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Commit | Line | Data |
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1c1af145 | 1 | /* |
2 | * Digital Signature Standard implementation for PuTTY. | |
3 | */ | |
4 | ||
5 | #include <stdio.h> | |
6 | #include <stdlib.h> | |
7 | #include <assert.h> | |
8 | ||
9 | #include "ssh.h" | |
10 | #include "misc.h" | |
11 | ||
12 | static void sha_mpint(SHA_State * s, Bignum b) | |
13 | { | |
14 | unsigned char lenbuf[4]; | |
15 | int len; | |
16 | len = (bignum_bitcount(b) + 8) / 8; | |
17 | PUT_32BIT(lenbuf, len); | |
18 | SHA_Bytes(s, lenbuf, 4); | |
19 | while (len-- > 0) { | |
20 | lenbuf[0] = bignum_byte(b, len); | |
21 | SHA_Bytes(s, lenbuf, 1); | |
22 | } | |
23 | memset(lenbuf, 0, sizeof(lenbuf)); | |
24 | } | |
25 | ||
26 | static void sha512_mpint(SHA512_State * s, Bignum b) | |
27 | { | |
28 | unsigned char lenbuf[4]; | |
29 | int len; | |
30 | len = (bignum_bitcount(b) + 8) / 8; | |
31 | PUT_32BIT(lenbuf, len); | |
32 | SHA512_Bytes(s, lenbuf, 4); | |
33 | while (len-- > 0) { | |
34 | lenbuf[0] = bignum_byte(b, len); | |
35 | SHA512_Bytes(s, lenbuf, 1); | |
36 | } | |
37 | memset(lenbuf, 0, sizeof(lenbuf)); | |
38 | } | |
39 | ||
40 | static void getstring(char **data, int *datalen, char **p, int *length) | |
41 | { | |
42 | *p = NULL; | |
43 | if (*datalen < 4) | |
44 | return; | |
45 | *length = GET_32BIT(*data); | |
46 | *datalen -= 4; | |
47 | *data += 4; | |
48 | if (*datalen < *length) | |
49 | return; | |
50 | *p = *data; | |
51 | *data += *length; | |
52 | *datalen -= *length; | |
53 | } | |
54 | static Bignum getmp(char **data, int *datalen) | |
55 | { | |
56 | char *p; | |
57 | int length; | |
58 | Bignum b; | |
59 | ||
60 | getstring(data, datalen, &p, &length); | |
61 | if (!p) | |
62 | return NULL; | |
63 | if (p[0] & 0x80) | |
64 | return NULL; /* negative mp */ | |
65 | b = bignum_from_bytes((unsigned char *)p, length); | |
66 | return b; | |
67 | } | |
68 | ||
69 | static Bignum get160(char **data, int *datalen) | |
70 | { | |
71 | Bignum b; | |
72 | ||
73 | b = bignum_from_bytes((unsigned char *)*data, 20); | |
74 | *data += 20; | |
75 | *datalen -= 20; | |
76 | ||
77 | return b; | |
78 | } | |
79 | ||
80 | static void *dss_newkey(char *data, int len) | |
81 | { | |
82 | char *p; | |
83 | int slen; | |
84 | struct dss_key *dss; | |
85 | ||
86 | dss = snew(struct dss_key); | |
87 | if (!dss) | |
88 | return NULL; | |
89 | getstring(&data, &len, &p, &slen); | |
90 | ||
91 | #ifdef DEBUG_DSS | |
92 | { | |
93 | int i; | |
94 | printf("key:"); | |
95 | for (i = 0; i < len; i++) | |
96 | printf(" %02x", (unsigned char) (data[i])); | |
97 | printf("\n"); | |
98 | } | |
99 | #endif | |
100 | ||
101 | if (!p || memcmp(p, "ssh-dss", 7)) { | |
102 | sfree(dss); | |
103 | return NULL; | |
104 | } | |
105 | dss->p = getmp(&data, &len); | |
106 | dss->q = getmp(&data, &len); | |
107 | dss->g = getmp(&data, &len); | |
108 | dss->y = getmp(&data, &len); | |
109 | ||
110 | return dss; | |
111 | } | |
112 | ||
113 | static void dss_freekey(void *key) | |
114 | { | |
115 | struct dss_key *dss = (struct dss_key *) key; | |
116 | freebn(dss->p); | |
117 | freebn(dss->q); | |
118 | freebn(dss->g); | |
119 | freebn(dss->y); | |
120 | sfree(dss); | |
121 | } | |
122 | ||
123 | static char *dss_fmtkey(void *key) | |
124 | { | |
125 | struct dss_key *dss = (struct dss_key *) key; | |
126 | char *p; | |
127 | int len, i, pos, nibbles; | |
