| 1 | #include <stdio.h> |
| 2 | #include <stdlib.h> |
| 3 | #include <assert.h> |
| 4 | |
| 5 | #include "ssh.h" |
| 6 | #include "misc.h" |
| 7 | |
| 8 | #define GET_32BIT(cp) \ |
| 9 | (((unsigned long)(unsigned char)(cp)[0] << 24) | \ |
| 10 | ((unsigned long)(unsigned char)(cp)[1] << 16) | \ |
| 11 | ((unsigned long)(unsigned char)(cp)[2] << 8) | \ |
| 12 | ((unsigned long)(unsigned char)(cp)[3])) |
| 13 | |
| 14 | #define PUT_32BIT(cp, value) { \ |
| 15 | (cp)[0] = (unsigned char)((value) >> 24); \ |
| 16 | (cp)[1] = (unsigned char)((value) >> 16); \ |
| 17 | (cp)[2] = (unsigned char)((value) >> 8); \ |
| 18 | (cp)[3] = (unsigned char)(value); } |
| 19 | |
| 20 | static void sha_mpint(SHA_State * s, Bignum b) |
| 21 | { |
| 22 | unsigned char lenbuf[4]; |
| 23 | int len; |
| 24 | len = (bignum_bitcount(b) + 8) / 8; |
| 25 | PUT_32BIT(lenbuf, len); |
| 26 | SHA_Bytes(s, lenbuf, 4); |
| 27 | while (len-- > 0) { |
| 28 | lenbuf[0] = bignum_byte(b, len); |
| 29 | SHA_Bytes(s, lenbuf, 1); |
| 30 | } |
| 31 | memset(lenbuf, 0, sizeof(lenbuf)); |
| 32 | } |
| 33 | |
| 34 | static void sha512_mpint(SHA512_State * s, Bignum b) |
| 35 | { |
| 36 | unsigned char lenbuf[4]; |
| 37 | int len; |
| 38 | len = (bignum_bitcount(b) + 8) / 8; |
| 39 | PUT_32BIT(lenbuf, len); |
| 40 | SHA512_Bytes(s, lenbuf, 4); |
| 41 | while (len-- > 0) { |
| 42 | lenbuf[0] = bignum_byte(b, len); |
| 43 | SHA512_Bytes(s, lenbuf, 1); |
| 44 | } |
| 45 | memset(lenbuf, 0, sizeof(lenbuf)); |
| 46 | } |
| 47 | |
| 48 | static void getstring(char **data, int *datalen, char **p, int *length) |
| 49 | { |
| 50 | *p = NULL; |
| 51 | if (*datalen < 4) |
| 52 | return; |
| 53 | *length = GET_32BIT(*data); |
| 54 | *datalen -= 4; |
| 55 | *data += 4; |
| 56 | if (*datalen < *length) |
| 57 | return; |
| 58 | *p = *data; |
| 59 | *data += *length; |
| 60 | *datalen -= *length; |
| 61 | } |
| 62 | static Bignum getmp(char **data, int *datalen) |
| 63 | { |
| 64 | char *p; |
| 65 | int length; |
| 66 | Bignum b; |
| 67 | |
| 68 | getstring(data, datalen, &p, &length); |
| 69 | if (!p) |
| 70 | return NULL; |
| 71 | if (p[0] & 0x80) |
| 72 | return NULL; /* negative mp */ |
| 73 | b = bignum_from_bytes(p, length); |
| 74 | return b; |
| 75 | } |
| 76 | |
| 77 | static Bignum get160(char **data, int *datalen) |
| 78 | { |
| 79 | Bignum b; |
| 80 | |
| 81 | b = bignum_from_bytes(*data, 20); |
| 82 | *data += 20; |
| 83 | *datalen -= 20; |
| 84 | |
| 85 | return b; |
| 86 | } |
| 87 | |
| 88 | static void *dss_newkey(char *data, int len) |
| 89 | { |
| 90 | char *p; |
| 91 | int slen; |
| 92 | struct dss_key *dss; |
| 93 | |
| 94 | dss = smalloc(sizeof(struct dss_key)); |
| 95 | if (!dss) |
| 96 | return NULL; |
| 97 | getstring(&data, &len, &p, &slen); |
| 98 | |
| 99 | #ifdef DEBUG_DSS |
| 100 | { |
| 101 | int i; |
| 102 | printf("key:"); |
| 103 | for (i = 0; i < len; i++) |
| 104 | printf(" %02x", (unsigned char) (data[i])); |
| 105 | printf("\n"); |
| 106 | } |
| 107 | #endif |
| 108 | |
| 109 | if (!p || memcmp(p, "ssh-dss", 7)) { |
