eaf1e20a |
1 | /* |
2 | * Digital Signature Standard implementation for PuTTY. |
3 | */ |
4 | |
7cca0d81 |
5 | #include <stdio.h> |
6 | #include <stdlib.h> |
65a22376 |
7 | #include <assert.h> |
7cca0d81 |
8 | |
e5574168 |
9 | #include "ssh.h" |
5c72ca61 |
10 | #include "misc.h" |
e5574168 |
11 | |
5c72ca61 |
12 | static void sha_mpint(SHA_State * s, Bignum b) |
13 | { |
5c72ca61 |
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 | } |
dfb88efd |
23 | smemclr(lenbuf, sizeof(lenbuf)); |
5c72ca61 |
24 | } |
25 | |
26 | static void sha512_mpint(SHA512_State * s, Bignum b) |
27 | { |
5c72ca61 |
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 | } |
dfb88efd |
37 | smemclr(lenbuf, sizeof(lenbuf)); |
5c72ca61 |
38 | } |
9c621433 |
39 | |
32874aea |
40 | static void getstring(char **data, int *datalen, char **p, int *length) |
41 | { |
7cca0d81 |
42 | *p = NULL; |
43 | if (*datalen < 4) |
32874aea |
44 | return; |
b1650067 |
45 | *length = toint(GET_32BIT(*data)); |
af1da246 |
46 | if (*length < 0) |
47 | return; |
32874aea |
48 | *datalen -= 4; |
49 | *data += 4; |
7cca0d81 |
50 | if (*datalen < *length) |
32874aea |
51 | return; |
7cca0d81 |
52 | *p = *data; |
32874aea |
53 | *data += *length; |
54 | *datalen -= *length; |
7cca0d81 |
55 | } |
32874aea |
56 | static Bignum getmp(char **data, int *datalen) |
57 | { |
7cca0d81 |
58 | char *p; |
3709bfe9 |
59 | int length; |
7cca0d81 |
60 | Bignum b; |
61 | |
62 | getstring(data, datalen, &p, &length); |
63 | if (!p) |
32874aea |
64 | return NULL; |
7cca0d81 |
65 | if (p[0] & 0x80) |
32874aea |
66 | return NULL; /* negative mp */ |
9202b1b2 |
67 | b = bignum_from_bytes((unsigned char *)p, length); |
7cca0d81 |
68 | return b; |
69 | } |
70 | |
32874aea |
71 | static Bignum get160(char **data, int *datalen) |
72 | { |
7cca0d81 |
73 | Bignum b; |
74 | |
9febf7ed |
75 | if (*datalen < 20) |
76 | return NULL; |
77 | |
9202b1b2 |
78 | b = bignum_from_bytes((unsigned char *)*data, 20); |
32874aea |
79 | *data += 20; |
80 | *datalen -= 20; |
7cca0d81 |
81 | |
7cca0d81 |
82 | return b; |
83 | } |
84 | |
36a70e5c |
85 | static void dss_freekey(void *key); /* forward reference */ |
86 | |
32874aea |
87 | static void *dss_newkey(char *data, int len) |
88 | { |
7cca0d81 |
89 | char *p; |
90 | int slen; |
e055a386 |
91 | struct dss_key *dss; |
92 | |
3d88e64d |
93 | dss = snew(struct dss_key); |
7cca0d81 |
94 | getstring(&data, &len, &p, &slen); |
9c621433 |
95 | |
96 | #ifdef DEBUG_DSS |
97 | { |
32874aea |
98 | int i; |
99 | printf("key:"); |
100 | for (i = 0; i < len; i++) |
101 | printf(" %02x", (unsigned char) (data[i])); |
102 | printf("\n"); |
9c621433 |
103 | } |
104 | #endif |
105 | |
af1da246 |
106 | if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) { |
dcbde236 |
107 | sfree(dss); |
e055a386 |
108 | return NULL; |
7cca0d81 |
109 | } |
e055a386 |
110 | dss->p = getmp(&data, &len); |
111 | dss->q = getmp(&data, &len); |
112 | dss->g = getmp(&data, &len); |
113 | dss->y = getmp(&data, &len); |
e6d62ec3 |
114 | dss->x = NULL; |
e055a386 |
115 | |
36a70e5c |
116 | if (!dss->p || !dss->q || !dss->g || !dss->y || |
117 | !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) { |
118 | /* Invalid key. */ |
119 | dss_freekey(dss); |
120 | return NULL; |
121 | } |
122 | |
e055a386 |
123 | return dss; |
