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Versions: 00                                                            
Security Working Group                               IPsec Working Group
INTERNET DRAFT                                       Naganand Doraswamy, Editor
                                                     November 1996
                                                     Expires in Six months

    Combined 3DES-CBC, HMAC and Replay Prevention Security Transform

Status of this Memo

   This document is a submission to the IETF Internet Protocol Security
   (IPSEC) Working Group. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@tis.com) or to the editor.

   This document is an Internet-Draft.  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 draft documents are 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."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.


   This draft describes a combination of privacy, authentication,
   integrity and replay prevention into a single packet format.

   This document is the result of significant work by several major con-
   tributors and the IPsec working group as a whole. These contributors,
   cited later in this document, provided many of the key technical
   details summarized in this document. [IB93] [IBK93]

Requirements Terminology

   In this document, the words that are used to define the  significance
   of  each particular requirement are usually capitalised.  These words

Naganand                     November 1, 1996                   [Page 1]

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   - MUST

      This word or the adjective "REQUIRED" means that the  item  is  an
      absolute requirement of the specification.


      This word or the adjective "RECOMMENDED" means  that  there  might
      exist  valid  reasons  in  particular circumstances to ignore this
      item, but the full implications should be understood and the  case
      carefully weighed before taking a different course.

   - MAY

      This word or the adjective "OPTIONAL" means that this item is tru-
      ly  optional.  One vendor might choose to include the item because
      a particular marketplace requires it or because  it  enhances  the
      product, for example; another vendor may omit the same item.

   For the purpose of this RFC, the terms conformance and compliance are

1.  Discussion

   This draft allows a combination of MD5 and 3DES-CBC. In addition to
   privacy, the goal of this transform is to ensure that the packet is
   authentic, can not be modified in transit, or replayed.

   The claims of privacy, integrity, authentication, and replay preven-
   tion are made in this draft. A good general text describing the
   methods and algorithm are in [Schneier95].

   Privacy is provided by 3DES-CBC [FIPS-46] [FIPS-46-1] [FIPS-74]

   Integrity is provided by HMAC [Krawczyk96].

   Authentication is provided since only the source and destination know
   the HMAC key. If the HMAC is correct, it proves that it must have
   been added by the source.

   Replay prevention is provided by the combination of a constantly
   increasing count, the SPI and the HMAC key. The integrity of the
   replay field is provided by the HMAC.

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2.  Packet Format

 |                Security Parameters Index (SPI)                | ^
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
 |                                                               | |
 +                Initialization Vector (Optional)               + |
 |                                                               | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |  ---
 |                 Replay Prevention Field (count)               | |   ^
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
 |                                                               | |   |
 ~                      Payload Data                             ~ |   |
 |                                                               |HMAC |
 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |  3DES
 |               |         Padding (0-7 bytes)                   | |  CBC
 +-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
 |                               |  Pad Length   | Payload Type  | v   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---  |
 |                                                               |     |
 ~                        HMAC digest                            ~     |
 |                                                               |     v
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    ---

2.1.  Security Parameters Index

   This field is negotiated at key setup and MUST not be 0 [RFC-1825]

2.2.  Initialization Vector

   The use of an explicit Initialization Vector MAY be negotiated. The
   purpose of this mode is to support devices that automatically gen-
   erate IVs and can not operate using a constant IV_key_.

   This field is optional and is only used when an explicit IV is nego-
   tiated during key exchange.  This field  contains random data or
   contains the last cyphertext block of a previous packet sent or

   For the packet which the explicit IV is received, the explicit IV is
   used in place of the constant IV_key_ described later in this docu-

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2.3.  Replay Prevention

   Replay Prevention is an unsigned 32 bit incrementing counter starting
   at a mutual agreed upon value (see Key Material) and is enforced to
   be within a mutually negotiated window size.

   The key (K, as described in a later section) MUST be changed fre-
   quently enough so that the counter is not allowed to return to the
   initial value; in other words, the key MUST be changed before 2^32
   packets are transmitted using this key. For a given SPI, counter
   wrapping MUST be considered to be a replay attack. (While a wrap is a
   replay attack, there is always the possibility that a packet can get
   duplicated, so the presence of a single or small number of duplicate
   packets is not an absolute indication of a replay attack.)

