release/src/router/nettle/gcm.c

   1 /* gcm.h
   2  *
   3  * Galois counter mode, specified by NIST,
   4  * http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf
   5  *
   6  * See also the gcm paper at
   7  * http://www.cryptobarn.com/papers/gcm-spec.pdf.
   8  */
   9
  10 /* NOTE: Tentative interface, subject to change. No effort will be
  11    made to avoid incompatible changes. */
  12
  13 /* nettle, low-level cryptographics library
  14  *
  15  * Copyright (C) 2011 Niels Möller
  16  * Copyright (C) 2011 Katholieke Universiteit Leuven
  17  *
  18  * Contributed by Nikos Mavrogiannopoulos
  19  *
  20  * The nettle library is free software; you can redistribute it and/or modify
  21  * it under the terms of the GNU Lesser General Public License as published by
  22  * the Free Software Foundation; either version 2.1 of the License, or (at your
  23  * option) any later version.
  24  *
  25  * The nettle library is distributed in the hope that it will be useful, but
  26  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  27  * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  28  * License for more details.
  29  *
  30  * You should have received a copy of the GNU Lesser General Public License
  31  * along with the nettle library; see the file COPYING.LIB.  If not, write to
  32  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
  33  * MA 02111-1301, USA.
  34  */
  35
  36 #if HAVE_CONFIG_H
  37 # include "config.h"
  38 #endif
  39
  40 #include <assert.h>
  41 #include <stdlib.h>
  42 #include <string.h>
  43
  44 #include "gcm.h"
  45
  46 #include "memxor.h"
  47 #include "nettle-internal.h"
  48 #include "macros.h"
  49
  50 #define GHASH_POLYNOMIAL 0xE1UL
  51
  52 static void
  53 gcm_gf_add (union gcm_block *r, const union gcm_block *x, const union gcm_block *y)
  54 {
  55   r->w[0] = x->w[0] ^ y->w[0];
  56   r->w[1] = x->w[1] ^ y->w[1];
  57 #if SIZEOF_LONG == 4
  58   r->w[2] = x->w[2] ^ y->w[2];
  59   r->w[3] = x->w[3] ^ y->w[3];
  60 #endif
  61 }
  62 /* Multiplication by 010...0; a big-endian shift right. If the bit
  63    shifted out is one, the defining polynomial is added to cancel it
  64    out. r == x is allowed. */
  65 static void
  66 gcm_gf_shift (union gcm_block *r, const union gcm_block *x)
  67 {
  68   long mask;
  69
  70   /* Shift uses big-endian representation. */
  71 #if WORDS_BIGENDIAN
  72 # if SIZEOF_LONG == 4
  73   mask = - (x->w[3] & 1);
  74   r->w[3] = (x->w[3] >> 1) | ((x->w[2] & 1) << 31);
  75   r->w[2] = (x->w[2] >> 1) | ((x->w[1] & 1) << 31);
  76   r->w[1] = (x->w[1] >> 1) | ((x->w[0] & 1) << 31);
  77   r->w[0] = (x->w[0] >> 1) ^ (mask & (GHASH_POLYNOMIAL << 24));
  78 # elif SIZEOF_LONG == 8
  79   mask = - (x->w[1] & 1);
  80   r->w[1] = (x->w[1] >> 1) | ((x->w[0] & 1) << 63);
  81   r->w[0] = (x->w[0] >> 1) ^ (mask & (GHASH_POLYNOMIAL << 56));
  82 # else
