[Docs] [txt|pdf] [draft-songlee-aes...] [Diff1] [Diff2]

INFORMATIONAL

Network Working Group                                           JH. Song
Request for Comments: 4493                                 R. Poovendran
Category: Informational                         University of Washington
                                                                  J. Lee
                                                     Samsung Electronics
                                                                T. Iwata
                                                       Nagoya University
                                                               June 2006


                         The AES-CMAC Algorithm

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 Internet Society (2006).

Abstract

   The National Institute of Standards and Technology (NIST) has
   recently specified the Cipher-based Message Authentication Code
   (CMAC), which is equivalent to the One-Key CBC MAC1 (OMAC1) submitted
   by Iwata and Kurosawa.  This memo specifies an authentication
   algorithm based on CMAC with the 128-bit Advanced Encryption Standard
   (AES).  This new authentication algorithm is named AES-CMAC.  The
   purpose of this document is to make the AES-CMAC algorithm
   conveniently available to the Internet Community.



















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Table of Contents

   1. Introduction ....................................................2
   2. Specification of AES-CMAC .......................................3
      2.1. Basic Definitions ..........................................3
      2.2. Overview ...................................................4
      2.3. Subkey Generation Algorithm ................................5
      2.4. MAC Generation Algorithm ...................................7
      2.5. MAC Verification Algorithm .................................9
   3. Security Considerations ........................................10
   4. Test Vectors ...................................................11
   5. Acknowledgement ................................................12
   6. References .....................................................12
      6.1. Normative References ......................................12
      6.2. Informative References ....................................12
   Appendix A. Test Code .............................................14

1.  Introduction

   The National Institute of Standards and Technology (NIST) has
   recently specified the Cipher-based Message Authentication Code
   (CMAC).  CMAC [NIST-CMAC] is a keyed hash function that is based on a
   symmetric key block cipher, such as the Advanced Encryption Standard
   [NIST-AES].  CMAC is equivalent to the One-Key CBC MAC1 (OMAC1)
   submitted by Iwata and Kurosawa [OMAC1a, OMAC1b].  OMAC1 is an
   improvement of the eXtended Cipher Block Chaining mode (XCBC)
   submitted by Black and Rogaway [XCBCa, XCBCb], which itself is an
   improvement of the basic Cipher Block Chaining-Message Authentication
   Code (CBC-MAC).  XCBC efficiently addresses the security deficiencies
   of CBC-MAC, and OMAC1 efficiently reduces the key size of XCBC.

   AES-CMAC provides stronger assurance of data integrity than a
   checksum or an error-detecting code.  The verification of a checksum
   or an error-detecting code detects only accidental modifications of
   the data, while CMAC is designed to detect intentional, unauthorized
   modifications of the data, as well as accidental modifications.

   AES-CMAC achieves a security goal similar to that of HMAC [RFC-HMAC].
   Since AES-CMAC is based on a symmetric key block cipher, AES, and
   HMAC is based on a hash function, such as SHA-1, AES-CMAC is
   appropriate for information systems in which AES is more readily
   available than a hash function.

   This memo specifies the authentication algorithm based on CMAC with
   AES-128.  This new authentication algorithm is named AES-CMAC.






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2.  Specification of AES-CMAC

2.1.  Basic Definitions

   The following table describes the basic definitions necessary to
   explain the specification of AES-CMAC.

   x || y          Concatenation.
                   x || y is the string x concatenated with the string
                   y.
                   0000 || 1111 is 00001111.

   x XOR y         Exclusive-OR operation.
                   For two equal length strings, x and y,
                   x XOR y is their bit-wise exclusive-OR.

   ceil(x)         Ceiling function.
                   The smallest integer no smaller than x.
                   ceil(3.5) is 4.  ceil(5) is 5.

   x << 1          Left-shift of the string x by 1 bit.
                   The most significant bit disappears, and a zero
                   comes into the least significant bit.
                   10010001 << 1 is 00100010.

   0^n             The string that consists of n zero-bits.
                   0^3 means 000 in binary format.
                   10^4 means 10000 in binary format.
                   10^i means 1 followed by i-times repeated
                   zeros.