   The receiver MUST verify that for a given SPI the packets received
   have non-repeating (non-duplicate) counter values. This can be imple-
   mented as a simple increasing count test or the receiver MAY choose
   to accept out-of-order packets as long as it is guaranteed that pack-
   ets can be received only once. For example, an implementation can use
   a sliding receive window. If such a receive window is supported, the
   receiver MUST ensure that it will accept packets within the current
   window only once, and reject any packets it receives with a value
   that is less than the lower bound of the window.

   Negotiated window sizes of 1 and 32 are suggested and larger multi-
   ples of 32 are allowed. 1 indicates that only constantly increasing
   replay numbers are allowed and packets which have replay values less
   than the highest received are always rejected. 32 indicates that are
   within 32 of the highest received, and are guaranteed not to have
   been received before, are allowed.

   A window size of 1 MUST be supported. A value of 32 SHOULD be sup-

   If a value of 32 is negotiated, then the most recent 32 packets are
   allowed to arrive out of order. That is, these 32 packets can arrive
   in any sequence relative to each other except that these packets are
   guaranteed to arrive only once. Appendix A has actual code that
   implement a 32 packet replay window and a test routine. The purpose
   of this routine is to show how it could be implemented.

2.4.  Payload

   The payload contains data that is described by the payload type
   field. This field is an integral number of bytes in length; the fol-
   lowing padding and pad length fields will help provide alignment to a
   four octet boundary.

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2.5.  Padding

   The padding (pad bytes and pad length field) is used to align the
   following "payload type" field to end on a four octet boundary (when
   counting from the start of the replay field).

   Padding bytes SHOULD be initialized with random data.

   At a minimum, the number of pad bytes added MUST be enough to align
   the payload type field on the next appropriate boundary. However, the
   sender MAY choose to include additional padding, provided that the
   alignment is maintained. In total, the sender can add 0-255 bytes of

2.6.  Pad Length

   The pad length field indicates the number of pad bytes immediately
   preceding it. The range of valid values is 0-255, where a value of
   zero indicates that the byte immediately preceding the pad length
   field is the last byte of the payload.

2.7.  Payload Type

   Describes what the payload is. The values are described in:


2.8.  HMAC Digest

   The HMAC digest is a 128 bit residue described in [Krawczyk96]. This
   covers the SPI, replay, payload, padding, pad length, payload type.

   HMAC is a keyed algorithm, where both directions are keyed
   separately.  The implementation MUST use the HMAC_key_ as described
   in the section on keys.

3.  Encryption Transform Procedure

   Triple encryption in Outer-CBC mode with a constant IV_key_ is used
   (IV_key_I for the initiator -> responder direction and IV_key_R for the
   responder -> initiator direction). The IV_key_ remains constant for all
   packets send in this direction.

   If an explicit IV is negotiated, 64 bits of random or the last
   cyphertext block of a previous packet send or receive can be used.

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   IV or
   IV_key_   count|x1             x2                x3
       |         |                 |                 |
       |--------(+)     ----------(+)     ----------(+)
                 |      |          |      |          |
             ---------  |      ---------  |      ---------
         k1--|  DES  |  |  k1--|  DES  |  |  k1--|  DES  |
             |encrypt|  |      |encrypt|  |      |encrypt|
             ---------  |      ---------  |      ---------
                 |      |          |      |          |
             ---------  |      ---------  |      ---------
         k2--|  DES  |  |  k2--|  DES  |  |  k2--|  DES  |
             |decrypt|  |      |decrypt|  |      |decrypt|
             ---------  |      ---------  |      ---------
                 |      |          |      |          |
             ---------  |      ---------  |      ---------
         k3--|  DES  |  |  k3--|  DES  |  |  k3--|  DES  |
             |encrypt|  |      |encrypt|  |      |encrypt|
             ---------  |      ---------  |      ---------
                 |------|          |------|          |----...
                 |                 |                 |
                y1                 y2                y3

   Where count is the Replay counter. x1, x2, x3 are the plaintext (x1
   is 32 bits, all others are 64 bits). y1, y2, y3 are the ciphertext.
   k1, k2, k3 corresponds to DES_KEY_[R|I]1, DES_KEY_[R|I]2, and

   This transformation is comprised of the following 3 steps.

