mdw@git.distorted.org.uk Git - tripe/blob - keyset.c

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Handling of symmetric keysets
   6  *
   7  * (c) 2001 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Trivial IP Encryption (TrIPE).
  13  *
  14  * TrIPE is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU General Public License as published by
  16  * the Free Software Foundation; either version 2 of the License, or
  17  * (at your option) any later version.
  18  *
  19  * TrIPE is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU General Public License
  25  * along with TrIPE; if not, write to the Free Software Foundation,
  26  * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  27  */
  28
  29 /*----- Header files ------------------------------------------------------*/
  30
  31 #include "tripe.h"
  32
  33 /*----- Tunable parameters ------------------------------------------------*/
  34
  35 /* --- Note on size limits --- *
  36  *
  37  * For a 64-bit block cipher (e.g., Blowfish), the probability of a collision
  38  * occurring after 32 MB is less than %$2^{-21}$%, and the probability of a
  39  * collision occurring after 64 MB is less than %$2^{-19}$%.  These could be
  40  * adjusted dependent on the encryption scheme, but it's too much pain.
  41  */
  42
  43 #define T_EXP MIN(60)                   /* Expiry time for a key */
  44 #define T_REGEN MIN(45)                 /* Regeneration time for a key */
  45 #define SZ_EXP MEG(64)                  /* Expiry data size for a key */
  46 #define SZ_REGEN MEG(32)                /* Data size threshold for regen */
  47
  48 /*----- Handy macros ------------------------------------------------------*/
  49
  50 #define KEYOK(ks, now) ((ks)->sz_exp > 0 && (ks)->t_exp > now)
  51
  52 #define SEQSZ 4                         /* Size of sequence number packet */
  53
  54 /*----- Low-level packet encryption and decryption ------------------------*/
  55
  56 /* --- Encrypted data format --- *
  57  *
  58  * Let %$p_i$% be the %$i$%-th plaintext message, with type %$t$%.  We first
  59  * compute
  60  *
  61  *   %$c_i = \mathcal{E}\textrm{-CBC}_{K_{\text{E}}}(p_i)$%
  62  *
  63  * as the CBC-ciphertext of %$p_i$%, and then
  64  *
  65  *   %$\sigma_i = \mathcal{T}_{K_{\text{M}}}(t, i, c_i)$%
  66  *
  67  * as a MAC on the %%\emph{ciphertext}%%.  The message sent is then the pair
  68  * %$(\sigma_i, c_i)$%.  This construction is provably secure in the NM-CCA
  69  * sense (assuming that the cipher is IND-CPA, and the MAC is SUF-CMA)
  70  * [Bellare and Namprempre].
  71  *
  72  * This also ensures that, assuming the key is good, we have a secure channel
  73  * [Krawczyk].  Actually, [Krawczyk] shows that, if the cipher is either a
  74  * simple stream cipher or a block cipher in CBC mode, we can use the MAC-
  75  * then-encrypt scheme and still have a secure channel.  However, I like the
  76  * NM-CCA guarantee from [Bellare and Namprempre].  I'm less worried about
  77  * the Horton Principle [Wagner and Schneier].
  78  */
  79
  80 /* --- @doencrypt@ --- *
  81  *
  82  * Arguments:   @keyset *ks@ = pointer to keyset to use
  83  *              @unsigned ty@ = type of message this is
  84  *              @buf *b@ = pointer to an input buffer
  85  *              @buf *bb@ = pointer to an output buffer
  86  *
  87  * Returns:     Zero if OK, nonzero if a new key is required.
  88  *
  89  * Use:         Encrypts a message with the given key.  We assume that the
  90  *              keyset is OK to use.