   MSB(x)          The most-significant bit of the string x.
                   MSB(10010000) means 1.

   padding(x)      10^i padded output of input x.
                   It is described in detail in section 2.4.

   Key             128-bit (16-octet) long key for AES-128.
                   Denoted by K.

   First subkey    128-bit (16-octet) long first subkey,
                   derived through the subkey
                   generation algorithm from the key K.
                   Denoted by K1.







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   Second subkey   128-bit (16-octet) long second subkey,
                   derived through the subkey
                   generation algorithm from the key K.
                   Denoted by K2.

   Message         A message to be authenticated.
                   Denoted by M.
                   The message can be null, which means that the length
                   of M is 0.

   Message length  The length of the message M in octets.
                   Denoted by len.
                   The minimum value of the length can be 0.  The
                   maximum value of the length is not specified in
                   this document.

   AES-128(K,M)    AES-128(K,M) is the 128-bit ciphertext of AES-128
                   for a 128-bit key, K, and a 128-bit message, M.

   MAC             A 128-bit string that is the output of AES-CMAC.
                   Denoted by T.
                   Validating the MAC provides assurance of the
                   integrity and authenticity of the message from
                   the source.

   MAC length      By default, the length of the output of AES-CMAC is
                   128 bits.  It is possible to truncate the MAC.
                   The result of the truncation should be taken in most
                   significant bits first order.  The MAC length must be
                   specified before the communication starts, and
                   it must not be changed during the lifetime of the
                   key.

2.2.  Overview

   AES-CMAC uses the Advanced Encryption Standard [NIST-AES] as a
   building block.  To generate a MAC, AES-CMAC takes a secret key, a
   message of variable length, and the length of the message in octets
   as inputs and returns a fixed-bit string called a MAC.

   The core of AES-CMAC is the basic CBC-MAC.  For a message, M, to be
   authenticated, the CBC-MAC is applied to M.  There are two cases of
   operation in CMAC.  Figure 2.1 illustrates the operation of CBC-MAC
   in both cases.  If the size of the input message block is equal to a
   positive multiple of the block size (namely, 128 bits), the last
   block shall be exclusive-OR'ed with K1 before processing.  Otherwise,
   the last block shall be padded with 10^i (notation is described in
   section 2.1) and exclusive-OR'ed with K2.  The result of the previous



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   process will be the input of the last encryption.  The output of
   AES-CMAC provides data integrity of the whole input message.

 +-----+     +-----+     +-----+     +-----+     +-----+     +---+----+
 | M_1 |     | M_2 |     | M_n |     | M_1 |     | M_2 |     |M_n|10^i|
 +-----+     +-----+     +-----+     +-----+     +-----+     +---+----+
    |           |           |   +--+    |           |           |   +--+
    |     +--->(+)    +--->(+)<-|K1|    |     +--->(+)    +--->(+)<-|K2|
    |     |     |     |     |   +--+    |     |     |     |     |   +--+
 +-----+  |  +-----+  |  +-----+     +-----+  |  +-----+  |  +-----+
 |AES_K|  |  |AES_K|  |  |AES_K|     |AES_K|  |  |AES_K|  |  |AES_K|
 +-----+  |  +-----+  |  +-----+     +-----+  |  +-----+  |  +-----+
    |     |     |     |     |           |     |     |     |     |
    +-----+     +-----+     |           +-----+     +-----+     |
                            |                                   |
                         +-----+                              +-----+
                         |  T  |                              |  T  |
                         +-----+                              +-----+

             (a) positive multiple block length         (b) otherwise

          Figure 2.1.  Illustration of the two cases of AES-CMAC

   AES_K is AES-128 with key K.
   The message M is divided into blocks M_1,...,M_n,
   where M_i is the i-th message block.
   The length of M_i is 128 bits for i = 1,...,n-1, and
   the length of the last block, M_n, is less than or equal to 128 bits.
   K1 is the subkey for the case (a), and
   K2 is the subkey for the case (b).
   K1 and K2 are generated by the subkey generation algorithm
   described in section 2.3.

2.3.  Subkey Generation Algorithm

   The subkey generation algorithm, Generate_Subkey(), takes a secret
   key, K, which is just the key for AES-128.

