mdw@git.distorted.org.uk Git - catacomb/blob - symm/ccm.c

   1 /* -*-c-*-
   2  *
   3  * CCM common code
   4  *
   5  * (c) 2018 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software: you can redistribute it and/or modify it
  13  * under the terms of the GNU Library General Public License as published
  14  * by the Free Software Foundation; either version 2 of the License, or
  15  * (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful, but
  18  * WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  20  * Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb.  If not, write to the Free Software
  24  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
  25  * USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include "ccm.h"
  31 #include "ccm-def.h"
  32
  33 /*----- MAC header and counter formatting ---------------------------------*
  34  *
  35  * CCM's most endearing feature is its complex formatting of the MAC
  36  * initialization vector and counter blocks.  This is only specified for
  37  * 128-bit blocks, so I've made up formats for 64-, 192-, and 256-bit blocks.
  38  * For completeness and ease of comparison, all of the encodings are defined
  39  * here.  Encoding of the rest of the MAC input data is common.
  40  *
  41  * The notation is taken from the definition of CCM in NIST SP800-38C.
  42  *
  43  *   * [x]_w = encoding of x, as w bits
  44  *   * P = plaintext/ciphertext
  45  *   * p = length of P in octets
  46  *   * Q = [p]_{8q} = encoding of p
  47  *   * q = length of Q in octets
  48  *   * N = nonce
  49  *   * H = B_0 = MAC header block
  50  *   * C_i = the i-th counter block
  51  *   * t = length of tag in octets
  52  *
  53  * 128-bit blocks (SP800-38C, appendix A):
  54  *
  55  * Let F be an octet:
  56  *
  57  *      Bits            7       6       6:3             3:0
  58  *      Meaning         0       AAD?    [t/2 - 1]_3     [q - 1]_3
  59  *
  60  * Then H = F || N || Q and C_i = 0^5 || [q - 1]_3 || N || [i]_{8q}.
  61  *
  62  * 64-bit blocks (new):
  63  *
  64  * Let F be an octet:
  65  *
  66  *      Bits            7       6       6:3             3:0
  67  *      Meaning         0       AAD?    [t - 1]_3       [q - 1]_3
  68  *
  69  * Then H = F || N || Q and C_i = 0^5 || [q - 1]_3 || N || [i]_{8q}.
  70  *
  71  * (i.e., almost exactly the same, except that the tag no longer needs to be
  72  * an even number of octets).
  73  *
  74  * n-bit blocks, for n > 128
  75  *
  76  * Let F be a 16-bit flags word:
  77  *
  78  *      Bits            15      15:8            7       7:0
  79  *      Meaning         0       [t]_7           AAD?    [q]_7
  80  *
  81  * Then H = F || N || Q and C_i = 0^9 || [q - 1]_7 || N || [i]_{8q}.
  82  */
  83
  84 /* --- @ccm_check@ --- *
  85  *
  86  * Arguments:   @const ccm_params *p@ = pointer to parameters
  87  *
  88  * Returns:     True (nonzero) if the parameters are OK; false (zero) if
  89  *              there's a problem.
  90  *
  91  * Use:         Verify that the CCM parameters are acceptable.
  92  */
  93
  94 int ccm_check(const ccm_params *p)
  95 {
  96   unsigned fsz = p->bsz <= 16 ? 1 : 2, q = p->bsz - p->nsz - fsz, i;
  97   size_t msz;
  98
  99   /* Check that the block size hasn't been bungled. */
 100   if (p->bsz < 16 && p->bsz != 8) return (0);
 101
 102   /* The length-of-the-length must be representable, and its encoding must
 103    * not be zero, since this is `reserved'.  The small-block encoding stores
 104    * %$q - 1$%, so we must have %$q \ge 2$%; the large-block encoding stores
 105    * %$q$% directly, so only %$q = 0$% is forbidden.