| 110 | sfree(dss); |
| 111 | return NULL; |
| 112 | } |
| 113 | dss->p = getmp(&data, &len); |
| 114 | dss->q = getmp(&data, &len); |
| 115 | dss->g = getmp(&data, &len); |
| 116 | dss->y = getmp(&data, &len); |
| 117 | |
| 118 | return dss; |
| 119 | } |
| 120 | |
| 121 | static void dss_freekey(void *key) |
| 122 | { |
| 123 | struct dss_key *dss = (struct dss_key *) key; |
| 124 | freebn(dss->p); |
| 125 | freebn(dss->q); |
| 126 | freebn(dss->g); |
| 127 | freebn(dss->y); |
| 128 | sfree(dss); |
| 129 | } |
| 130 | |
| 131 | static char *dss_fmtkey(void *key) |
| 132 | { |
| 133 | struct dss_key *dss = (struct dss_key *) key; |
| 134 | char *p; |
| 135 | int len, i, pos, nibbles; |
| 136 | static const char hex[] = "0123456789abcdef"; |
| 137 | if (!dss->p) |
| 138 | return NULL; |
| 139 | len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */ |
| 140 | len += 4 * (bignum_bitcount(dss->p) + 15) / 16; |
| 141 | len += 4 * (bignum_bitcount(dss->q) + 15) / 16; |
| 142 | len += 4 * (bignum_bitcount(dss->g) + 15) / 16; |
| 143 | len += 4 * (bignum_bitcount(dss->y) + 15) / 16; |
| 144 | p = smalloc(len); |
| 145 | if (!p) |
| 146 | return NULL; |
| 147 | |
| 148 | pos = 0; |
| 149 | pos += sprintf(p + pos, "0x"); |
| 150 | nibbles = (3 + bignum_bitcount(dss->p)) / 4; |
| 151 | if (nibbles < 1) |
| 152 | nibbles = 1; |
| 153 | for (i = nibbles; i--;) |
| 154 | p[pos++] = |
| 155 | hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF]; |
| 156 | pos += sprintf(p + pos, ",0x"); |
| 157 | nibbles = (3 + bignum_bitcount(dss->q)) / 4; |
| 158 | if (nibbles < 1) |
| 159 | nibbles = 1; |
| 160 | for (i = nibbles; i--;) |
| 161 | p[pos++] = |
| 162 | hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF]; |
| 163 | pos += sprintf(p + pos, ",0x"); |
| 164 | nibbles = (3 + bignum_bitcount(dss->g)) / 4; |
| 165 | if (nibbles < 1) |
| 166 | nibbles = 1; |
| 167 | for (i = nibbles; i--;) |
| 168 | p[pos++] = |
| 169 | hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF]; |
| 170 | pos += sprintf(p + pos, ",0x"); |
| 171 | nibbles = (3 + bignum_bitcount(dss->y)) / 4; |
| 172 | if (nibbles < 1) |
| 173 | nibbles = 1; |
| 174 | for (i = nibbles; i--;) |
| 175 | p[pos++] = |
| 176 | hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF]; |
| 177 | p[pos] = '\0'; |
| 178 | return p; |
| 179 | } |
| 180 | |
| 181 | static char *dss_fingerprint(void *key) |
| 182 | { |
| 183 | struct dss_key *dss = (struct dss_key *) key; |
| 184 | struct MD5Context md5c; |
| 185 | unsigned char digest[16], lenbuf[4]; |
| 186 | char buffer[16 * 3 + 40]; |
| 187 | char *ret; |
| 188 | int numlen, i; |
| 189 | |
| 190 | MD5Init(&md5c); |
| 191 | MD5Update(&md5c, "\0\0\0\7ssh-dss", 11); |
| 192 | |
| 193 | #define ADD_BIGNUM(bignum) \ |
| 194 | numlen = (bignum_bitcount(bignum)+8)/8; \ |
| 195 | PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \ |
| 196 | for (i = numlen; i-- ;) { \ |
| 197 | unsigned char c = bignum_byte(bignum, i); \ |
| 198 | MD5Update(&md5c, &c, 1); \ |
| 199 | } |