   The outputs of the subkey generation algorithm are two subkeys, K1
   and K2.  We write (K1,K2) := Generate_Subkey(K).

   Subkeys K1 and K2 are used in both MAC generation and MAC
   verification algorithms.  K1 is used for the case where the length of
   the last block is equal to the block length.  K2 is used for the case
   where the length of the last block is less than the block length.






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   Figure 2.2 specifies the subkey generation algorithm.

   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                    Algorithm Generate_Subkey                      +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                                                                   +
   +   Input    : K (128-bit key)                                      +
   +   Output   : K1 (128-bit first subkey)                            +
   +              K2 (128-bit second subkey)                           +
   +-------------------------------------------------------------------+
   +                                                                   +
   +   Constants: const_Zero is 0x00000000000000000000000000000000     +
   +              const_Rb   is 0x00000000000000000000000000000087     +
   +   Variables: L          for output of AES-128 applied to 0^128    +
   +                                                                   +
   +   Step 1.  L := AES-128(K, const_Zero);                           +
   +   Step 2.  if MSB(L) is equal to 0                                +
   +            then    K1 := L << 1;                                  +
   +            else    K1 := (L << 1) XOR const_Rb;                   +
   +   Step 3.  if MSB(K1) is equal to 0                               +
   +            then    K2 := K1 << 1;                                 +
   +            else    K2 := (K1 << 1) XOR const_Rb;                  +
   +   Step 4.  return K1, K2;                                         +
   +                                                                   +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

                Figure 2.2.  Algorithm Generate_Subkey

   In step 1, AES-128 with key K is applied to an all-zero input block.

   In step 2, K1 is derived through the following operation:

   If the most significant bit of L is equal to 0, K1 is the left-shift
   of L by 1 bit.

   Otherwise, K1 is the exclusive-OR of const_Rb and the left-shift of L
   by 1 bit.

   In step 3, K2 is derived through the following operation:

   If the most significant bit of K1 is equal to 0, K2 is the left-shift
   of K1 by 1 bit.

   Otherwise, K2 is the exclusive-OR of const_Rb and the left-shift of
   K1 by 1 bit.

   In step 4, (K1,K2) := Generate_Subkey(K) is returned.




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   The mathematical meaning of the procedures in steps 2 and 3,
   including const_Rb, can be found in [OMAC1a].

2.4.  MAC Generation Algorithm

   The MAC generation algorithm, AES-CMAC(), takes three inputs, a
   secret key, a message, and the length of the message in octets.  The
   secret key, denoted by K, is just the key for AES-128.  The message
   and its length in octets are denoted by M and len, respectively.  The
   message M is denoted by the sequence of M_i, where M_i is the i-th
   message block.  That is, if M consists of n blocks, then M is written
   as

    -   M = M_1 || M_2 || ... || M_{n-1} || M_n

   The length of M_i is 128 bits for i = 1,...,n-1, and the length of
   the last block M_n is less than or equal to 128 bits.

   The output of the MAC generation algorithm is a 128-bit string,
   called a MAC, which is used to validate the input message.  The MAC
   is denoted by T, and we write T := AES-CMAC(K,M,len).  Validating the
   MAC provides assurance of the integrity and authenticity of the
   message from the source.

   It is possible to truncate the MAC.  According to [NIST-CMAC], at
   least a 64-bit MAC should be used as protection against guessing
   attacks.  The result of truncation should be taken in most
   significant bits first order.

   The block length of AES-128 is 128 bits (16 octets).  There is a
   special treatment if the length of the message is not a positive
   multiple of the block length.  The special treatment is to pad M with
   the bit-string 10^i to adjust the length of the last block up to the
   block length.

   For an input string x of r-octets, where 0 <= r < 16, the padding
   function, padding(x), is defined as follows:

   -   padding(x) = x || 10^i      where i is 128-8*r-1

   That is, padding(x) is the concatenation of x and a single '1',
   followed by the minimum number of '0's, so that the total length is
   equal to 128 bits.

   Figure 2.3 describes the MAC generation algorithm.