128 | static const char hex[] = "0123456789abcdef"; | |
129 | if (!dss->p) | |
130 | return NULL; | |
131 | len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */ | |
132 | len += 4 * (bignum_bitcount(dss->p) + 15) / 16; | |
133 | len += 4 * (bignum_bitcount(dss->q) + 15) / 16; | |
134 | len += 4 * (bignum_bitcount(dss->g) + 15) / 16; | |
135 | len += 4 * (bignum_bitcount(dss->y) + 15) / 16; | |
136 | p = snewn(len, char); | |
137 | if (!p) | |
138 | return NULL; | |
139 | ||
140 | pos = 0; | |
141 | pos += sprintf(p + pos, "0x"); | |
142 | nibbles = (3 + bignum_bitcount(dss->p)) / 4; | |
143 | if (nibbles < 1) | |
144 | nibbles = 1; | |
145 | for (i = nibbles; i--;) | |
146 | p[pos++] = | |
147 | hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF]; | |
148 | pos += sprintf(p + pos, ",0x"); | |
149 | nibbles = (3 + bignum_bitcount(dss->q)) / 4; | |
150 | if (nibbles < 1) | |
151 | nibbles = 1; | |
152 | for (i = nibbles; i--;) | |
153 | p[pos++] = | |
154 | hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF]; | |
155 | pos += sprintf(p + pos, ",0x"); | |
156 | nibbles = (3 + bignum_bitcount(dss->g)) / 4; | |
157 | if (nibbles < 1) | |
158 | nibbles = 1; | |
159 | for (i = nibbles; i--;) | |
160 | p[pos++] = | |
161 | hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF]; | |
162 | pos += sprintf(p + pos, ",0x"); | |
163 | nibbles = (3 + bignum_bitcount(dss->y)) / 4; | |
164 | if (nibbles < 1) | |
165 | nibbles = 1; | |
166 | for (i = nibbles; i--;) | |
167 | p[pos++] = | |
168 | hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF]; | |
169 | p[pos] = '\0'; | |
170 | return p; | |
171 | } | |
172 | ||
173 | static char *dss_fingerprint(void *key) | |
174 | { | |
175 | struct dss_key *dss = (struct dss_key *) key; | |
176 | struct MD5Context md5c; | |
177 | unsigned char digest[16], lenbuf[4]; | |
178 | char buffer[16 * 3 + 40]; | |
179 | char *ret; | |
180 | int numlen, i; | |
181 | ||
182 | MD5Init(&md5c); | |
183 | MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11); | |
184 | ||
185 | #define ADD_BIGNUM(bignum) \ | |
186 | numlen = (bignum_bitcount(bignum)+8)/8; \ | |
187 | PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \ | |
188 | for (i = numlen; i-- ;) { \ | |
189 | unsigned char c = bignum_byte(bignum, i); \ | |
190 | MD5Update(&md5c, &c, 1); \ | |
191 | } | |
192 | ADD_BIGNUM(dss->p); | |
193 | ADD_BIGNUM(dss->q); | |
194 | ADD_BIGNUM(dss->g); | |
195 | ADD_BIGNUM(dss->y); | |
196 | #undef ADD_BIGNUM | |
197 | ||
198 | MD5Final(digest, &md5c); | |
199 | ||
200 | sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p)); | |
201 | for (i = 0; i < 16; i++) | |
202 | sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "", | |
203 | digest[i]); | |
204 | ret = snewn(strlen(buffer) + 1, char); | |
205 | if (ret) | |
206 | strcpy(ret, buffer); | |
207 | return ret; | |
208 | } | |
209 | ||
210 | static int dss_verifysig(void *key, char *sig, int siglen, | |
211 | char *data, int datalen) | |
212 | { | |
213 | struct dss_key *dss = (struct dss_key *) key; | |
214 | char *p; | |
215 | int slen; | |
216 | char hash[20]; | |
217 | Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v; | |
218 | int ret; | |
219 | ||
220 | if (!dss->p) | |
221 | return 0; | |
222 | ||
223 | #ifdef DEBUG_DSS | |
224 | { | |
225 | int i; | |
226 | printf("sig:"); | |
227 | for (i = 0; i < siglen; i++) | |
228 | printf(" %02x", (unsigned char) (sig[i])); | |
229 | printf("\n"); | |
230 | } | |
231 | #endif | |