| 200 | ADD_BIGNUM(dss->p); |
| 201 | ADD_BIGNUM(dss->q); |
| 202 | ADD_BIGNUM(dss->g); |
| 203 | ADD_BIGNUM(dss->y); |
| 204 | #undef ADD_BIGNUM |
| 205 | |
| 206 | MD5Final(digest, &md5c); |
| 207 | |
| 208 | sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p)); |
| 209 | for (i = 0; i < 16; i++) |
| 210 | sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "", |
| 211 | digest[i]); |
| 212 | ret = smalloc(strlen(buffer) + 1); |
| 213 | if (ret) |
| 214 | strcpy(ret, buffer); |
| 215 | return ret; |
| 216 | } |
| 217 | |
| 218 | static int dss_verifysig(void *key, char *sig, int siglen, |
| 219 | char *data, int datalen) |
| 220 | { |
| 221 | struct dss_key *dss = (struct dss_key *) key; |
| 222 | char *p; |
| 223 | int slen; |
| 224 | char hash[20]; |
| 225 | Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v; |
| 226 | int ret; |
| 227 | |
| 228 | if (!dss->p) |
| 229 | return 0; |
| 230 | |
| 231 | #ifdef DEBUG_DSS |
| 232 | { |
| 233 | int i; |
| 234 | printf("sig:"); |
| 235 | for (i = 0; i < siglen; i++) |
| 236 | printf(" %02x", (unsigned char) (sig[i])); |
| 237 | printf("\n"); |
| 238 | } |
| 239 | #endif |
| 240 | /* |
| 241 | * Commercial SSH (2.0.13) and OpenSSH disagree over the format |
| 242 | * of a DSA signature. OpenSSH is in line with the IETF drafts: |
| 243 | * it uses a string "ssh-dss", followed by a 40-byte string |
| 244 | * containing two 160-bit integers end-to-end. Commercial SSH |
| 245 | * can't be bothered with the header bit, and considers a DSA |
| 246 | * signature blob to be _just_ the 40-byte string containing |
| 247 | * the two 160-bit integers. We tell them apart by measuring |
| 248 | * the length: length 40 means the commercial-SSH bug, anything |
| 249 | * else is assumed to be IETF-compliant. |
| 250 | */ |
| 251 | if (siglen != 40) { /* bug not present; read admin fields */ |
| 252 | getstring(&sig, &siglen, &p, &slen); |
| 253 | if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) { |
| 254 | return 0; |
| 255 | } |
| 256 | sig += 4, siglen -= 4; /* skip yet another length field */ |
| 257 | } |
| 258 | r = get160(&sig, &siglen); |
| 259 | s = get160(&sig, &siglen); |
| 260 | if (!r || !s) |
| 261 | return 0; |
| 262 | |
| 263 | /* |
| 264 | * Step 1. w <- s^-1 mod q. |
| 265 | */ |
| 266 | w = modinv(s, dss->q); |
| 267 | |
| 268 | /* |
| 269 | * Step 2. u1 <- SHA(message) * w mod q. |
| 270 | */ |
| 271 | SHA_Simple(data, datalen, hash); |
| 272 | p = hash; |
| 273 | slen = 20; |
| 274 | sha = get160(&p, &slen); |
| 275 | u1 = modmul(sha, w, dss->q); |
| 276 | |
| 277 | /* |
| 278 | * Step 3. u2 <- r * w mod q. |
| 279 | */ |
| 280 | u2 = modmul(r, w, dss->q); |
| 281 | |
| 282 | /* |
| 283 | * Step 4. v <- (g^u1 * y^u2 mod p) mod q. |
| 284 | */ |
| 285 | gu1p = modpow(dss->g, u1, dss->p); |
| 286 | yu2p = modpow(dss->y, u2, dss->p); |
| 287 | gu1yu2p = modmul(gu1p, yu2p, dss->p); |
| 288 | v = modmul(gu1yu2p, One, dss->q); |
| 289 | |
| 290 | /* |
| 291 | * Step 5. v should now be equal to r. |
| 292 | */ |