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   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                   Algorithm AES-CMAC                              +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                                                                   +
   +   Input    : K    ( 128-bit key )                                 +
   +            : M    ( message to be authenticated )                 +
   +            : len  ( length of the message in octets )             +
   +   Output   : T    ( message authentication code )                 +
   +                                                                   +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +   Constants: const_Zero is 0x00000000000000000000000000000000     +
   +              const_Bsize is 16                                    +
   +                                                                   +
   +   Variables: K1, K2 for 128-bit subkeys                           +
   +              M_i is the i-th block (i=1..ceil(len/const_Bsize))   +
   +              M_last is the last block xor-ed with K1 or K2        +
   +              n      for number of blocks to be processed          +
   +              r      for number of octets of last block            +
   +              flag   for denoting if last block is complete or not +
   +                                                                   +
   +   Step 1.  (K1,K2) := Generate_Subkey(K);                         +
   +   Step 2.  n := ceil(len/const_Bsize);                            +
   +   Step 3.  if n = 0                                               +
   +            then                                                   +
   +                 n := 1;                                           +
   +                 flag := false;                                    +
   +            else                                                   +
   +                 if len mod const_Bsize is 0                       +
   +                 then flag := true;                                +
   +                 else flag := false;                               +
   +                                                                   +
   +   Step 4.  if flag is true                                        +
   +            then M_last := M_n XOR K1;                             +
   +            else M_last := padding(M_n) XOR K2;                    +
   +   Step 5.  X := const_Zero;                                       +
   +   Step 6.  for i := 1 to n-1 do                                   +
   +                begin                                              +
   +                  Y := X XOR M_i;                                  +
   +                  X := AES-128(K,Y);                               +
   +                end                                                +
   +            Y := M_last XOR X;                                     +
   +            T := AES-128(K,Y);                                     +
   +   Step 7.  return T;                                              +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

                      Figure 2.3.  Algorithm AES-CMAC





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   In step 1, subkeys K1 and K2 are derived from K through the subkey
   generation algorithm.

   In step 2, the number of blocks, n, is calculated.  The number of
   blocks is the smallest integer value greater than or equal to the
   quotient determined by dividing the length parameter by the block
   length, 16 octets.

   In step 3, the length of the input message is checked.  If the input
   length is 0 (null), the number of blocks to be processed shall be 1,
   and the flag shall be marked as not-complete-block (false).
   Otherwise, if the last block length is 128 bits, the flag is marked
   as complete-block (true); else mark the flag as not-complete-block
   (false).

   In step 4, M_last is calculated by exclusive-OR'ing M_n and one of
   the previously calculated subkeys.  If the last block is a complete
   block (true), then M_last is the exclusive-OR of M_n and K1.
   Otherwise, M_last is the exclusive-OR of padding(M_n) and K2.

   In step 5, the variable X is initialized.

   In step 6, the basic CBC-MAC is applied to M_1,...,M_{n-1},M_last.

   In step 7, the 128-bit MAC, T := AES-CMAC(K,M,len), is returned.

   If necessary, the MAC is truncated before it is returned.

2.5.  MAC Verification Algorithm

   The verification of the MAC is simply done by a MAC recomputation.
   We use the MAC generation algorithm, which is described in section
   2.4.

   The MAC verification algorithm, Verify_MAC(), takes four inputs, a
   secret key, a message, the length of the message in octets, and the
   received MAC.  These are denoted by K, M, len, and T', respectively.

   The output of the MAC verification algorithm is either INVALID or
   VALID.

   Figure 2.4 describes the MAC verification algorithm.









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   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                      Algorithm Verify_MAC                         +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   +                                                                   +
   +   Input    : K    ( 128-bit Key )                                 +
   +            : M    ( message to be verified )                      +
   +            : len  ( length of the message in octets )             +
   +            : T'   ( the received MAC to be verified )             +
   +   Output   : INVALID or VALID                                     +
   +                                                                   +
   +-------------------------------------------------------------------+
   +                                                                   +
   +   Step 1.  T* := AES-CMAC(K,M,len);                               +
   +   Step 2.  if T* is equal to T'                                   +
   +            then                                                   +
   +                 return VALID;                                     +
   +            else                                                   +
   +                 return INVALID;                                   +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

                    Figure 2.4.  Algorithm Verify_MAC

   In step 1, T* is derived from K, M, and len through the MAC
   generation algorithm.