232 | /* | |
233 | * Commercial SSH (2.0.13) and OpenSSH disagree over the format | |
234 | * of a DSA signature. OpenSSH is in line with RFC 4253: | |
235 | * it uses a string "ssh-dss", followed by a 40-byte string | |
236 | * containing two 160-bit integers end-to-end. Commercial SSH | |
237 | * can't be bothered with the header bit, and considers a DSA | |
238 | * signature blob to be _just_ the 40-byte string containing | |
239 | * the two 160-bit integers. We tell them apart by measuring | |
240 | * the length: length 40 means the commercial-SSH bug, anything | |
241 | * else is assumed to be RFC-compliant. | |
242 | */ | |
243 | if (siglen != 40) { /* bug not present; read admin fields */ | |
244 | getstring(&sig, &siglen, &p, &slen); | |
245 | if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) { | |
246 | return 0; | |
247 | } | |
248 | sig += 4, siglen -= 4; /* skip yet another length field */ | |
249 | } | |
250 | r = get160(&sig, &siglen); | |
251 | s = get160(&sig, &siglen); | |
252 | if (!r || !s) | |
253 | return 0; | |
254 | ||
255 | /* | |
256 | * Step 1. w <- s^-1 mod q. | |
257 | */ | |
258 | w = modinv(s, dss->q); | |
259 | ||
260 | /* | |
261 | * Step 2. u1 <- SHA(message) * w mod q. | |
262 | */ | |
263 | SHA_Simple(data, datalen, (unsigned char *)hash); | |
264 | p = hash; | |
265 | slen = 20; | |
266 | sha = get160(&p, &slen); | |
267 | u1 = modmul(sha, w, dss->q); | |
268 | ||
269 | /* | |
270 | * Step 3. u2 <- r * w mod q. | |
271 | */ | |
272 | u2 = modmul(r, w, dss->q); | |
273 | ||
274 | /* | |
275 | * Step 4. v <- (g^u1 * y^u2 mod p) mod q. | |
276 | */ | |
277 | gu1p = modpow(dss->g, u1, dss->p); | |
278 | yu2p = modpow(dss->y, u2, dss->p); | |
279 | gu1yu2p = modmul(gu1p, yu2p, dss->p); | |
280 | v = modmul(gu1yu2p, One, dss->q); | |
281 | ||
282 | /* | |
283 | * Step 5. v should now be equal to r. | |
284 | */ | |
285 | ||
286 | ret = !bignum_cmp(v, r); | |
287 | ||
288 | freebn(w); | |
289 | freebn(sha); | |
290 | freebn(gu1p); | |
291 | freebn(yu2p); | |
292 | freebn(gu1yu2p); | |
293 | freebn(v); | |
294 | freebn(r); | |
295 | freebn(s); | |
296 | ||
297 | return ret; | |
298 | } | |
299 | ||
300 | static unsigned char *dss_public_blob(void *key, int *len) | |
301 | { | |
302 | struct dss_key *dss = (struct dss_key *) key; | |
303 | int plen, qlen, glen, ylen, bloblen; | |
304 | int i; | |
305 | unsigned char *blob, *p; | |
306 | ||
307 | plen = (bignum_bitcount(dss->p) + 8) / 8; | |
308 | qlen = (bignum_bitcount(dss->q) + 8) / 8; | |
309 | glen = (bignum_bitcount(dss->g) + 8) / 8; | |
310 | ylen = (bignum_bitcount(dss->y) + 8) / 8; | |
311 | ||
312 | /* | |
313 | * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total | |
314 | * 27 + sum of lengths. (five length fields, 20+7=27). | |
315 | */ | |
316 | bloblen = 27 + plen + qlen + glen + ylen; | |
317 | blob = snewn(bloblen, unsigned char); | |
318 | p = blob; | |
319 | PUT_32BIT(p, 7); | |
320 | p += 4; | |
321 | memcpy(p, "ssh-dss", 7); | |
322 | p += 7; | |
323 | PUT_32BIT(p, plen); | |
324 | p += 4; | |
325 | for (i = plen; i--;) | |
326 | *p++ = bignum_byte(dss->p, i); | |
327 | PUT_32BIT(p, qlen); | |
328 | p += 4; | |
329 | for (i = qlen; i--;) | |
330 | *p++ = bignum_byte(dss->q, i); | |
331 | PUT_32BIT(p, glen); | |
332 | p += 4; | |
333 | for (i = glen; i--;) | |
334 | *p++ = bignum_byte(dss->g, i); | |
335 | PUT_32BIT(p, ylen); | |
336 | p += 4; | |
337 | for (i = ylen; i--;) | |