      1. Taking the data and encapsulating it with the SPI, IV (if
      present), count, pad, pad length, and payload type.

      2. Calculating the HMAC using the HMAC_key_ and creating the dig-
      est from the SPI, IV (if present), count, data, pad, pad length,
      and payload type and placing the result into the HMAC digest

      3. Encrypting the count, data, pad, pad length, payload type, and
      HMAC digest using 3DES and the appropriate DES_key_ and IV_key_.
      (Note that the first DES block is a combination of the count and
      the first word of plaintext.)

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4.  Decryption Transform Procedure

   Triple encryption in Outer-CBC with  constant IV_key_ is used. If an IV is
   present in the packet, then the IV_key_ is not used and is replaced by the

   IV or
   IV_key_      y1                y2                y3
     |          |                 |                 |
     |          |-------          |-------          |----...
     |          |      |          |      |          |
     |      ---------  |      ---------  |      ---------
     |  k3--|  DES  |  |  k3--|  DES  |  |  k3--|  DES  |
     |      |decrypt|  |      |decrypt|  |      |decrypt|
     |      ---------  |      ---------  |      ---------
     |          |      |          |      |          |
     |      ---------  |      ---------  |      ---------
     |  k2--|  DES  |  |  k2--|  DES  |  |  k2--|  DES  |
     |      |encrypt|  |      |encrypt|  |      |encrypt|
     |      ---------  |      ---------  |      ---------
     |          |      |          |      |          |
     |      ---------  |      ---------  |      ---------
     |  k1--|  DES  |  |  k1--|  DES  |  |  k1--|  DES  |
     |      |decrypt|  |      |decrypt|  |      |decrypt|
     |      ---------  |      ---------  |      ---------
     |          |      |          |      |          |
     |---------(+)     |---------(+)     |---------(+)
                |                 |                 |
           (count|x1)             x2                x3

   Decryption is comprised of the following 4 steps.

      1. (Optional step) Decrypt the first bock of data using the
      appropriate DES_key_ and IV_key_ (or IV) and then do a quick "san-
      ity check" of the count. If the count has decreased below the win-
      dow or has increased by more than 65k, then it is safe to discard
      this packet as either a replay, non-authentic or too old. If the
      count is within 65K, then the probability that the packet is
      authentic is 65535/65536. (The following replay check and HMAC
      check are both still required).

      2. Decrypt the count (if not already done), data, pad, pad length,
      and payload type using DES and the appropriate DES_key_ and
      IV_key_ (or IV).

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      3. Calculate the HMAC using the HMAC_key_ and create the digest
      from the SPI, IV (if present) count, data, pad, pad length, and
      payload type and checking the result at digest at the end of the
      packet. If the digest is incorrect, discard the packet.

      4. Check the count using the window algorithm discussed above. If
      the packet is duplicate or too old, discard the packet.

5.  Key Material

   The key K is provided by the key management layer. This key is used
   to derive the symmetric keys, they are:

      DES_Key_I1, DES_KEY_I2, and DES_KEY_I3 are the keys used for
      3DES for traffic from the initiator -> responder. DES_KEY_I1 is the inner
      most key and DES_KEY_I3 is the outermost key.

      DES_Key_R1, DES_KEY_R2, and DES_KEY_R3 are the keys used for
      3DES for traffic from the initiator -> responder. DES_KEY_R3 is the inner
      most key and DES_KEY_R1 is the outermost key.

      HMAC_Key_I is the key for the HMAC Algorithm for traffic from the
      initiator -> responder.

      HMAC_Key_R is the key for the HMAC Algorithm for traffic from the
      responder -> initiator.