   In step 2, T* and T' are compared.  If T* is equal to T', then return
   VALID; otherwise return INVALID.

   If the output is INVALID, then the message is definitely not
   authentic, i.e., it did not originate from a source that executed the
   generation process on the message to produce the purported MAC.

   If the output is VALID, then the design of the AES-CMAC provides
   assurance that the message is authentic and, hence, was not corrupted
   in transit; however, this assurance, as for any MAC algorithm, is not
   absolute.

3.  Security Considerations

   The security provided by AES-CMAC is built on the strong
   cryptographic algorithm AES.  However, as is true with any
   cryptographic algorithm, part of its strength lies in the secret key,
   K, and the correctness of the implementation in all of the
   participating systems.  If the secret key is compromised or
   inappropriately shared, it guarantees neither authentication nor
   integrity of message at all.  The secret key shall be generated in a
   way that meets the pseudo randomness requirement of RFC 4086
   [RFC4086] and should be kept safe.  If and only if AES-CMAC is used



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   properly it provides the authentication and integrity that meet the
   best current practice of message authentication.

4.  Test Vectors

   The following test vectors are the same as those of [NIST-CMAC].  The
   following vectors are also the output of the test program in Appendix
   A.

   --------------------------------------------------
   Subkey Generation
   K              2b7e1516 28aed2a6 abf71588 09cf4f3c
   AES-128(key,0) 7df76b0c 1ab899b3 3e42f047 b91b546f
   K1             fbeed618 35713366 7c85e08f 7236a8de
   K2             f7ddac30 6ae266cc f90bc11e e46d513b
   --------------------------------------------------

   --------------------------------------------------
   Example 1: len = 0
   M              <empty string>
   AES-CMAC       bb1d6929 e9593728 7fa37d12 9b756746
   --------------------------------------------------

   Example 2: len = 16
   M              6bc1bee2 2e409f96 e93d7e11 7393172a
   AES-CMAC       070a16b4 6b4d4144 f79bdd9d d04a287c
   --------------------------------------------------

   Example 3: len = 40
   M              6bc1bee2 2e409f96 e93d7e11 7393172a
                  ae2d8a57 1e03ac9c 9eb76fac 45af8e51
                  30c81c46 a35ce411
   AES-CMAC       dfa66747 de9ae630 30ca3261 1497c827
   --------------------------------------------------

   Example 4: len = 64
   M              6bc1bee2 2e409f96 e93d7e11 7393172a
                  ae2d8a57 1e03ac9c 9eb76fac 45af8e51
                  30c81c46 a35ce411 e5fbc119 1a0a52ef
                  f69f2445 df4f9b17 ad2b417b e66c3710
   AES-CMAC       51f0bebf 7e3b9d92 fc497417 79363cfe
   --------------------------------------------------









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5.  Acknowledgement

   Portions of the text herein are borrowed from [NIST-CMAC].  We
   appreciate the OMAC1 authors, the SP 800-38B author, and Russ Housley
   for his useful comments and guidance, which have been incorporated
   herein.  We also thank Alfred Hoenes for many useful comments.  This
   memo was prepared while Tetsu Iwata was at Ibaraki University, Japan.

   We acknowledge the support from the following grants:  Collaborative
   Technology Alliance (CTA) from US Army Research Laboratory, DAAD19-
   01-2-0011; Presidential Award from Army Research Office, W911NF-05-
   1-0491; NSF CAREER ANI-0093187.  Results do not reflect any position
   of the funding agencies.

6.  References

6.1.  Normative References

   [NIST-CMAC] NIST, Special Publication 800-38B, "Recommendation for
               Block Cipher Modes of Operation: The CMAC Mode for
               Authentication", May 2005.

   [NIST-AES]  NIST, FIPS 197, "Advanced Encryption Standard (AES)",
               November 2001.
               http://csrc.nist.gov/publications/fips/fips197/fips-
               197.pdf

   [RFC4086]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
               "Randomness Requirements for Security", BCP 106, RFC
               4086, June 2005.