338 | *p++ = bignum_byte(dss->y, i); | |
339 | assert(p == blob + bloblen); | |
340 | *len = bloblen; | |
341 | return blob; | |
342 | } | |
343 | ||
344 | static unsigned char *dss_private_blob(void *key, int *len) | |
345 | { | |
346 | struct dss_key *dss = (struct dss_key *) key; | |
347 | int xlen, bloblen; | |
348 | int i; | |
349 | unsigned char *blob, *p; | |
350 | ||
351 | xlen = (bignum_bitcount(dss->x) + 8) / 8; | |
352 | ||
353 | /* | |
354 | * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen. | |
355 | */ | |
356 | bloblen = 4 + xlen; | |
357 | blob = snewn(bloblen, unsigned char); | |
358 | p = blob; | |
359 | PUT_32BIT(p, xlen); | |
360 | p += 4; | |
361 | for (i = xlen; i--;) | |
362 | *p++ = bignum_byte(dss->x, i); | |
363 | assert(p == blob + bloblen); | |
364 | *len = bloblen; | |
365 | return blob; | |
366 | } | |
367 | ||
368 | static void *dss_createkey(unsigned char *pub_blob, int pub_len, | |
369 | unsigned char *priv_blob, int priv_len) | |
370 | { | |
371 | struct dss_key *dss; | |
372 | char *pb = (char *) priv_blob; | |
373 | char *hash; | |
374 | int hashlen; | |
375 | SHA_State s; | |
376 | unsigned char digest[20]; | |
377 | Bignum ytest; | |
378 | ||
379 | dss = dss_newkey((char *) pub_blob, pub_len); | |
380 | dss->x = getmp(&pb, &priv_len); | |
381 | ||
382 | /* | |
383 | * Check the obsolete hash in the old DSS key format. | |
384 | */ | |
385 | hashlen = -1; | |
386 | getstring(&pb, &priv_len, &hash, &hashlen); | |
387 | if (hashlen == 20) { | |
388 | SHA_Init(&s); | |
389 | sha_mpint(&s, dss->p); | |
390 | sha_mpint(&s, dss->q); | |
391 | sha_mpint(&s, dss->g); | |
392 | SHA_Final(&s, digest); | |
393 | if (0 != memcmp(hash, digest, 20)) { | |
394 | dss_freekey(dss); | |
395 | return NULL; | |
396 | } | |
397 | } | |
398 | ||
399 | /* | |
400 | * Now ensure g^x mod p really is y. | |
401 | */ | |
402 | ytest = modpow(dss->g, dss->x, dss->p); | |
403 | if (0 != bignum_cmp(ytest, dss->y)) { | |
404 | dss_freekey(dss); | |
405 | return NULL; | |
406 | } | |
407 | freebn(ytest); | |
408 | ||
409 | return dss; | |
410 | } | |
411 | ||
412 | static void *dss_openssh_createkey(unsigned char **blob, int *len) | |
413 | { | |
414 | char **b = (char **) blob; | |
415 | struct dss_key *dss; | |
416 | ||
417 | dss = snew(struct dss_key); | |
418 | if (!dss) | |
419 | return NULL; | |
420 | ||
421 | dss->p = getmp(b, len); | |
422 | dss->q = getmp(b, len); | |
423 | dss->g = getmp(b, len); | |
424 | dss->y = getmp(b, len); | |
425 | dss->x = getmp(b, len); | |
426 | ||
427 | if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x) { | |
428 | sfree(dss->p); | |
429 | sfree(dss->q); | |
430 | sfree(dss->g); | |
431 | sfree(dss->y); | |
432 | sfree(dss->x); | |
433 | sfree(dss); | |
434 | return NULL; | |
435 | } | |
436 | ||
437 | return dss; | |
438 | } | |
439 | ||
440 | static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len) | |
441 | { | |
442 | struct dss_key *dss = (struct dss_key *) key; | |
443 | int bloblen, i; | |
444 | ||
445 | bloblen = | |
446 | ssh2_bignum_length(dss->p) + | |
447 | ssh2_bignum_length(dss->q) + | |
448 | ssh2_bignum_length(dss->g) + | |
449 | ssh2_bignum_length(dss->y) + | |
450 | ssh2_bignum_length(dss->x); | |
451 | ||
452 | if (bloblen > len) | |
453 | return bloblen; | |
454 | ||
455 | bloblen = 0; | |
456 | #define ENC(x) \ | |
457 | PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \ | |
458 | for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i); | |
