                  the hex value 01 01 in this field.
        2..3           SGN_ALG    Integrity algorithm indicator.
                                  11 00 - HMAC
        4..7           Filler     Contains ff ff ff ff
        8..15          SND_SEQ    Sequence number field.
        16..23         SGN_CKSUM  Checksum of "to-be-signed data",
                                  calculated according to algorithm
                                  specified in SGN_ALG field.

   The MIC mechanism used for GSS-MIC-based messages is as follows:

           GetMIC(Kss, direction, export, seq_num, data)
           {
                   struct Token {
                          struct Header {
                                 OCTET TOK_ID[2];
                                 OCTET SGN_ALG[2];
                                 OCTET Filler[4];
                            };
                          OCTET SND_SEQ[8];
                          OCTET SGN_CKSUM[8];
                   } Token;


                   Token.TOK_ID = 01 01;
                   Token.SGN_SLG = 11 00;
                   Token.Filler = ff ff ff ff;

                   // Create the sequence number

                   if (direction == sender_is_initiator)
                   {
                           memset(Token.SEND_SEQ+4, 0xff, 4)
                   }
                   else if (direction == sender_is_acceptor)
                   {
                           memset(Token.SEND_SEQ+4, 0, 4)
                   }
                   Token.SEND_SEQ[0] = (seq_num & 0xff000000) >> 24;
                   Token.SEND_SEQ[1] = (seq_num & 0x00ff0000) >> 16;
                   Token.SEND_SEQ[2] = (seq_num & 0x0000ff00) >> 8;
                   Token.SEND_SEQ[3] = (seq_num & 0x000000ff);




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                   // Derive signing key from session key

                   Ksign = HMAC(Kss, "signaturekey");
                                     // length includes terminating null

                   // Generate checksum of message - SGN_CKSUM
                   //   Key derivation salt = 15

                   Sgn_Cksum = MD5((int32)15, Token.Header, data);

                   // Save first 8 octets of HMAC Sgn_Cksum

                   Sgn_Cksum = HMAC(Ksign, Sgn_Cksum);
                   memcpy(Token.SGN_CKSUM, Sgn_Cksum, 8);

                   // Encrypt the sequence number

                   // Derive encryption key for the sequence number
                   //   Key derivation salt = 0

                   if (exportable)
                   {
                           Kseq = HMAC(Kss, "fortybits", (int32)0);
                                        // len includes terminating null
                           memset(Kseq+7, 0xab, 7)
                   }
                   else
                   {
                            Kseq = HMAC(Kss, (int32)0);
                   }
                   Kseq = HMAC(Kseq, Token.SGN_CKSUM);

                   // Encrypt the sequence number

                   RC4(Kseq, Token.SND_SEQ);
           }

7.3.   GSS-API WRAP Semantics

   There are two encryption keys for GSS-API message tokens, one that is
   128 bits in strength and one that is 56 bits in strength as defined
   in Section 6.

   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 [RFC1964] Section 1.2.2.3.





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   The RC4-HMAC GSS_Wrap() token has the following format:


      Byte no          Name         Description
        0..1           TOK_ID       Identification field.
                                    Tokens emitted by GSS_Wrap() contain
                                    the hex value 02 01 in this field.
        2..3           SGN_ALG      Checksum algorithm indicator.
                                    11 00 - HMAC
        4..5           SEAL_ALG     ff ff - none
                                    00 00 - DES-CBC
                                    10 00 - RC4
        6..7           Filler       Contains ff ff
        8..15          SND_SEQ      Encrypted sequence number field.
        16..23         SGN_CKSUM    Checksum of plaintext padded data,
                                    calculated according to algorithm
                                    specified in SGN_ALG field.
        24..31         Confounder   Random confounder.
        32..last       Data         Encrypted or plaintext padded data.

   The encryption mechanism used for GSS-wrap-based messages is as
   follows:


           WRAP(Kss, encrypt, direction, export, seq_num, data)
           {
                   struct Token {          // 32 octets
                          struct Header {
                                 OCTET TOK_ID[2];
                                 OCTET SGN_ALG[2];
                                 OCTET SEAL_ALG[2];
                                 OCTET Filler[2];
                          };
                          OCTET SND_SEQ[8];
                          OCTET SGN_CKSUM[8];
                            OCTET Confounder[8];
                   } Token;


                   Token.TOK_ID = 02 01;
                   Token.SGN_SLG = 11 00;
                   Token.SEAL_ALG = (no_encrypt)? ff ff : 10 00;
                   Token.Filler = ff ff;

                   // Create the sequence number

                   if (direction == sender_is_initiator)
                   {



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                           memset(&Token.SEND_SEQ[4], 0xff, 4)
                   }
                   else if (direction == sender_is_acceptor)
                   {
                           memset(&Token.SEND_SEQ[4], 0, 4)
                   }
                   Token.SEND_SEQ[0] = (seq_num & 0xff000000) >> 24;
                   Token.SEND_SEQ[1] = (seq_num & 0x00ff0000) >> 16;
                   Token.SEND_SEQ[2] = (seq_num & 0x0000ff00) >> 8;
                   Token.SEND_SEQ[3] = (seq_num & 0x000000ff);

                   // Generate random confounder

                   nonce(&Token.Confounder, 8);

                   // Derive signing key from session key

                   Ksign = HMAC(Kss, "signaturekey");

                   // Generate checksum of message -
                   //  SGN_CKSUM + Token.Confounder
                   //   Key derivation salt = 15

                   Sgn_Cksum = MD5((int32)15, Token.Header,
                                   Token.Confounder);