  91  */
  92
  93 static int doencrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
  94 {
  95   ghash *h;
  96   gcipher *c = ks->cout;
  97   const octet *p = BCUR(b);
  98   size_t sz = BLEFT(b);
  99   octet *qmac, *qseq, *qiv, *qpk;
 100   uint32 oseq;
 101   size_t ivsz = GC_CLASS(c)->blksz;
 102   size_t tagsz = ks->tagsz;
 103   size_t osz, nsz;
 104   octet t[4];
 105   int rc = 0;
 106
 107   /* --- Allocate the required buffer space --- */
 108
 109   if (buf_ensure(bb, tagsz + SEQSZ + ivsz + sz))
 110     return (0); /* Caution! */
 111   qmac = BCUR(bb); qseq = qmac + tagsz; qiv = qseq + SEQSZ; qpk = qiv + ivsz;
 112   BSTEP(bb, tagsz + SEQSZ + ivsz + sz);
 113   STORE32(t, ty);
 114
 115   oseq = ks->oseq++; STORE32(qseq, oseq);
 116   IF_TRACING(T_KEYSET, {
 117     trace(T_KEYSET, "keyset: encrypting packet %lu using keyset %u",
 118           (unsigned long)oseq, ks->seq);
 119     trace_block(T_CRYPTO, "crypto: plaintext packet", p, sz);
 120   })
 121
 122   /* --- Encrypt the packet --- */
 123
 124   if (ivsz) {
 125     rand_get(RAND_GLOBAL, qiv, ivsz);
 126     GC_SETIV(c, qiv);
 127     IF_TRACING(T_KEYSET, {
 128       trace_block(T_CRYPTO, "crypto: initialization vector", qiv, ivsz);
 129     })
 130   }
 131   GC_ENCRYPT(c, p, qpk, sz);
 132   IF_TRACING(T_KEYSET, {
 133     trace_block(T_CRYPTO, "crypto: encrypted packet", qpk, sz);
 134   })
 135
 136   /* --- Now compute the MAC --- */
 137
 138   if (tagsz) {
 139     h = GM_INIT(ks->mout);
 140     GH_HASH(h, t, sizeof(t));
 141     GH_HASH(h, qseq, SEQSZ + ivsz + sz);
 142     memcpy(qmac, GH_DONE(h, 0), tagsz);
 143     GH_DESTROY(h);
 144     IF_TRACING(T_KEYSET, {
 145       trace_block(T_CRYPTO, "crypto: computed MAC", qmac, tagsz);
 146     })
 147   }
 148
 149   /* --- Deduct the packet size from the key's data life --- */
 150
 151   osz = ks->sz_exp;
 152   if (osz > sz)
 153     nsz = osz - sz;
 154   else
 155     nsz = 0;
 156   if (osz >= SZ_REGEN && nsz < SZ_REGEN) {
 157     T( trace(T_KEYSET, "keyset: keyset %u data regen limit exceeded -- "
 158              "forcing exchange", ks->seq); )
 159     rc = -1;
 160   }
 161   ks->sz_exp = nsz;
 162   return (rc);
 163 }
 164
 165 /* --- @dodecrypt@ --- *
 166  *
 167  * Arguments:   @keyset *ks@ = pointer to keyset to use
 168  *              @unsigned ty@ = expected type code
 169  *              @buf *b@ = pointer to an input buffer
 170  *              @buf *bb@ = pointer to an output buffer
 171  *              @uint32 *seq@ = where to store the sequence number
 172  *
 173  * Returns:     Zero if OK, nonzero if it failed.
 174  *
 175  * Use:         Attempts to decrypt a message with the given key.  No other
 176  *              checking (e.g., sequence number checks) is performed.  We
 177  *              assume that the keyset is OK to use, and that there is
 178  *              sufficient output buffer space reserved.  If the decryption
 179  *              is successful, the buffer pointer is moved past the decrypted
 180  *              packet, and the packet's sequence number is stored in @*seq@.