459 | ENC(dss->p); | |
460 | ENC(dss->q); | |
461 | ENC(dss->g); | |
462 | ENC(dss->y); | |
463 | ENC(dss->x); | |
464 | ||
465 | return bloblen; | |
466 | } | |
467 | ||
468 | static int dss_pubkey_bits(void *blob, int len) | |
469 | { | |
470 | struct dss_key *dss; | |
471 | int ret; | |
472 | ||
473 | dss = dss_newkey((char *) blob, len); | |
474 | ret = bignum_bitcount(dss->p); | |
475 | dss_freekey(dss); | |
476 | ||
477 | return ret; | |
478 | } | |
479 | ||
480 | static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen) | |
481 | { | |
482 | /* | |
483 | * The basic DSS signing algorithm is: | |
484 | * | |
485 | * - invent a random k between 1 and q-1 (exclusive). | |
486 | * - Compute r = (g^k mod p) mod q. | |
487 | * - Compute s = k^-1 * (hash + x*r) mod q. | |
488 | * | |
489 | * This has the dangerous properties that: | |
490 | * | |
491 | * - if an attacker in possession of the public key _and_ the | |
492 | * signature (for example, the host you just authenticated | |
493 | * to) can guess your k, he can reverse the computation of s | |
494 | * and work out x = r^-1 * (s*k - hash) mod q. That is, he | |
495 | * can deduce the private half of your key, and masquerade | |
496 | * as you for as long as the key is still valid. | |
497 | * | |
498 | * - since r is a function purely of k and the public key, if | |
499 | * the attacker only has a _range of possibilities_ for k | |
500 | * it's easy for him to work through them all and check each | |
501 | * one against r; he'll never be unsure of whether he's got | |
502 | * the right one. | |
503 | * | |
504 | * - if you ever sign two different hashes with the same k, it | |
505 | * will be immediately obvious because the two signatures | |
506 | * will have the same r, and moreover an attacker in | |
507 | * possession of both signatures (and the public key of | |
508 | * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q, | |
509 | * and from there deduce x as before. | |
510 | * | |
511 | * - the Bleichenbacher attack on DSA makes use of methods of | |
512 | * generating k which are significantly non-uniformly | |
513 | * distributed; in particular, generating a 160-bit random | |
514 | * number and reducing it mod q is right out. | |
515 | * | |
516 | * For this reason we must be pretty careful about how we | |
517 | * generate our k. Since this code runs on Windows, with no | |
518 | * particularly good system entropy sources, we can't trust our | |
519 | * RNG itself to produce properly unpredictable data. Hence, we | |
520 | * use a totally different scheme instead. | |
521 | * | |
522 | * What we do is to take a SHA-512 (_big_) hash of the private | |
523 | * key x, and then feed this into another SHA-512 hash that | |
524 | * also includes the message hash being signed. That is: | |
525 | * | |
526 | * proto_k = SHA512 ( SHA512(x) || SHA160(message) ) | |
527 | * | |
528 | * This number is 512 bits long, so reducing it mod q won't be | |
529 | * noticeably non-uniform. So | |
530 | * | |
531 | * k = proto_k mod q | |
532 | * | |
533 | * This has the interesting property that it's _deterministic_: | |
534 | * signing the same hash twice with the same key yields the | |
535 | * same signature. | |
536 | * | |
537 | * Despite this determinism, it's still not predictable to an | |
538 | * attacker, because in order to repeat the SHA-512 | |
539 | * construction that created it, the attacker would have to | |