      IV_key_I is used to stop code book attacks on the first block for
      traffic from the initiator -> responder.

      IV_key_R is used to stop code book attacks on the first block for
      traffic from the responder -> initiator.

      RP_key_I is the initial value and wrap point for the replay
      prevention field for traffic from the initiator -> responder.

      RP_key_R is the initial value and wrap point for the replay
      prevention field for traffic from the responder -> initiator.

   The vertical bar symbol "|" is used to denote concatenation of bit

   MD5(x|y) denotes the result of applying the MD5 function to the con-
   catenated bit strings x and y.

   Truncate(x,n) denotes the result of truncating x to its first n bits.

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       DES_Key_I1  = Truncate(MD5( 0 | D_PAD_I | K ),64)
       DES_Key_I2  = Truncate(MD5( 1 | D_PAD_I | K ),64)
       DES_Key_I3  = Truncate(MD5( 2 | D_PAD_I | K ),64)
       DES_Key_R1  = Truncate(MD5( 0 | D_PAD_R | K ),64)
       DES_Key_R2  = Truncate(MD5( 1 | D_PAD_R | K ),64)
       DES_Key_R3  = Truncate(MD5( 2 | D_PAD_R | K ),64)
        IV_Key_I  = Truncate(MD5( I_Pad_I | K ),64)
        IV_Key_R  = Truncate(MD5( I_Pad_R | K ),64)
      HMAC_Key_I  =          MD5( H_Pad_I | K )
      HMAC_Key_R  =          MD5( H_Pad_R | K )
        RP_Key_I  = Truncate(MD5( R_Pad_I | K ),32)
        RP_Key_R  = Truncate(MD5( R_Pad_R | K ),32)

   where each _Pad_is 512 bit string.

      D_Pad_I = 0x5C repeated 63 times.
      D_Pad_R = 0x3A repeated 63 times.
      I_Pad_I = 0xAC repeated 64 times.
      I_Pad_R = 0x55 repeated 64 times.
      H_Pad_I = 0x53 repeated 64 times.
      H_Pad_R = 0x3C repeated 64 times.
      R_Pad_I = 0x35 repeated 64 times.
      R_Pad_R = 0xCC repeated 64 times.

   (Implementation note, The 16 byte intermediate residuals can be  pre-
   calculated from these constants and stored to reduce processing over-

6.  Security Considerations

   The ESP-3DES-HMAC-RP transform described in this draft is immune to
   the [Bellovin96] attacks. (AH [RFC-1826], in some modes, can also
   provide immunity to these attack.)

   The implications of the size of K can be found in [Blaze96].

7.  References

   [Bellovin96] Bellovin, S., "Problem Areas for the IP Security Proto-
   cols", AT&T Research, ftp://ftp.research.att.com/dist/smb/badesp.ps,
   July, 1996.

   [FIPS-46] US National Bureau of Standards, "Data Encryption Stan-
   dard", Federal Information Processing Standard (FIPS) Publication 46,
   January 1977.

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   [FIPS-46-1] US National Bureau of Standards, "Data Encryption Stan-
   dard", Federal Information Processing Standard (FIPS) Publication
   46-1, January 1988.

   [FIPS-74] US National Bureau of Standards, "Guidelines for Implement-
   ing and Using the Data Encryption Standard", Federal Information Pro-
   cessing Standard (FIPS) Publication 74, April 1981.

   [FIPS-81] US National Bureau of Standards, "DES Modes of Operation"
   Federal Information Processing Standard (FIPS) Publication 81,
   December 1980.

   [Krawczyk96] Krawczyk, H., Bellare, M., Canetti, R., "HMAC-MD5:
   Keyed-MD5 for Message Authentication", work-in-progress,
   hmac-md5-00.txt, March, 1996

   [Maughan96] Maughan, D., Schertler, M. Internet Security Association
   and Key Management Protocol (ISAKMP), work-in-progress,
   isakmp-04.txt, February, 1996

   [Orman96] Orman, H., "The Oakley Key Determination Protocol", work-
   in-progress, http://info.internet.isi.edu:80/in-drafts/files/draft-
   ietf-ipsec-oakley-00.txt, February, 1996.