 181  */
 182
 183 static int dodecrypt(keyset *ks, unsigned ty, buf *b, buf *bb, uint32 *seq)
 184 {
 185   const octet *pmac, *piv, *pseq, *ppk;
 186   size_t psz = BLEFT(b);
 187   size_t sz;
 188   octet *q = BCUR(bb);
 189   ghash *h;
 190   gcipher *c = ks->cin;
 191   size_t ivsz = GC_CLASS(c)->blksz;
 192   size_t tagsz = ks->tagsz;
 193   octet *mac;
 194   int eq;
 195   octet t[4];
 196
 197   /* --- Break up the packet into its components --- */
 198
 199   if (psz < ivsz + SEQSZ + tagsz) {
 200     T( trace(T_KEYSET, "keyset: block too small for keyset %u", ks->seq); )
 201     return (-1);
 202   }
 203   sz = psz - ivsz - SEQSZ - tagsz;
 204   pmac = BCUR(b); pseq = pmac + tagsz; piv = pseq + SEQSZ; ppk = piv + ivsz;
 205   STORE32(t, ty);
 206
 207   IF_TRACING(T_KEYSET, {
 208     trace(T_KEYSET, "keyset: decrypting using keyset %u", ks->seq);
 209     trace_block(T_CRYPTO, "crypto: ciphertext packet", ppk, sz);
 210   })
 211
 212   /* --- Verify the MAC on the packet --- */
 213
 214   if (tagsz) {
 215     h = GM_INIT(ks->min);
 216     GH_HASH(h, t, sizeof(t));
 217     GH_HASH(h, pseq, SEQSZ + ivsz + sz);
 218     mac = GH_DONE(h, 0);
 219     eq = !memcmp(mac, pmac, tagsz);
 220     IF_TRACING(T_KEYSET, {
 221       trace_block(T_CRYPTO, "crypto: computed MAC", mac, tagsz);
 222     })
 223     GH_DESTROY(h);
 224     if (!eq) {
 225       IF_TRACING(T_KEYSET, {
 226         trace(T_KEYSET, "keyset: incorrect MAC: decryption failed");
 227         trace_block(T_CRYPTO, "crypto: expected MAC", pmac, tagsz);
 228       })
 229       return (-1);
 230     }
 231   }
 232
 233   /* --- Decrypt the packet --- */
 234
 235   if (ivsz) {
 236     GC_SETIV(c, piv);
 237     IF_TRACING(T_KEYSET, {
 238       trace_block(T_CRYPTO, "crypto: initialization vector", piv, ivsz);
 239     })
 240   }
 241   GC_DECRYPT(c, ppk, q, sz);
 242   if (seq)
 243     *seq = LOAD32(pseq);
 244   IF_TRACING(T_KEYSET, {
 245     trace(T_KEYSET, "keyset: decrypted OK (sequence = %lu)",
 246           (unsigned long)LOAD32(pseq));
 247     trace_block(T_CRYPTO, "crypto: decrypted packet", q, sz);
 248   })
 249   BSTEP(bb, sz);
 250   return (0);
 251 }
 252
 253 /*----- Operations on a single keyset -------------------------------------*/
 254
 255 /* --- @ks_drop@ --- *
 256  *
 257  * Arguments:   @keyset *ks@ = pointer to a keyset
 258  *
 259  * Returns:     ---
 260  *
 261  * Use:         Decrements a keyset's reference counter.  If the counter hits
 262  *              zero, the keyset is freed.
 263  */
 264
 265 void ks_drop(keyset *ks)
 266 {
 267   if (--ks->ref)
 268     return;
 269   GC_DESTROY(ks->cin);
 270   GC_DESTROY(ks->cout);
 271   GM_DESTROY(ks->min);
 272   GM_DESTROY(ks->mout);
 273   DESTROY(ks);
 274 }
 275
 276 /* --- @ks_gen@ --- *
 277  *
 278  * Arguments:   @const void *k@ = pointer to key material
 279  *              @size_t x, y, z@ = offsets into key material (see below)
 280  *              @peer *p@ = pointer to peer information
 281  *
 282  * Returns:     A pointer to the new keyset.