6.2.  Informative References

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

   [OMAC1a]    Tetsu Iwata and Kaoru Kurosawa, "OMAC: One-Key CBC MAC",
               Fast Software Encryption, FSE 2003, LNCS 2887, pp. 129-
               153, Springer-Verlag, 2003.

   [OMAC1b]    Tetsu Iwata and Kaoru Kurosawa, "OMAC: One-Key CBC MAC",
               Submission to NIST, December 2002.  Available from the
               NIST modes of operation web site at
               http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/
               omac/omac-spec.pdf





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   [XCBCa]     John Black and Phillip Rogaway, "A Suggestion for
               Handling Arbitrary-Length Messages with the CBC MAC",
               NIST Second Modes of Operation Workshop, August 2001.
               Available from the NIST modes of operation web site at
               http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/
               xcbc-mac/xcbc-mac-spec.pdf

   [XCBCb]     John Black and Phillip Rogaway, "CBC MACs for Arbitrary-
               Length Messages: The Three-Key Constructions", Journal of
               Cryptology, Vol. 18, No. 2, pp. 111-132, Springer-Verlag,
               Spring 2005.








































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Appendix A.  Test Code

  This C source is designed to generate the test vectors that appear in
  this memo to verify correctness of the algorithm.  The source code is
  not intended for use in commercial products.

  /****************************************************************/
  /* AES-CMAC with AES-128 bit                                    */
  /* CMAC     Algorithm described in SP800-38B                    */
  /* Author: Junhyuk Song (junhyuk.song@samsung.com)              */
  /*         Jicheol Lee  (jicheol.lee@samsung.com)               */
  /****************************************************************/

  #include <stdio.h>

  /* For CMAC Calculation */
  unsigned char const_Rb[16] = {
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x87
  };
  unsigned char const_Zero[16] = {
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
  };

  /* Basic Functions */

  void xor_128(unsigned char *a, unsigned char *b, unsigned char *out)
  {
      int i;
      for (i=0;i<16; i++)
      {
          out[i] = a[i] ^ b[i];
      }
  }

  void print_hex(char *str, unsigned char *buf, int len)
  {
      int     i;

      for ( i=0; i<len; i++ ) {
          if ( (i % 16) == 0 && i != 0 ) printf(str);
          printf("%02x", buf[i]);
          if ( (i % 4) == 3 ) printf(" ");
          if ( (i % 16) == 15 ) printf("\n");
      }
      if ( (i % 16) != 0 ) printf("\n");
  }



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  void print128(unsigned char *bytes)
  {
      int         j;
      for (j=0; j<16;j++) {
          printf("%02x",bytes[j]);
          if ( (j%4) == 3 ) printf(" ");
      }
  }

  void print96(unsigned char *bytes)
  {
      int         j;
      for (j=0; j<12;j++) {
          printf("%02x",bytes[j]);
          if ( (j%4) == 3 ) printf(" ");
      }
  }

  /* AES-CMAC Generation Function */

  void leftshift_onebit(unsigned char *input,unsigned char *output)
  {
      int         i;
      unsigned char overflow = 0;

      for ( i=15; i>=0; i-- ) {
          output[i] = input[i] << 1;
          output[i] |= overflow;
          overflow = (input[i] & 0x80)?1:0;
      }
      return;
  }

  void generate_subkey(unsigned char *key, unsigned char *K1, unsigned
                       char *K2)
  {
      unsigned char L[16];
      unsigned char Z[16];
      unsigned char tmp[16];
      int i;

      for ( i=0; i<16; i++ ) Z[i] = 0;

      AES_128(key,Z,L);

      if ( (L[0] & 0x80) == 0 ) { /* If MSB(L) = 0, then K1 = L << 1 */
          leftshift_onebit(L,K1);
      } else {    /* Else K1 = ( L << 1 ) (+) Rb */



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RFC 4493                 The AES-CMAC Algorithm                June 2006


          leftshift_onebit(L,tmp);
          xor_128(tmp,const_Rb,K1);
      }

      if ( (K1[0] & 0x80) == 0 ) {
          leftshift_onebit(K1,K2);
      } else {
          leftshift_onebit(K1,tmp);
          xor_128(tmp,const_Rb,K2);
      }
      return;
  }

  void padding ( unsigned char *lastb, unsigned char *pad, int length )
  {
      int         j;