   [RFC-1825] Atkinson, R, "Security Architecture for the Internet Pro-
   tocol", ftp://ds.internic.net/rfc/rfc1825.txt, August 1995.

   [RFC-1826] Atkinson, R, "IP Authentication Header",
   ftp://ds.internic.net/rfc/rfc1826.txt, August 1995.

   [Schneier95] Schneier, B., "Applied Cryptography Second Edition",
   John Wiley & Sons, New York, NY, 1995.  ISBN 0-471-12845-7

   [Blaze96] Blaze M., Diffie, W., Rivest, R., Schneier, B., Shimomura,
   T., Thompson, E., Wiener, M., "Minimal Key Lengths for Symmetric
   Ciphers to Provide Adequate Commercial Security",
   http://theory.lcs.mit.edu/~rivest/bsa-final-report.ascii, January,

   [IB93] John Ioannidis and Matt Blaze, "Architecture and Implementa-
   tion of Network-layer Security Under Unix", Proceedings of USENIX
   Security Symposium, Santa Clara, CA, October 1993.

   [IBK93] John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network-
   Layer Security for IP", presentation at the Spring 1993 IETF Meeting,
   Columbus, Ohio.

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8.  Acknowledgements

   This document is based on the document "Combined DES-CBC, HMAC, and
   Replay Prevention Security Transform," by the IPsec Working Group, J.
   Naganand, editor [Naganand96].  Much of the text of that document is
   repeated here, with the details of DES replaced with 3DES. I would like to
   thank Bob Baldwin, Steve Bellovin, Hugo Krawcyzk, Hilarie Orman, and Bill
   Sommerfeld for their suggestions on key generation for 3DES.

   The IPsec working group can be contacted through the chairs:

        Ran Atkinson
        Cisco Systems

        Paul Lambert
        Oracle Corporation

9.  Editor's Address

        Naganand Doraswamy
        Ftp Software, Inc.

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INTERNET DRAFT                                            November 1996

Appendix A

   This is a routine that implements a 32 packet window. This is intend-
   ed on being an implementation sample.

   #include <stdio.h>
   #include <stdlib.h>
   typedef unsigned long u_long;

   enum {
       ReplayWindowSize = 32

   u_long bitmap = 0;          /* session state - must be 32 bits */
   u_long lastSeq = 0;         /* session state */

   /* Returns 0 if packet disallowed, 1 if packet permitted */
   int ChkReplayWindow(u_long seq);

   int ChkReplayWindow(u_long seq) {
       u_long diff;

       if (seq == 0) return 0; /* first == 0 or wrapped */
       if (seq > lastSeq) {    /* new larger sequence number */
           diff = seq - lastSeq;
           if (diff < ReplayWindowSize) { /* In window */
               bitmap = (bitmap << diff) | 1; /* set bit for this packet */
           } else bitmap = 1;  /* This packet has a "way larger" */
           lastSeq = seq;
           return 1;           /* larger is good */
       diff = lastSeq - seq;
       if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */
       if (bitmap & (1l << diff)) return 0; /* this packet already seen */
       bitmap |= (1l << diff);  /* mark as seen */
       return 1;               /* out of order but good */

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char string_buffer[512];
#define STRING_BUFFER_SIZE sizeof(string_buffer)

int main() {
    int result;
    u_long last, current, bits;

    printf("Input initial state (bits in hex, last msgnum):\n");
    if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0);
    sscanf(string_buffer, "%lx %lu", &bits, &last);
    if (last != 0)
    bits |= 1;
    bitmap = bits;
    lastSeq = last;
    printf("bits:%08lx last:%lu\n", bitmap, lastSeq);
    printf("Input value to test (current):\n");

    while (1) {
        if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break;
        sscanf(string_buffer, "%lu", &current);
        result = ChkReplayWindow(current);
        printf("%-3s", result ? "OK" : "BAD");
        printf(" bits:%08lx last:%lu\n", bitmap, lastSeq);
    return 0;

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