7cca0d81 |
124 | } |
125 | |
32874aea |
126 | static void dss_freekey(void *key) |
127 | { |
128 | struct dss_key *dss = (struct dss_key *) key; |
e6d62ec3 |
129 | if (dss->p) |
130 | freebn(dss->p); |
131 | if (dss->q) |
132 | freebn(dss->q); |
133 | if (dss->g) |
134 | freebn(dss->g); |
135 | if (dss->y) |
136 | freebn(dss->y); |
137 | if (dss->x) |
138 | freebn(dss->x); |
dcbde236 |
139 | sfree(dss); |
e055a386 |
140 | } |
141 | |
32874aea |
142 | static char *dss_fmtkey(void *key) |
143 | { |
144 | struct dss_key *dss = (struct dss_key *) key; |
7cca0d81 |
145 | char *p; |
d5859615 |
146 | int len, i, pos, nibbles; |
147 | static const char hex[] = "0123456789abcdef"; |
e055a386 |
148 | if (!dss->p) |
32874aea |
149 | return NULL; |
150 | len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */ |
151 | len += 4 * (bignum_bitcount(dss->p) + 15) / 16; |
152 | len += 4 * (bignum_bitcount(dss->q) + 15) / 16; |
153 | len += 4 * (bignum_bitcount(dss->g) + 15) / 16; |
154 | len += 4 * (bignum_bitcount(dss->y) + 15) / 16; |
3d88e64d |
155 | p = snewn(len, char); |
32874aea |
156 | if (!p) |
157 | return NULL; |
d5859615 |
158 | |
159 | pos = 0; |
32874aea |
160 | pos += sprintf(p + pos, "0x"); |
161 | nibbles = (3 + bignum_bitcount(dss->p)) / 4; |
162 | if (nibbles < 1) |
163 | nibbles = 1; |
164 | for (i = nibbles; i--;) |
165 | p[pos++] = |
166 | hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF]; |
167 | pos += sprintf(p + pos, ",0x"); |
168 | nibbles = (3 + bignum_bitcount(dss->q)) / 4; |
169 | if (nibbles < 1) |
170 | nibbles = 1; |
171 | for (i = nibbles; i--;) |
172 | p[pos++] = |
173 | hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF]; |
174 | pos += sprintf(p + pos, ",0x"); |
175 | nibbles = (3 + bignum_bitcount(dss->g)) / 4; |
176 | if (nibbles < 1) |
177 | nibbles = 1; |
178 | for (i = nibbles; i--;) |
179 | p[pos++] = |
180 | hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF]; |
181 | pos += sprintf(p + pos, ",0x"); |
182 | nibbles = (3 + bignum_bitcount(dss->y)) / 4; |
183 | if (nibbles < 1) |
184 | nibbles = 1; |
185 | for (i = nibbles; i--;) |
186 | p[pos++] = |
187 | hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF]; |
d5859615 |
188 | p[pos] = '\0'; |
7cca0d81 |
189 | return p; |
190 | } |
191 | |
32874aea |
192 | static char *dss_fingerprint(void *key) |
193 | { |
194 | struct dss_key *dss = (struct dss_key *) key; |
d5859615 |
195 | struct MD5Context md5c; |
196 | unsigned char digest[16], lenbuf[4]; |
32874aea |
197 | char buffer[16 * 3 + 40]; |
d5859615 |
198 | char *ret; |
199 | int numlen, i; |
200 | |
201 | MD5Init(&md5c); |
9202b1b2 |
202 | MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11); |
d5859615 |
203 | |
204 | #define ADD_BIGNUM(bignum) \ |
ddecd643 |
205 | numlen = (bignum_bitcount(bignum)+8)/8; \ |
d5859615 |
206 | PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \ |
207 | for (i = numlen; i-- ;) { \ |
208 | unsigned char c = bignum_byte(bignum, i); \ |
209 | MD5Update(&md5c, &c, 1); \ |
210 | } |
e055a386 |
211 | ADD_BIGNUM(dss->p); |
212 | ADD_BIGNUM(dss->q); |
213 | ADD_BIGNUM(dss->g); |
214 | ADD_BIGNUM(dss->y); |
d5859615 |
215 | #undef ADD_BIGNUM |
216 | |
217 | MD5Final(digest, &md5c); |
218 | |
ddecd643 |
219 | sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p)); |