| 293 | |
| 294 | ret = !bignum_cmp(v, r); |
| 295 | |
| 296 | freebn(w); |
| 297 | freebn(sha); |
| 298 | freebn(gu1p); |
| 299 | freebn(yu2p); |
| 300 | freebn(gu1yu2p); |
| 301 | freebn(v); |
| 302 | freebn(r); |
| 303 | freebn(s); |
| 304 | |
| 305 | return ret; |
| 306 | } |
| 307 | |
| 308 | static unsigned char *dss_public_blob(void *key, int *len) |
| 309 | { |
| 310 | struct dss_key *dss = (struct dss_key *) key; |
| 311 | int plen, qlen, glen, ylen, bloblen; |
| 312 | int i; |
| 313 | unsigned char *blob, *p; |
| 314 | |
| 315 | plen = (bignum_bitcount(dss->p) + 8) / 8; |
| 316 | qlen = (bignum_bitcount(dss->q) + 8) / 8; |
| 317 | glen = (bignum_bitcount(dss->g) + 8) / 8; |
| 318 | ylen = (bignum_bitcount(dss->y) + 8) / 8; |
| 319 | |
| 320 | /* |
| 321 | * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total |
| 322 | * 27 + sum of lengths. (five length fields, 20+7=27). |
| 323 | */ |
| 324 | bloblen = 27 + plen + qlen + glen + ylen; |
| 325 | blob = smalloc(bloblen); |
| 326 | p = blob; |
| 327 | PUT_32BIT(p, 7); |
| 328 | p += 4; |
| 329 | memcpy(p, "ssh-dss", 7); |
| 330 | p += 7; |
| 331 | PUT_32BIT(p, plen); |
| 332 | p += 4; |
| 333 | for (i = plen; i--;) |
| 334 | *p++ = bignum_byte(dss->p, i); |
| 335 | PUT_32BIT(p, qlen); |
| 336 | p += 4; |
| 337 | for (i = qlen; i--;) |
| 338 | *p++ = bignum_byte(dss->q, i); |
| 339 | PUT_32BIT(p, glen); |
| 340 | p += 4; |
| 341 | for (i = glen; i--;) |
| 342 | *p++ = bignum_byte(dss->g, i); |
| 343 | PUT_32BIT(p, ylen); |
| 344 | p += 4; |
| 345 | for (i = ylen; i--;) |
| 346 | *p++ = bignum_byte(dss->y, i); |
| 347 | assert(p == blob + bloblen); |
| 348 | *len = bloblen; |
| 349 | return blob; |
| 350 | } |
| 351 | |
| 352 | static unsigned char *dss_private_blob(void *key, int *len) |
| 353 | { |
| 354 | struct dss_key *dss = (struct dss_key *) key; |
| 355 | int xlen, bloblen; |
| 356 | int i; |
| 357 | unsigned char *blob, *p; |
| 358 | SHA_State s; |
| 359 | unsigned char digest[20]; |
| 360 | |
| 361 | xlen = (bignum_bitcount(dss->x) + 8) / 8; |
| 362 | |
| 363 | /* |
| 364 | * mpint x, string[20] the SHA of p||q||g. Total 28 + xlen. |
| 365 | * (two length fields and twenty bytes, 20+8=28). |
| 366 | */ |
| 367 | bloblen = 28 + xlen; |
| 368 | blob = smalloc(bloblen); |
| 369 | p = blob; |
| 370 | PUT_32BIT(p, xlen); |
| 371 | p += 4; |
| 372 | for (i = xlen; i--;) |
| 373 | *p++ = bignum_byte(dss->x, i); |
| 374 | PUT_32BIT(p, 20); |
| 375 | SHA_Init(&s); |
| 376 | sha_mpint(&s, dss->p); |
| 377 | sha_mpint(&s, dss->q); |
| 378 | sha_mpint(&s, dss->g); |
| 379 | SHA_Final(&s, digest); |
| 380 | p += 4; |
| 381 | for (i = 0; i < 20; i++) |
| 382 | *p++ = digest[i]; |
| 383 | assert(p == blob + bloblen); |
| 384 | *len = bloblen; |
| 385 | return blob; |
| 386 | } |
| 387 | |
| 388 | static void *dss_createkey(unsigned char *pub_blob, int pub_len, |
| 389 | unsigned char *priv_blob, int priv_len) |
| 390 | { |
| 391 | struct dss_key *dss; |
| 392 | char *pb = (char *) priv_blob; |
| 393 | char *hash; |