  83 #  error Unsupported word size. */
  84 #endif
  85 #else /* ! WORDS_BIGENDIAN */
  86 # if SIZEOF_LONG == 4
  87 #define RSHIFT_WORD(x) \
  88   ((((x) & 0xfefefefeUL) >> 1) \
  89    | (((x) & 0x00010101) << 15))
  90   mask = - ((x->w[3] >> 24) & 1);
  91   r->w[3] = RSHIFT_WORD(x->w[3]) | ((x->w[2] >> 17) & 0x80);
  92   r->w[2] = RSHIFT_WORD(x->w[2]) | ((x->w[1] >> 17) & 0x80);
  93   r->w[1] = RSHIFT_WORD(x->w[1]) | ((x->w[0] >> 17) & 0x80);
  94   r->w[0] = RSHIFT_WORD(x->w[0]) ^ (mask & GHASH_POLYNOMIAL);
  95 # elif SIZEOF_LONG == 8
  96 #define RSHIFT_WORD(x) \
  97   ((((x) & 0xfefefefefefefefeUL) >> 1) \
  98    | (((x) & 0x0001010101010101UL) << 15))
  99   mask = - ((x->w[1] >> 56) & 1);
 100   r->w[1] = RSHIFT_WORD(x->w[1]) | ((x->w[0] >> 49) & 0x80);
 101   r->w[0] = RSHIFT_WORD(x->w[0]) ^ (mask & GHASH_POLYNOMIAL);
 102 # else
 103 #  error Unsupported word size. */
 104 # endif
 105 # undef RSHIFT_WORD
 106 #endif /* ! WORDS_BIGENDIAN */
 107 }
 108
 109 #if GCM_TABLE_BITS == 0
 110 /* Sets x <- x * y mod r, using the plain bitwise algorithm from the
 111    specification. y may be shorter than a full block, missing bytes
 112    are assumed zero. */
 113 static void
 114 gcm_gf_mul (union gcm_block *x, const union gcm_block *y)
 115 {
 116   union gcm_block V;
 117   union gcm_block Z;
 118   unsigned i;
 119
 120   memcpy(V.b, x, sizeof(V));
 121   memset(Z.b, 0, sizeof(Z));
 122
 123   for (i = 0; i < GCM_BLOCK_SIZE; i++)
 124     {
 125       uint8_t b = y->b[i];
 126       unsigned j;
 127       for (j = 0; j < 8; j++, b <<= 1)
 128         {
 129           if (b & 0x80)
 130             gcm_gf_add(&Z, &Z, &V);
 131
 132           gcm_gf_shift(&V, &V);
 133         }
 134     }
 135   memcpy (x->b, Z.b, sizeof(Z));
 136 }
 137 #else /* GCM_TABLE_BITS != 0 */
 138
 139 # if WORDS_BIGENDIAN
 140 #  define W(left,right) (0x##left##right)
 141 # else
 142 #  define W(left,right) (0x##right##left)
 143 # endif
 144
 145 # if GCM_TABLE_BITS == 4
 146 static const uint16_t
 147 shift_table[0x10] = {
 148   W(00,00),W(1c,20),W(38,40),W(24,60),W(70,80),W(6c,a0),W(48,c0),W(54,e0),
 149   W(e1,00),W(fd,20),W(d9,40),W(c5,60),W(91,80),W(8d,a0),W(a9,c0),W(b5,e0),
 150 };
 151
 152 static void
 153 gcm_gf_shift_4(union gcm_block *x)
 154 {
 155   unsigned long *w = x->w;
 156   unsigned long reduce;
 157
 158   /* Shift uses big-endian representation. */
 159 #if WORDS_BIGENDIAN
 160 # if SIZEOF_LONG == 4
 161   reduce = shift_table[w[3] & 0xf];
 162   w[3] = (w[3] >> 4) | ((w[2] & 0xf) << 28);
 163   w[2] = (w[2] >> 4) | ((w[1] & 0xf) << 28);
 164   w[1] = (w[1] >> 4) | ((w[0] & 0xf) << 28);
 165   w[0] = (w[0] >> 4) ^ (reduce << 16);
 166 # elif SIZEOF_LONG == 8
 167   reduce = shift_table[w[1] & 0xf];
 168   w[1] = (w[1] >> 4) | ((w[0] & 0xf) << 60);
 169   w[0] = (w[0] >> 4) ^ (reduce << 48);
 170 # else
 171 #  error Unsupported word size. */