 106    */
 107   if (p->nsz > p->bsz - fsz - (p->bsz <= 16 ? 2 : 1)) return (0);
 108   if (q > (p->bsz <= 16 ? 8 : 127)) return (0);
 109
 110   /* Verify that the message length will fit in the space allotted. */
 111   for (i = 1, msz = p->msz >> 8; msz; i++, msz >>= 8);
 112   if (i > q) return (0);
 113
 114   /* The tag can't be larger than the block size.  Also, it must be
 115    * representable, and its encoding must not be zero, because otherwise it'd
 116    * be possible for a MAC header block to look like a counter block.  The
 117    * tag encoding is fiddly: for 64-bit blocks, we store %$t - 1$%, so we
 118    * must have %$t \ge 2$%; for 128-bit blocks, we store %$t/2 - 2$%, so
 119    * we must have %$t \ge 4$% with %$t$% even; otherwise, we store %$t$%
 120    * directly, so the only requirement is that %$t \ge 1$%.
 121    */
 122   if (p->tsz > p->bsz) return (0);
 123   if (p->tsz < (p->bsz == 8 ? 2 : p->bsz == 16 ? 4 : 1)) return (0);
 124   if (p->bsz == 16 && p->tsz%2 != 0) return (0);
 125
 126   /* All looks good. */
 127   return (1);
 128 }
 129
 130 /* --- @ccm_fmthdr@ --- *
 131  *
 132  * Arguments:   @const ccm_params *p@ = pointer to parameters
 133  *              @octet *b@ = block-size buffer to write header
 134  *              @const void *n@ = pointer to nonce
 135  *
 136  * Returns:     ---
 137  *
 138  * Use:         Format a MAC header block.
 139  */
 140
 141 void ccm_fmthdr(const ccm_params *p, octet *b, const void *n)
 142 {
 143   size_t fsz = p->bsz <= 16 ? 1 : 2, q = p->bsz - p->nsz - fsz;
 144   unsigned f0 = 0, f1 = 0;
 145   size_t i, t;
 146
 147   /* Encode whether the AAD is empty. */
 148   if (!p->hsz) /* do nothing */;
 149   else if (p->bsz <= 16) f0 |= 0x40;
 150   else f1 |= 0x80;
 151
 152   /* Encode the tag size. */
 153   switch (p->bsz) {
 154     case  8: f0 |= (p->tsz - 1) << 3; break;
 155     case 16: f0 |= (p->tsz - 2) << 2; break;
 156     default: f0 |=  p->tsz;           break;
 157   }
 158
 159   /* Encode the length-of-the-length.  (This is the most bletcherous part of
 160    * CCM.)
 161    */
 162   if (p->bsz <= 16) f0 |= q - 1;
 163   else f1 |= q;
 164
 165   /* Insert the flags and nonce. */
 166   b[0] = f0; memcpy(b + fsz, n, p->nsz);
 167   if (p->bsz > 16) b[1] = f1;
 168
 169   /* Write the message length. */
 170   for (i = 0, t = p->msz; i < q; i++, t >>= 8) b[p->bsz - i - 1] = U8(t);
 171 }
 172
 173 /* --- @ccm_fmtctr@ --- *
 174  *
 175  * Arguments:   @const ccm_params *p@ = pointer to parameters
 176  *              @octet *b@ = block-size buffer to write header
 177  *              @const void *n@ = pointer to nonce
 178  *
 179  * Returns:     ---
 180  *
 181  * Use:         Format an initial counter block.
 182  */
 183
 184 void ccm_fmtctr(const ccm_params *p, octet *b, const void *n)
 185 {
 186   size_t fsz = p->bsz <= 16 ? 1 : 2, q = p->bsz - p->nsz - fsz;
 187   unsigned f0 = 0, f1 = 0;
 188
 189   /* Encode the message length length.  (Did I complain about this?) */
 190   if (p->bsz <= 16) f0 |= q - 1;
 191   else f1 |= q;
 192
 193   /* Insert the flags and nonce. */
 194   b[0] = f0; memcpy(b + fsz, n, p->nsz);
 195   if (p->bsz > 16) b[1] = f1;
 196
 197   /* Zero out the initial counter. */
 198   memset(b + fsz + p->nsz, 0, q);
 199 }
 200
 201 /*----- That's all, folks -------------------------------------------------*/