| 394 | int hashlen; |
| 395 | SHA_State s; |
| 396 | unsigned char digest[20]; |
| 397 | Bignum ytest; |
| 398 | |
| 399 | dss = dss_newkey((char *) pub_blob, pub_len); |
| 400 | dss->x = getmp(&pb, &priv_len); |
| 401 | getstring(&pb, &priv_len, &hash, &hashlen); |
| 402 | |
| 403 | /* |
| 404 | * Verify details of the key. First check that the hash is |
| 405 | * indeed a hash of p||q||g. |
| 406 | */ |
| 407 | if (hashlen != 20) { |
| 408 | dss_freekey(dss); |
| 409 | return NULL; |
| 410 | } |
| 411 | SHA_Init(&s); |
| 412 | sha_mpint(&s, dss->p); |
| 413 | sha_mpint(&s, dss->q); |
| 414 | sha_mpint(&s, dss->g); |
| 415 | SHA_Final(&s, digest); |
| 416 | if (0 != memcmp(hash, digest, 20)) { |
| 417 | dss_freekey(dss); |
| 418 | return NULL; |
| 419 | } |
| 420 | |
| 421 | /* |
| 422 | * Now ensure g^x mod p really is y. |
| 423 | */ |
| 424 | ytest = modpow(dss->g, dss->x, dss->p); |
| 425 | if (0 != bignum_cmp(ytest, dss->y)) { |
| 426 | dss_freekey(dss); |
| 427 | return NULL; |
| 428 | } |
| 429 | freebn(ytest); |
| 430 | |
| 431 | return dss; |
| 432 | } |
| 433 | |
| 434 | static void *dss_openssh_createkey(unsigned char **blob, int *len) |
| 435 | { |
| 436 | char **b = (char **) blob; |
| 437 | struct dss_key *dss; |
| 438 | |
| 439 | dss = smalloc(sizeof(struct dss_key)); |
| 440 | if (!dss) |
| 441 | return NULL; |
| 442 | |
| 443 | dss->p = getmp(b, len); |
| 444 | dss->q = getmp(b, len); |
| 445 | dss->g = getmp(b, len); |
| 446 | dss->y = getmp(b, len); |
| 447 | dss->x = getmp(b, len); |
| 448 | |
| 449 | if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x) { |
| 450 | sfree(dss->p); |
| 451 | sfree(dss->q); |
| 452 | sfree(dss->g); |
| 453 | sfree(dss->y); |
| 454 | sfree(dss->x); |
| 455 | sfree(dss); |
| 456 | return NULL; |
| 457 | } |
| 458 | |
| 459 | return dss; |
| 460 | } |
| 461 | |
| 462 | static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len) |
| 463 | { |
| 464 | struct dss_key *dss = (struct dss_key *) key; |
| 465 | int bloblen, i; |
| 466 | |
| 467 | bloblen = |
| 468 | ssh2_bignum_length(dss->p) + |
| 469 | ssh2_bignum_length(dss->q) + |
| 470 | ssh2_bignum_length(dss->g) + |
| 471 | ssh2_bignum_length(dss->y) + |
| 472 | ssh2_bignum_length(dss->x); |
| 473 | |
| 474 | if (bloblen > len) |
| 475 | return bloblen; |
| 476 | |
| 477 | bloblen = 0; |
| 478 | #define ENC(x) \ |
| 479 | PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \ |
| 480 | for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i); |
| 481 | ENC(dss->p); |
| 482 | ENC(dss->q); |
| 483 | ENC(dss->g); |
| 484 | ENC(dss->y); |
| 485 | ENC(dss->x); |
| 486 | |
| 487 | return bloblen; |
| 488 | } |
| 489 | |
| 490 | unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen) |
| 491 | { |
| 492 | /* |
| 493 | * The basic DSS signing algorithm is: |
| 494 | * |
| 495 | * - invent a random k between 1 and q-1 (exclusive). |
| 496 | * - Compute r = (g^k mod p) mod q. |
| 497 | * - Compute s = k^-1 * (hash + x*r) mod q. |
| 498 | * |
| 499 | * This has the dangerous properties that: |