      /* original last block */
      for ( j=0; j<16; j++ ) {
          if ( j < length ) {
              pad[j] = lastb[j];
          } else if ( j == length ) {
              pad[j] = 0x80;
          } else {
              pad[j] = 0x00;
          }
      }
  }

  void AES_CMAC ( unsigned char *key, unsigned char *input, int length,
                  unsigned char *mac )
  {
      unsigned char       X[16],Y[16], M_last[16], padded[16];
      unsigned char       K1[16], K2[16];
      int         n, i, flag;
      generate_subkey(key,K1,K2);

      n = (length+15) / 16;       /* n is number of rounds */

      if ( n == 0 ) {
          n = 1;
          flag = 0;
      } else {
          if ( (length%16) == 0 ) { /* last block is a complete block */
              flag = 1;
          } else { /* last block is not complete block */
              flag = 0;
          }



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RFC 4493                 The AES-CMAC Algorithm                June 2006


      }

      if ( flag ) { /* last block is complete block */
          xor_128(&input[16*(n-1)],K1,M_last);
      } else {
          padding(&input[16*(n-1)],padded,length%16);
          xor_128(padded,K2,M_last);
      }

      for ( i=0; i<16; i++ ) X[i] = 0;
      for ( i=0; i<n-1; i++ ) {
          xor_128(X,&input[16*i],Y); /* Y := Mi (+) X  */
          AES_128(key,Y,X);      /* X := AES-128(KEY, Y); */
      }

      xor_128(X,M_last,Y);
      AES_128(key,Y,X);

      for ( i=0; i<16; i++ ) {
          mac[i] = X[i];
      }
  }

  int main()
  {
      unsigned char L[16], K1[16], K2[16], T[16], TT[12];
      unsigned char M[64] = {
          0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96,
          0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
          0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c,
          0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
          0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11,
          0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
          0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17,
          0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10
      };
      unsigned char key[16] = {
          0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6,
          0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c
      };

      printf("--------------------------------------------------\n");
      printf("K              "); print128(key); printf("\n");

      printf("\nSubkey Generation\n");
      AES_128(key,const_Zero,L);
      printf("AES_128(key,0) "); print128(L); printf("\n");
      generate_subkey(key,K1,K2);



Song, et al.                 Informational                     [Page 17]

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      printf("K1             "); print128(K1); printf("\n");
      printf("K2             "); print128(K2); printf("\n");

      printf("\nExample 1: len = 0\n");
      printf("M              "); printf("<empty string>\n");

      AES_CMAC(key,M,0,T);
      printf("AES_CMAC       "); print128(T); printf("\n");

      printf("\nExample 2: len = 16\n");
      printf("M              "); print_hex("                ",M,16);
      AES_CMAC(key,M,16,T);
      printf("AES_CMAC       "); print128(T); printf("\n");
      printf("\nExample 3: len = 40\n");
      printf("M              "); print_hex("               ",M,40);
      AES_CMAC(key,M,40,T);
      printf("AES_CMAC       "); print128(T); printf("\n");

      printf("\nExample 4: len = 64\n");
      printf("M              "); print_hex("               ",M,64);
      AES_CMAC(key,M,64,T);
      printf("AES_CMAC       "); print128(T); printf("\n");

      printf("--------------------------------------------------\n");

      return 0;
  }
























Song, et al.                 Informational                     [Page 18]

RFC 4493                 The AES-CMAC Algorithm                June 2006


Authors' Addresses

  Junhyuk Song
  University of Washington
  Samsung Electronics

  Phone: (206) 853-5843
  EMail: songlee@ee.washington.edu, junhyuk.song@samsung.com


  Jicheol Lee
  Samsung Electronics

  Phone: +82-31-279-3605
  EMail: jicheol.lee@samsung.com


  Radha Poovendran
  Network Security Lab
  University of Washington

  Phone: (206) 221-6512
  EMail: radha@ee.washington.edu


  Tetsu Iwata
  Nagoya University

  EMail: iwata@cse.nagoya-u.ac.jp






















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RFC 4493                 The AES-CMAC Algorithm                June 2006


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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Song, et al.                 Informational                     [Page 20]


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