                   // Derive encryption key for data
                   //   Key derivation salt = 0

                   for (i = 0; i < 16; i++) Klocal[i] = Kss[i] ^ 0xF0;
                                                            // XOR
                   if (exportable)
                   {
                           Kcrypt = HMAC(Klocal, "fortybits", (int32)0);
                                       // len includes terminating null
                           memset(Kcrypt+7, 0xab, 7);
                   }
                   else
                   {
                           Kcrypt = HMAC(Klocal, (int32)0);
                     }

                   // new encryption key salted with seq

                   Kcrypt = HMAC(Kcrypt, (int32)seq);






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                   // Encrypt confounder (if encrypting)

                   if (encrypt)
                           RC4(Kcrypt, Token.Confounder);

                   // Sum the data buffer

                   Sgn_Cksum += MD5(data);         // Append to checksum

                   // Encrypt the data (if encrypting)

                   if (encrypt)
                           RC4(Kcrypt, data);

                   // Save first 8 octets of HMAC Sgn_Cksum

                   Sgn_Cksum = HMAC(Ksign, Sgn_Cksum);
                   memcpy(Token.SGN_CKSUM, Sgn_Cksum, 8);

                   // Derive encryption key for the sequence number
                   //   Key derivation salt = 0

                   if (exportable)
                   {
                           Kseq = HMAC(Kss, "fortybits", (int32)0);
                                       // len includes terminating null
                           memset(Kseq+7, 0xab, 7)
                   }
                   else
                   {
                           Kseq = HMAC(Kss, (int32)0);
                   }
                   Kseq = HMAC(Kseq, Token.SGN_CKSUM);

                   // Encrypt the sequence number

                   RC4(Kseq, Token.SND_SEQ);

                   // Encrypted message = Token + Data
           }

   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.






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8.  Security Considerations

   Care must be taken in implementing these encryption types because
   they use a stream cipher.  If a different IV is not used in each
   direction when using a session key, the encryption is weak.  By using
   the sequence number as an IV, this is avoided.

   There are two classes of attack on RC4 described in [MIRONOV].
   Strong distinguishers distinguish an RC4 keystream from randomness at
   the start of the stream.  Weak distinguishers can operate on any part
   of the keystream, and the best ones, described in [FMcG] and
   [MANTIN05], can exploit data from multiple, different keystreams.  A
   consequence of these is that encrypting the same data (for instance,
   a password) sufficiently many times in separate RC4 keystreams can be
   sufficient to leak information to an adversary.  The encryption types
   defined in this document defend against these by constructing a new
   keystream for every message.  However, it is RECOMMENDED not to use
   the RC4 encryption types defined in this document for high-volume
   connections.

   Weaknesses in MD4 [BOER91] were demonstrated by den Boer and
   Bosselaers in 1991.  In August 2004, Xiaoyun Wang, et al., reported
   MD4 collisions generated using hand calculation [WANG04].
   Implementations based on Wang's algorithm can find collisions in real
   time.  However, the intended usage of MD4 described in this document
   does not rely on the collision-resistant property of MD4.
   Furthermore, MD4 is always used in the context of a keyed hash in
   this document.  Although no evidence has suggested keyed MD4 hashes
   are vulnerable to collision-based attacks, no study has directly
   proved that the HMAC-MD4 is secure: the existing study simply assumed
   that the hash function used in HMAC is collision proof.  It is thus
   RECOMMENDED not to use the RC4 encryption types defined in this
   document if alternative stronger encryption types, such as
   aes256-cts-hmac-sha1-96 [RFC3962], are available.

9.  IANA Considerations

   Section 5 of this document defines two Kerberos encryption types
   rc4-hmac (23) and rc4-hmac-exp (24).  The Kerberos parameters
   registration page at <http://www.iana.org/assignments/kerberos-
   parameters> has been updated to reference this document for these two
   encryption types.

10.  Acknowledgements

   The authors wish to thank Sam Hartman, Ken Raeburn, and Qunli Li for
   their insightful comments.




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11.  References

11.1.  Normative References

   [RFC1320]  Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320,
              April 1992.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
              Encryption for Kerberos 5", RFC 3962, February 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4537]  Zhu, L., Leach, P., and K. Jaganathan, "Kerberos
              Cryptosystem Negotiation Extension", RFC 4537, June 2006.

11.2.  Informative References

   [BOER91]   den Boer, B. and A. Bosselaers, "An Attack on the Last Two
              Rounds of MD4", Proceedings of the 11th Annual
              International Cryptology Conference on Advances in
              Cryptology, pages: 194 - 203, 1991.

   [FMcG]     Fluhrer, S. and D. McGrew, "Statistical Analysis of the
              Alleged RC4 Keystream Generator", Fast Software
              Encryption:  7th International Workshop, FSE 2000, April
              2000, <http://www.mindspring.com/~dmcgrew/rc4-03.pdf>.







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   [MANTIN05] Mantin, I., "Predicting and Distinguishing Attacks on RC4
              Keystream Generator", Advances in Cryptology -- EUROCRYPT
              2005: 24th Annual International Conference on the Theory
              and Applications of Cryptographic Techniques, May 2005.

   [MIRONOV]  Mironov, I., "(Not So) Random Shuffles of RC4", Advances
              in Cryptology -- CRYPTO 2002: 22nd Annual International
              Cryptology Conference, August 2002,
              <http://eprint.iacr.org/2002/067.pdf>.

   [WANG04]   Wang, X., Lai, X., Feng, D., Chen, H., and X. Yu,
              "Cryptanalysis of Hash functions MD4 and RIPEMD", August
              2004, <http://www.infosec.sdu.edu.cn/paper/md4-ripemd-
              attck.pdf>.

Authors' Addresses

   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: karthikj@microsoft.com


   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: lzhu@microsoft.com


   John Brezak
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: jbrezak@microsoft.com









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