 172 #endif
 173 #else /* ! WORDS_BIGENDIAN */
 174 # if SIZEOF_LONG == 4
 175 #define RSHIFT_WORD(x) \
 176   ((((x) & 0xf0f0f0f0UL) >> 4)                  \
 177    | (((x) & 0x000f0f0f) << 12))
 178   reduce = shift_table[(w[3] >> 24) & 0xf];
 179   w[3] = RSHIFT_WORD(w[3]) | ((w[2] >> 20) & 0xf0);
 180   w[2] = RSHIFT_WORD(w[2]) | ((w[1] >> 20) & 0xf0);
 181   w[1] = RSHIFT_WORD(w[1]) | ((w[0] >> 20) & 0xf0);
 182   w[0] = RSHIFT_WORD(w[0]) ^ reduce;
 183 # elif SIZEOF_LONG == 8
 184 #define RSHIFT_WORD(x) \
 185   ((((x) & 0xf0f0f0f0f0f0f0f0UL) >> 4) \
 186    | (((x) & 0x000f0f0f0f0f0f0fUL) << 12))
 187   reduce = shift_table[(w[1] >> 56) & 0xf];
 188   w[1] = RSHIFT_WORD(w[1]) | ((w[0] >> 52) & 0xf0);
 189   w[0] = RSHIFT_WORD(w[0]) ^ reduce;
 190 # else
 191 #  error Unsupported word size. */
 192 # endif
 193 # undef RSHIFT_WORD
 194 #endif /* ! WORDS_BIGENDIAN */
 195 }
 196
 197 static void
 198 gcm_gf_mul (union gcm_block *x, const union gcm_block *table)
 199 {
 200   union gcm_block Z;
 201   unsigned i;
 202
 203   memset(Z.b, 0, sizeof(Z));
 204
 205   for (i = GCM_BLOCK_SIZE; i-- > 0;)
 206     {
 207       uint8_t b = x->b[i];
 208
 209       gcm_gf_shift_4(&Z);
 210       gcm_gf_add(&Z, &Z, &table[b & 0xf]);
 211       gcm_gf_shift_4(&Z);
 212       gcm_gf_add(&Z, &Z, &table[b >> 4]);
 213     }
 214   memcpy (x->b, Z.b, sizeof(Z));
 215 }
 216 # elif GCM_TABLE_BITS == 8
 217 static const uint16_t
 218 shift_table[0x100] = {
 219   W(00,00),W(01,c2),W(03,84),W(02,46),W(07,08),W(06,ca),W(04,8c),W(05,4e),
 220   W(0e,10),W(0f,d2),W(0d,94),W(0c,56),W(09,18),W(08,da),W(0a,9c),W(0b,5e),
 221   W(1c,20),W(1d,e2),W(1f,a4),W(1e,66),W(1b,28),W(1a,ea),W(18,ac),W(19,6e),
 222   W(12,30),W(13,f2),W(11,b4),W(10,76),W(15,38),W(14,fa),W(16,bc),W(17,7e),
 223   W(38,40),W(39,82),W(3b,c4),W(3a,06),W(3f,48),W(3e,8a),W(3c,cc),W(3d,0e),
 224   W(36,50),W(37,92),W(35,d4),W(34,16),W(31,58),W(30,9a),W(32,dc),W(33,1e),
 225   W(24,60),W(25,a2),W(27,e4),W(26,26),W(23,68),W(22,aa),W(20,ec),W(21,2e),
 226   W(2a,70),W(2b,b2),W(29,f4),W(28,36),W(2d,78),W(2c,ba),W(2e,fc),W(2f,3e),
 227   W(70,80),W(71,42),W(73,04),W(72,c6),W(77,88),W(76,4a),W(74,0c),W(75,ce),
 228   W(7e,90),W(7f,52),W(7d,14),W(7c,d6),W(79,98),W(78,5a),W(7a,1c),W(7b,de),
 229   W(6c,a0),W(6d,62),W(6f,24),W(6e,e6),W(6b,a8),W(6a,6a),W(68,2c),W(69,ee),
 230   W(62,b0),W(63,72),W(61,34),W(60,f6),W(65,b8),W(64,7a),W(66,3c),W(67,fe),
 231   W(48,c0),W(49,02),W(4b,44),W(4a,86),W(4f,c8),W(4e,0a),W(4c,4c),W(4d,8e),
 232   W(46,d0),W(47,12),W(45,54),W(44,96),W(41,d8),W(40,1a),W(42,5c),W(43,9e),
 233   W(54,e0),W(55,22),W(57,64),W(56,a6),W(53,e8),W(52,2a),W(50,6c),W(51,ae),
 234   W(5a,f0),W(5b,32),W(59,74),W(58,b6),W(5d,f8),W(5c,3a),W(5e,7c),W(5f,be),
 235   W(e1,00),W(e0,c2),W(e2,84),W(e3,46),W(e6,08),W(e7,ca),W(e5,8c),W(e4,4e),
 236   W(ef,10),W(ee,d2),W(ec,94),W(ed,56),W(e8,18),W(e9,da),W(eb,9c),W(ea,5e),