 283  *
 284  * Use:         Derives a new keyset from the given key material.  The
 285  *              offsets @x@, @y@ and @z@ separate the key material into three
 286  *              parts.  Between the @k@ and @k + x@ is `my' contribution to
 287  *              the key material; between @k + x@ and @k + y@ is `your'
 288  *              contribution; and between @k + y@ and @k + z@ is a shared
 289  *              value we made together.  These are used to construct two
 290  *              pairs of symmetric keys.  Each pair consists of an encryption
 291  *              key and a message authentication key.  One pair is used for
 292  *              outgoing messages, the other for incoming messages.
 293  *
 294  *              The new key is marked so that it won't be selected for output
 295  *              by @ksl_encrypt@.  You can still encrypt data with it by
 296  *              calling @ks_encrypt@ directly.
 297  */
 298
 299 keyset *ks_gen(const void *k, size_t x, size_t y, size_t z, peer *p)
 300 {
 301   ghash *h;
 302   const octet *hh;
 303   keyset *ks = CREATE(keyset);
 304   time_t now = time(0);
 305   const octet *pp = k;
 306   T( static unsigned seq = 0; )
 307
 308   T( trace(T_KEYSET, "keyset: adding new keyset %u", seq); )
 309
 310   /* --- Construct the various keys --- *
 311    *
 312    * This is done with macros, because it's quite tedious.
 313    */
 314
 315 #define MINE GH_HASH(h, pp, x)
 316 #define YOURS GH_HASH(h, pp + x, y - x)
 317 #define OURS GH_HASH(h, pp + y, z - y)
 318
 319 #define HASH_in MINE; YOURS; OURS
 320 #define HASH_out YOURS; MINE; OURS
 321 #define INIT_c(k) GC_INIT(algs.c, (k), algs.cksz)
 322 #define INIT_m(k) GM_KEY(algs.m, (k), algs.mksz)
 323 #define STR_c "encryption"
 324 #define STR_m "integrity"
 325 #define STR_in "incoming"
 326 #define STR_out "outgoing"
 327
 328 #define SETKEY(a, dir) do {                                             \
 329   h = GH_INIT(algs.h);                                                  \
 330   HASH_STRING(h, "tripe-" STR_##a);                                     \
 331   HASH_##dir;                                                           \
 332   hh = GH_DONE(h, 0);                                                   \
 333   IF_TRACING(T_KEYSET, {                                                \
 334     trace_block(T_CRYPTO, "crypto: " STR_##dir " key " STR_##a,         \
 335                 hh, algs.a##ksz);                                       \
 336   })                                                                    \
 337   ks->a##dir = INIT_##a(hh);                                            \
 338   GH_DESTROY(h);                                                        \
 339 } while (0)
 340
 341   SETKEY(c, in); SETKEY(c, out);
 342   SETKEY(m, in); SETKEY(m, out);
 343
 344 #undef MINE
 345 #undef YOURS
 346 #undef OURS
 347 #undef STR_c
 348 #undef STR_m
 349 #undef STR_in
 350 #undef STR_out
 351 #undef INIT_c
 352 #undef INIT_m
 353 #undef HASH_in
 354 #undef HASH_out
 355 #undef SETKEY
 356
 357   T( ks->seq = seq++; )
 358   ks->ref = 1;
 359   ks->t_exp = now + T_EXP;
 360   ks->sz_exp = SZ_EXP;
 361   ks->oseq = 0;
 362   seq_reset(&ks->iseq);
 363   ks->next = 0;
 364   ks->p = p;
 365   ks->f = KSF_LISTEN;
 366   ks->tagsz = algs.tagsz;
 367   return (ks);
 368 }
 369
 370 /* --- @ks_tregen@ --- *
 371  *
 372  * Arguments:   @keyset *ks@ = pointer to a keyset
 373  *
 374  * Returns:     The time at which moves ought to be made to replace this key.