| 500 | * |
| 501 | * - if an attacker in possession of the public key _and_ the |
| 502 | * signature (for example, the host you just authenticated |
| 503 | * to) can guess your k, he can reverse the computation of s |
| 504 | * and work out x = r^-1 * (s*k - hash) mod q. That is, he |
| 505 | * can deduce the private half of your key, and masquerade |
| 506 | * as you for as long as the key is still valid. |
| 507 | * |
| 508 | * - since r is a function purely of k and the public key, if |
| 509 | * the attacker only has a _range of possibilities_ for k |
| 510 | * it's easy for him to work through them all and check each |
| 511 | * one against r; he'll never be unsure of whether he's got |
| 512 | * the right one. |
| 513 | * |
| 514 | * - if you ever sign two different hashes with the same k, it |
| 515 | * will be immediately obvious because the two signatures |
| 516 | * will have the same r, and moreover an attacker in |
| 517 | * possession of both signatures (and the public key of |
| 518 | * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q, |
| 519 | * and from there deduce x as before. |
| 520 | * |
| 521 | * - the Bleichenbacher attack on DSA makes use of methods of |
| 522 | * generating k which are significantly non-uniformly |
| 523 | * distributed; in particular, generating a 160-bit random |
| 524 | * number and reducing it mod q is right out. |
| 525 | * |
| 526 | * For this reason we must be pretty careful about how we |
| 527 | * generate our k. Since this code runs on Windows, with no |
| 528 | * particularly good system entropy sources, we can't trust our |
| 529 | * RNG itself to produce properly unpredictable data. Hence, we |
| 530 | * use a totally different scheme instead. |
| 531 | * |
| 532 | * What we do is to take a SHA-512 (_big_) hash of the private |
| 533 | * key x, and then feed this into another SHA-512 hash that |
| 534 | * also includes the message hash being signed. That is: |
| 535 | * |
| 536 | * proto_k = SHA512 ( SHA512(x) || SHA160(message) ) |
| 537 | * |
| 538 | * This number is 512 bits long, so reducing it mod q won't be |
| 539 | * noticeably non-uniform. So |
| 540 | * |
| 541 | * k = proto_k mod q |
| 542 | * |
| 543 | * This has the interesting property that it's _deterministic_: |
| 544 | * signing the same hash twice with the same key yields the |
| 545 | * same signature. |
| 546 | * |
| 547 | * Despite this determinism, it's still not predictable to an |
| 548 | * attacker, because in order to repeat the SHA-512 |
| 549 | * construction that created it, the attacker would have to |
| 550 | * know the private key value x - and by assumption he doesn't, |
| 551 | * because if he knew that he wouldn't be attacking k! |
| 552 | * |
| 553 | * (This trick doesn't, _per se_, protect against reuse of k. |
| 554 | * Reuse of k is left to chance; all it does is prevent |
| 555 | * _excessively high_ chances of reuse of k due to entropy |
| 556 | * problems.) |
| 557 | * |
| 558 | * Thanks to Colin Plumb for the general idea of using x to |