 237   W(fd,20),W(fc,e2),W(fe,a4),W(ff,66),W(fa,28),W(fb,ea),W(f9,ac),W(f8,6e),
 238   W(f3,30),W(f2,f2),W(f0,b4),W(f1,76),W(f4,38),W(f5,fa),W(f7,bc),W(f6,7e),
 239   W(d9,40),W(d8,82),W(da,c4),W(db,06),W(de,48),W(df,8a),W(dd,cc),W(dc,0e),
 240   W(d7,50),W(d6,92),W(d4,d4),W(d5,16),W(d0,58),W(d1,9a),W(d3,dc),W(d2,1e),
 241   W(c5,60),W(c4,a2),W(c6,e4),W(c7,26),W(c2,68),W(c3,aa),W(c1,ec),W(c0,2e),
 242   W(cb,70),W(ca,b2),W(c8,f4),W(c9,36),W(cc,78),W(cd,ba),W(cf,fc),W(ce,3e),
 243   W(91,80),W(90,42),W(92,04),W(93,c6),W(96,88),W(97,4a),W(95,0c),W(94,ce),
 244   W(9f,90),W(9e,52),W(9c,14),W(9d,d6),W(98,98),W(99,5a),W(9b,1c),W(9a,de),
 245   W(8d,a0),W(8c,62),W(8e,24),W(8f,e6),W(8a,a8),W(8b,6a),W(89,2c),W(88,ee),
 246   W(83,b0),W(82,72),W(80,34),W(81,f6),W(84,b8),W(85,7a),W(87,3c),W(86,fe),
 247   W(a9,c0),W(a8,02),W(aa,44),W(ab,86),W(ae,c8),W(af,0a),W(ad,4c),W(ac,8e),
 248   W(a7,d0),W(a6,12),W(a4,54),W(a5,96),W(a0,d8),W(a1,1a),W(a3,5c),W(a2,9e),
 249   W(b5,e0),W(b4,22),W(b6,64),W(b7,a6),W(b2,e8),W(b3,2a),W(b1,6c),W(b0,ae),
 250   W(bb,f0),W(ba,32),W(b8,74),W(b9,b6),W(bc,f8),W(bd,3a),W(bf,7c),W(be,be),
 251 };
 252
 253 static void
 254 gcm_gf_shift_8(union gcm_block *x)
 255 {
 256   unsigned long *w = x->w;
 257   unsigned long reduce;
 258
 259   /* Shift uses big-endian representation. */
 260 #if WORDS_BIGENDIAN
 261 # if SIZEOF_LONG == 4
 262   reduce = shift_table[w[3] & 0xff];
 263   w[3] = (w[3] >> 8) | ((w[2] & 0xff) << 24);
 264   w[2] = (w[2] >> 8) | ((w[1] & 0xff) << 24);
 265   w[1] = (w[1] >> 8) | ((w[0] & 0xff) << 24);
 266   w[0] = (w[0] >> 8) ^ (reduce << 16);
 267 # elif SIZEOF_LONG == 8
 268   reduce = shift_table[w[1] & 0xff];
 269   w[1] = (w[1] >> 8) | ((w[0] & 0xff) << 56);
 270   w[0] = (w[0] >> 8) ^ (reduce << 48);
 271 # else
 272 #  error Unsupported word size. */
 273 #endif
 274 #else /* ! WORDS_BIGENDIAN */
 275 # if SIZEOF_LONG == 4
 276   reduce = shift_table[(w[3] >> 24) & 0xff];
 277   w[3] = (w[3] << 8) | (w[2] >> 24);
 278   w[2] = (w[2] << 8) | (w[1] >> 24);
 279   w[1] = (w[1] << 8) | (w[0] >> 24);
 280   w[0] = (w[0] << 8) ^ reduce;
 281 # elif SIZEOF_LONG == 8
 282   reduce = shift_table[(w[1] >> 56) & 0xff];
 283   w[1] = (w[1] << 8) | (w[0] >> 56);
 284   w[0] = (w[0] << 8) ^ reduce;
 285 # else
 286 #  error Unsupported word size. */
 287 # endif
 288 #endif /* ! WORDS_BIGENDIAN */
 289 }
 290
 291 static void
 292 gcm_gf_mul (union gcm_block *x, const union gcm_block *table)
 293 {
 294   union gcm_block Z;
 295   unsigned i;
 296
 297   memcpy(Z.b, table[x->b[GCM_BLOCK_SIZE-1]].b, GCM_BLOCK_SIZE);
 298
 299   for (i = GCM_BLOCK_SIZE-2; i > 0; i--)
 300     {
 301       gcm_gf_shift_8(&Z);
 302       gcm_gf_add(&Z, &Z, &table[x->b[i]]);
 303     }
 304   gcm_gf_shift_8(&Z);
 305   gcm_gf_add(x, &Z, &table[x->b[0]]);
 306 }
 307
 308 # else /* GCM_TABLE_BITS != 8 */
 309 #  error Unsupported table size.