 375  */
 376
 377 time_t ks_tregen(keyset *ks) { return (ks->t_exp - T_EXP + T_REGEN); }
 378
 379 /* --- @ks_activate@ --- *
 380  *
 381  * Arguments:   @keyset *ks@ = pointer to a keyset
 382  *
 383  * Returns:     ---
 384  *
 385  * Use:         Activates a keyset, so that it can be used for encrypting
 386  *              outgoing messages.
 387  */
 388
 389 void ks_activate(keyset *ks)
 390 {
 391   if (ks->f & KSF_LISTEN) {
 392     T( trace(T_KEYSET, "keyset: activating keyset %u", ks->seq); )
 393     ks->f &= ~KSF_LISTEN;
 394   }
 395 }
 396
 397 /* --- @ks_encrypt@ --- *
 398  *
 399  * Arguments:   @keyset *ks@ = pointer to a keyset
 400  *              @unsigned ty@ = message type
 401  *              @buf *b@ = pointer to input buffer
 402  *              @buf *bb@ = pointer to output buffer
 403  *
 404  * Returns:     Zero if OK, nonzero if the key needs replacing.  If the
 405  *              encryption failed, the output buffer is broken and zero is
 406  *              returned.
 407  *
 408  * Use:         Encrypts a block of data using the key.  Note that the `key
 409  *              ought to be replaced' notification is only ever given once
 410  *              for each key.  Also note that this call forces a keyset to be
 411  *              used even if it's marked as not for data output.
 412  */
 413
 414 int ks_encrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
 415 {
 416   time_t now = time(0);
 417
 418   if (!KEYOK(ks, now)) {
 419     buf_break(bb);
 420     return (0);
 421   }
 422   return (doencrypt(ks, ty, b, bb));
 423 }
 424
 425 /* --- @ks_decrypt@ --- *
 426  *
 427  * Arguments:   @keyset *ks@ = pointer to a keyset
 428  *              @unsigned ty@ = expected type code
 429  *              @buf *b@ = pointer to an input buffer
 430  *              @buf *bb@ = pointer to an output buffer
 431  *
 432  * Returns:     Zero on success, or nonzero if there was some problem.
 433  *
 434  * Use:         Attempts to decrypt a message using a given key.  Note that
 435  *              requesting decryption with a key directly won't clear a
 436  *              marking that it's not for encryption.
 437  */
 438
 439 int ks_decrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
 440 {
 441   time_t now = time(0);
 442   uint32 seq;
 443
 444   if (!KEYOK(ks, now) ||
 445       buf_ensure(bb, BLEN(b)) ||
 446       dodecrypt(ks, ty, b, bb, &seq) ||
 447       seq_check(&ks->iseq, seq, "SYMM"))
 448     return (-1);
 449   return (0);
 450 }
 451
 452 /*----- Keyset list handling ----------------------------------------------*/
 453
 454 /* --- @ksl_free@ --- *
 455  *
 456  * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 457  *
 458  * Returns:     ---
 459  *
 460  * Use:         Frees (releases references to) all of the keys in a keyset.
 461  */
 462
 463 void ksl_free(keyset **ksroot)
 464 {
 465   keyset *ks, *ksn;
 466   for (ks = *ksroot; ks; ks = ksn) {
 467     ksn = ks->next;
 468     ks->f &= ~KSF_LINK;
 469     ks_drop(ks);
 470   }
 471 }
 472
 473 /* --- @ksl_link@ --- *
 474  *
 475  * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 476  *              @keyset *ks@ = pointer to a keyset
 477  *
 478  * Returns:     ---
 479  *
 480  * Use:         Links a keyset into a list.  A keyset can only be on one list
 481  *              at a time.  Bad things happen otherwise.