540 | * know the private key value x - and by assumption he doesn't, | |
541 | * because if he knew that he wouldn't be attacking k! | |
542 | * | |
543 | * (This trick doesn't, _per se_, protect against reuse of k. | |
544 | * Reuse of k is left to chance; all it does is prevent | |
545 | * _excessively high_ chances of reuse of k due to entropy | |
546 | * problems.) | |
547 | * | |
548 | * Thanks to Colin Plumb for the general idea of using x to | |
549 | * ensure k is hard to guess, and to the Cambridge University | |
550 | * Computer Security Group for helping to argue out all the | |
551 | * fine details. | |
552 | */ | |
553 | struct dss_key *dss = (struct dss_key *) key; | |
554 | SHA512_State ss; | |
555 | unsigned char digest[20], digest512[64]; | |
556 | Bignum proto_k, k, gkp, hash, kinv, hxr, r, s; | |
557 | unsigned char *bytes; | |
558 | int nbytes, i; | |
559 | ||
560 | SHA_Simple(data, datalen, digest); | |
561 | ||
562 | /* | |
563 | * Hash some identifying text plus x. | |
564 | */ | |
565 | SHA512_Init(&ss); | |
566 | SHA512_Bytes(&ss, "DSA deterministic k generator", 30); | |
567 | sha512_mpint(&ss, dss->x); | |
568 | SHA512_Final(&ss, digest512); | |
569 | ||
570 | /* | |
571 | * Now hash that digest plus the message hash. | |
572 | */ | |
573 | SHA512_Init(&ss); | |
574 | SHA512_Bytes(&ss, digest512, sizeof(digest512)); | |
575 | SHA512_Bytes(&ss, digest, sizeof(digest)); | |
576 | SHA512_Final(&ss, digest512); | |
577 | ||
578 | memset(&ss, 0, sizeof(ss)); | |
579 | ||
580 | /* | |
581 | * Now convert the result into a bignum, and reduce it mod q. | |
582 | */ | |
583 | proto_k = bignum_from_bytes(digest512, 64); | |
584 | k = bigmod(proto_k, dss->q); | |
585 | freebn(proto_k); | |
586 | ||
587 | memset(digest512, 0, sizeof(digest512)); | |
588 | ||
589 | /* | |
590 | * Now we have k, so just go ahead and compute the signature. | |
591 | */ | |
592 | gkp = modpow(dss->g, k, dss->p); /* g^k mod p */ | |
593 | r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */ | |
594 | freebn(gkp); | |
595 | ||
596 | hash = bignum_from_bytes(digest, 20); | |
597 | kinv = modinv(k, dss->q); /* k^-1 mod q */ | |
598 | hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */ | |
599 | s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */ | |
600 | freebn(hxr); | |
601 | freebn(kinv); | |
602 | freebn(hash); | |
603 | ||
604 | /* | |
605 | * Signature blob is | |
606 | * | |
607 | * string "ssh-dss" | |
608 | * string two 20-byte numbers r and s, end to end | |
609 | * | |
610 | * i.e. 4+7 + 4+40 bytes. | |
611 | */ | |
612 | nbytes = 4 + 7 + 4 + 40; | |
613 | bytes = snewn(nbytes, unsigned char); | |
614 | PUT_32BIT(bytes, 7); | |
615 | memcpy(bytes + 4, "ssh-dss", 7); | |
616 | PUT_32BIT(bytes + 4 + 7, 40); | |
617 | for (i = 0; i < 20; i++) { | |
618 | bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i); | |
619 | bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i); | |
620 | } | |
621 | freebn(r); | |
622 | freebn(s); | |
623 | ||
624 | *siglen = nbytes; | |
625 | return bytes; | |
626 | } | |
627 | ||
628 | const struct ssh_signkey ssh_dss = { | |
629 | dss_newkey, | |
630 | dss_freekey, | |
631 | dss_fmtkey, | |
632 | dss_public_blob, | |
633 | dss_private_blob, | |
634 | dss_createkey, | |
635 | dss_openssh_createkey, | |
636 | dss_openssh_fmtkey, | |
637 | dss_pubkey_bits, | |
638 | dss_fingerprint, | |
639 | dss_verifysig, | |
640 | dss_sign, | |
641 | "ssh-dss", | |
642 | "dss" | |
643 | }; |