 310 # endif /* GCM_TABLE_BITS != 8 */
 311
 312 #undef W
 313
 314 #endif /* GCM_TABLE_BITS */
 315
 316 /* Increment the rightmost 32 bits. */
 317 #define INC32(block) INCREMENT(4, (block.b) + GCM_BLOCK_SIZE - 4)
 318
 319 /* Initialization of GCM.
 320  * @ctx: The context of GCM
 321  * @cipher: The context of the underlying block cipher
 322  * @f: The underlying cipher encryption function
 323  */
 324 void
 325 gcm_set_key(struct gcm_key *key,
 326             void *cipher, nettle_crypt_func *f)
 327 {
 328   /* Middle element if GCM_TABLE_BITS > 0, otherwise the first
 329      element */
 330   unsigned i = (1<<GCM_TABLE_BITS)/2;
 331
 332   /* H */
 333   memset(key->h[0].b, 0, GCM_BLOCK_SIZE);
 334   f (cipher, GCM_BLOCK_SIZE, key->h[i].b, key->h[0].b);
 335
 336 #if GCM_TABLE_BITS
 337   /* Algorithm 3 from the gcm paper. First do powers of two, then do
 338      the rest by adding. */
 339   while (i /= 2)
 340     gcm_gf_shift(&key->h[i], &key->h[2*i]);
 341   for (i = 2; i < 1<<GCM_TABLE_BITS; i *= 2)
 342     {
 343       unsigned j;
 344       for (j = 1; j < i; j++)
 345         gcm_gf_add(&key->h[i+j], &key->h[i],&key->h[j]);
 346     }
 347 #endif
 348 }
 349
 350 static void
 351 gcm_hash(const struct gcm_key *key, union gcm_block *x,
 352          unsigned length, const uint8_t *data)
 353 {
 354   for (; length >= GCM_BLOCK_SIZE;
 355        length -= GCM_BLOCK_SIZE, data += GCM_BLOCK_SIZE)
 356     {
 357       memxor (x->b, data, GCM_BLOCK_SIZE);
 358       gcm_gf_mul (x, key->h);
 359     }
 360   if (length > 0)
 361     {
 362       memxor (x->b, data, length);
 363       gcm_gf_mul (x, key->h);
 364     }
 365 }
 366
 367 static void
 368 gcm_hash_sizes(const struct gcm_key *key, union gcm_block *x,
 369                uint64_t auth_size, uint64_t data_size)
 370 {
 371   uint8_t buffer[GCM_BLOCK_SIZE];
 372
 373   data_size *= 8;
 374   auth_size *= 8;
 375
 376   WRITE_UINT64 (buffer, auth_size);
 377   WRITE_UINT64 (buffer + 8, data_size);
 378
 379   gcm_hash(key, x, GCM_BLOCK_SIZE, buffer);
 380 }
 381
 382 /*
 383  * @length: The size of the iv (fixed for now to GCM_NONCE_SIZE)
 384  * @iv: The iv
 385  */
 386 void
 387 gcm_set_iv(struct gcm_ctx *ctx, const struct gcm_key *key,
 388            unsigned length, const uint8_t *iv)
 389 {
 390   if (length == GCM_IV_SIZE)
 391     {
 392       memcpy (ctx->iv.b, iv, GCM_BLOCK_SIZE - 4);
 393       ctx->iv.b[GCM_BLOCK_SIZE - 4] = 0;
 394       ctx->iv.b[GCM_BLOCK_SIZE - 3] = 0;
 395       ctx->iv.b[GCM_BLOCK_SIZE - 2] = 0;
 396       ctx->iv.b[GCM_BLOCK_SIZE - 1] = 1;
 397     }
 398   else
 399     {
 400       memset(ctx->iv.b, 0, GCM_BLOCK_SIZE);
 401       gcm_hash(key, &ctx->iv, length, iv);
 402       gcm_hash_sizes(key, &ctx->iv, 0, length);
 403     }
 404
 405   memcpy (ctx->ctr.b, ctx->iv.b, GCM_BLOCK_SIZE);
 406   INC32 (ctx->ctr);
 407
 408   /* Reset the rest of the message-dependent state. */