 482  */
 483
 484 void ksl_link(keyset **ksroot, keyset *ks)
 485 {
 486   assert(!(ks->f & KSF_LINK));
 487   ks->next = *ksroot;
 488   *ksroot = ks;
 489   ks->f |= KSF_LINK;
 490   ks->ref++;
 491 }
 492
 493 /* --- @ksl_prune@ --- *
 494  *
 495  * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 496  *
 497  * Returns:     ---
 498  *
 499  * Use:         Prunes the keyset list by removing keys which mustn't be used
 500  *              any more.
 501  */
 502
 503 void ksl_prune(keyset **ksroot)
 504 {
 505   time_t now = time(0);
 506
 507   while (*ksroot) {
 508     keyset *ks = *ksroot;
 509
 510     if (ks->t_exp <= now) {
 511       T( trace(T_KEYSET, "keyset: expiring keyset %u (time limit reached)",
 512                ks->seq); )
 513       goto kill;
 514     } else if (ks->sz_exp == 0) {
 515       T( trace(T_KEYSET, "keyset: expiring keyset %u (data limit reached)",
 516                ks->seq); )
 517       goto kill;
 518     } else {
 519       ksroot = &ks->next;
 520       continue;
 521     }
 522
 523   kill:
 524     *ksroot = ks->next;
 525     ks->f &= ~KSF_LINK;
 526     ks_drop(ks);
 527   }
 528 }
 529
 530 /* --- @ksl_encrypt@ --- *
 531  *
 532  * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 533  *              @unsigned ty@ = message type
 534  *              @buf *b@ = pointer to input buffer
 535  *              @buf *bb@ = pointer to output buffer
 536  *
 537  * Returns:     Nonzero if a new key is needed.
 538  *
 539  * Use:         Encrypts a packet.
 540  */
 541
 542 int ksl_encrypt(keyset **ksroot, unsigned ty, buf *b, buf *bb)
 543 {
 544   time_t now = time(0);
 545   keyset *ks = *ksroot;
 546
 547   for (;;) {
 548     if (!ks) {
 549       T( trace(T_KEYSET, "keyset: no suitable keysets found"); )
 550       buf_break(bb);
 551       return (-1);
 552     }
 553     if (KEYOK(ks, now) && !(ks->f & KSF_LISTEN))
 554       break;
 555     ks = ks->next;
 556   }
 557
 558   return (doencrypt(ks, ty, b, bb));
 559 }
 560
 561 /* --- @ksl_decrypt@ --- *
 562  *
 563  * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 564  *              @unsigned ty@ = expected type code
 565  *              @buf *b@ = pointer to input buffer
 566  *              @buf *bb@ = pointer to output buffer
 567  *
 568  * Returns:     Nonzero if the packet couldn't be decrypted.
 569  *
 570  * Use:         Decrypts a packet.
 571  */
 572
 573 int ksl_decrypt(keyset **ksroot, unsigned ty, buf *b, buf *bb)
 574 {
 575   time_t now = time(0);
 576   keyset *ks;
 577   uint32 seq;
 578
 579   if (buf_ensure(bb, BLEN(b)))
 580     return (-1);
 581
 582   for (ks = *ksroot; ks; ks = ks->next) {
 583     if (!KEYOK(ks, now))
 584       continue;
 585     if (!dodecrypt(ks, ty, b, bb, &seq)) {
 586       if (ks->f & KSF_LISTEN) {
 587         T( trace(T_KEYSET, "keyset: implicitly activating keyset %u",
 588                  ks->seq); )
 589         ks->f &= ~KSF_LISTEN;
 590       }
 591       return (seq_check(&ks->iseq, seq, "SYMM"));
 592     }
 593   }
 594   T( trace(T_KEYSET, "keyset: no matching keys, or incorrect MAC"); )
 595   return (-1);
 596 }
 597
 598 /*----- That's all, folks -------------------------------------------------*/