| 559 | * ensure k is hard to guess, and to the Cambridge University |
| 560 | * Computer Security Group for helping to argue out all the |
| 561 | * fine details. |
| 562 | */ |
| 563 | struct dss_key *dss = (struct dss_key *) key; |
| 564 | SHA512_State ss; |
| 565 | unsigned char digest[20], digest512[64]; |
| 566 | Bignum proto_k, k, gkp, hash, kinv, hxr, r, s; |
| 567 | unsigned char *bytes; |
| 568 | int nbytes, i; |
| 569 | |
| 570 | SHA_Simple(data, datalen, digest); |
| 571 | |
| 572 | /* |
| 573 | * Hash some identifying text plus x. |
| 574 | */ |
| 575 | SHA512_Init(&ss); |
| 576 | SHA512_Bytes(&ss, "DSA deterministic k generator", 30); |
| 577 | sha512_mpint(&ss, dss->x); |
| 578 | SHA512_Final(&ss, digest512); |
| 579 | |
| 580 | /* |
| 581 | * Now hash that digest plus the message hash. |
| 582 | */ |
| 583 | SHA512_Init(&ss); |
| 584 | SHA512_Bytes(&ss, digest512, sizeof(digest512)); |
| 585 | SHA512_Bytes(&ss, digest, sizeof(digest)); |
| 586 | SHA512_Final(&ss, digest512); |
| 587 | |
| 588 | memset(&ss, 0, sizeof(ss)); |
| 589 | |
| 590 | /* |
| 591 | * Now convert the result into a bignum, and reduce it mod q. |
| 592 | */ |
| 593 | proto_k = bignum_from_bytes(digest512, 64); |
| 594 | k = bigmod(proto_k, dss->q); |
| 595 | freebn(proto_k); |
| 596 | |
| 597 | memset(digest512, 0, sizeof(digest512)); |
| 598 | |
| 599 | /* |
| 600 | * Now we have k, so just go ahead and compute the signature. |
| 601 | */ |
| 602 | gkp = modpow(dss->g, k, dss->p); /* g^k mod p */ |
| 603 | r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */ |
| 604 | freebn(gkp); |
| 605 | |
| 606 | hash = bignum_from_bytes(digest, 20); |
| 607 | kinv = modinv(k, dss->q); /* k^-1 mod q */ |
| 608 | hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */ |
| 609 | s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */ |
| 610 | freebn(hxr); |
| 611 | freebn(kinv); |
| 612 | freebn(hash); |
| 613 | |
| 614 | /* |
| 615 | * Signature blob is |
| 616 | * |
| 617 | * string "ssh-dss" |
| 618 | * string two 20-byte numbers r and s, end to end |
| 619 | * |
| 620 | * i.e. 4+7 + 4+40 bytes. |
| 621 | */ |
| 622 | nbytes = 4 + 7 + 4 + 40; |
| 623 | bytes = smalloc(nbytes); |
| 624 | PUT_32BIT(bytes, 7); |
| 625 | memcpy(bytes + 4, "ssh-dss", 7); |
| 626 | PUT_32BIT(bytes + 4 + 7, 40); |
| 627 | for (i = 0; i < 20; i++) { |
| 628 | bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i); |
| 629 | bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i); |
| 630 | } |
| 631 | freebn(r); |
| 632 | freebn(s); |
| 633 | |
| 634 | *siglen = nbytes; |
| 635 | return bytes; |
| 636 | } |
| 637 | |
| 638 | const struct ssh_signkey ssh_dss = { |
| 639 | dss_newkey, |
| 640 | dss_freekey, |
| 641 | dss_fmtkey, |
| 642 | dss_public_blob, |
| 643 | dss_private_blob, |
| 644 | dss_createkey, |
| 645 | dss_openssh_createkey, |
| 646 | dss_openssh_fmtkey, |
| 647 | dss_fingerprint, |
| 648 | dss_verifysig, |
| 649 | dss_sign, |
| 650 | "ssh-dss", |
| 651 | "dss" |
| 652 | }; |