 409   memset(ctx->x.b, 0, sizeof(ctx->x));
 410   ctx->auth_size = ctx->data_size = 0;
 411 }
 412
 413 void
 414 gcm_update(struct gcm_ctx *ctx, const struct gcm_key *key,
 415            unsigned length, const uint8_t *data)
 416 {
 417   assert(ctx->auth_size % GCM_BLOCK_SIZE == 0);
 418   assert(ctx->data_size == 0);
 419
 420   gcm_hash(key, &ctx->x, length, data);
 421
 422   ctx->auth_size += length;
 423 }
 424
 425 static void
 426 gcm_crypt(struct gcm_ctx *ctx, void *cipher, nettle_crypt_func *f,
 427           unsigned length, uint8_t *dst, const uint8_t *src)
 428 {
 429   uint8_t buffer[GCM_BLOCK_SIZE];
 430
 431   if (src != dst)
 432     {
 433       for (; length >= GCM_BLOCK_SIZE;
 434            (length -= GCM_BLOCK_SIZE,
 435             src += GCM_BLOCK_SIZE, dst += GCM_BLOCK_SIZE))
 436         {
 437           f (cipher, GCM_BLOCK_SIZE, dst, ctx->ctr.b);
 438           memxor (dst, src, GCM_BLOCK_SIZE);
 439           INC32 (ctx->ctr);
 440         }
 441     }
 442   else
 443     {
 444       for (; length >= GCM_BLOCK_SIZE;
 445            (length -= GCM_BLOCK_SIZE,
 446             src += GCM_BLOCK_SIZE, dst += GCM_BLOCK_SIZE))
 447         {
 448           f (cipher, GCM_BLOCK_SIZE, buffer, ctx->ctr.b);
 449           memxor3 (dst, src, buffer, GCM_BLOCK_SIZE);
 450           INC32 (ctx->ctr);
 451         }
 452     }
 453   if (length > 0)
 454     {
 455       /* A final partial block */
 456       f (cipher, GCM_BLOCK_SIZE, buffer, ctx->ctr.b);
 457       memxor3 (dst, src, buffer, length);
 458       INC32 (ctx->ctr);
 459     }
 460 }
 461
 462 void
 463 gcm_encrypt (struct gcm_ctx *ctx, const struct gcm_key *key,
 464              void *cipher, nettle_crypt_func *f,
 465              unsigned length, uint8_t *dst, const uint8_t *src)
 466 {
 467   assert(ctx->data_size % GCM_BLOCK_SIZE == 0);
 468
 469   gcm_crypt(ctx, cipher, f, length, dst, src);
 470   gcm_hash(key, &ctx->x, length, dst);
 471
 472   ctx->data_size += length;
 473 }
 474
 475 void
 476 gcm_decrypt(struct gcm_ctx *ctx, const struct gcm_key *key,
 477             void *cipher, nettle_crypt_func *f,
 478             unsigned length, uint8_t *dst, const uint8_t *src)
 479 {
 480   assert(ctx->data_size % GCM_BLOCK_SIZE == 0);
 481
 482   gcm_hash(key, &ctx->x, length, src);
 483   gcm_crypt(ctx, cipher, f, length, dst, src);
 484
 485   ctx->data_size += length;
 486 }
 487
 488 void
 489 gcm_digest(struct gcm_ctx *ctx, const struct gcm_key *key,
 490            void *cipher, nettle_crypt_func *f,
 491            unsigned length, uint8_t *digest)
 492 {
 493   uint8_t buffer[GCM_BLOCK_SIZE];
 494
 495   assert (length <= GCM_BLOCK_SIZE);
 496
 497   gcm_hash_sizes(key, &ctx->x, ctx->auth_size, ctx->data_size);
 498
 499   f (cipher, GCM_BLOCK_SIZE, buffer, ctx->iv.b);
 500   memxor3 (digest, ctx->x.b, buffer, length);
 501
 502   return;
 503 }