d5859615 |
220 | for (i = 0; i < 16; i++) |
32874aea |
221 | sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "", |
222 | digest[i]); |
3d88e64d |
223 | ret = snewn(strlen(buffer) + 1, char); |
d5859615 |
224 | if (ret) |
32874aea |
225 | strcpy(ret, buffer); |
d5859615 |
226 | return ret; |
227 | } |
228 | |
e055a386 |
229 | static int dss_verifysig(void *key, char *sig, int siglen, |
32874aea |
230 | char *data, int datalen) |
231 | { |
232 | struct dss_key *dss = (struct dss_key *) key; |
7cca0d81 |
233 | char *p; |
c5fbd713 |
234 | int slen; |
7cca0d81 |
235 | char hash[20]; |
59600f67 |
236 | Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v; |
7cca0d81 |
237 | int ret; |
238 | |
e055a386 |
239 | if (!dss->p) |
32874aea |
240 | return 0; |
7cca0d81 |
241 | |
9c621433 |
242 | #ifdef DEBUG_DSS |
243 | { |
32874aea |
244 | int i; |
245 | printf("sig:"); |
246 | for (i = 0; i < siglen; i++) |
247 | printf(" %02x", (unsigned char) (sig[i])); |
248 | printf("\n"); |
9c621433 |
249 | } |
250 | #endif |
7f7837c8 |
251 | /* |
252 | * Commercial SSH (2.0.13) and OpenSSH disagree over the format |
bccbeb71 |
253 | * of a DSA signature. OpenSSH is in line with RFC 4253: |
7f7837c8 |
254 | * it uses a string "ssh-dss", followed by a 40-byte string |
255 | * containing two 160-bit integers end-to-end. Commercial SSH |
256 | * can't be bothered with the header bit, and considers a DSA |
257 | * signature blob to be _just_ the 40-byte string containing |
258 | * the two 160-bit integers. We tell them apart by measuring |
259 | * the length: length 40 means the commercial-SSH bug, anything |
bccbeb71 |
260 | * else is assumed to be RFC-compliant. |
7f7837c8 |
261 | */ |
32874aea |
262 | if (siglen != 40) { /* bug not present; read admin fields */ |
263 | getstring(&sig, &siglen, &p, &slen); |
264 | if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) { |
265 | return 0; |
266 | } |
267 | sig += 4, siglen -= 4; /* skip yet another length field */ |
7cca0d81 |
268 | } |
7cca0d81 |
269 | r = get160(&sig, &siglen); |
270 | s = get160(&sig, &siglen); |
43472509 |
271 | if (!r || !s) { |
272 | if (r) |
273 | freebn(r); |
274 | if (s) |
275 | freebn(s); |
32874aea |
276 | return 0; |
43472509 |
277 | } |
7cca0d81 |
278 | |
919e9a53 |
279 | if (!bignum_cmp(s, Zero)) { |
280 | freebn(r); |
281 | freebn(s); |
282 | return 0; |
283 | } |
284 | |
7cca0d81 |
285 | /* |
286 | * Step 1. w <- s^-1 mod q. |
287 | */ |
e055a386 |
288 | w = modinv(s, dss->q); |
de81309d |
289 | if (!w) { |
290 | freebn(r); |
291 | freebn(s); |
292 | return 0; |
293 | } |
7cca0d81 |
294 | |
295 | /* |
296 | * Step 2. u1 <- SHA(message) * w mod q. |
297 | */ |
9202b1b2 |
298 | SHA_Simple(data, datalen, (unsigned char *)hash); |
32874aea |
299 | p = hash; |
300 | slen = 20; |
301 | sha = get160(&p, &slen); |
e055a386 |
302 | u1 = modmul(sha, w, dss->q); |
7cca0d81 |
303 | |
304 | /* |
305 | * Step 3. u2 <- r * w mod q. |
306 | */ |
e055a386 |
307 | u2 = modmul(r, w, dss->q); |
7cca0d81 |
308 | |
309 | /* |
310 | * Step 4. v <- (g^u1 * y^u2 mod p) mod q. |
311 | */ |
e055a386 |
312 | gu1p = modpow(dss->g, u1, dss->p); |
e055a386 |
313 | yu2p = modpow(dss->y, u2, dss->p); |
e055a386 |
314 | gu1yu2p = modmul(gu1p, yu2p, dss->p); |
e055a386 |
315 | v = modmul(gu1yu2p, One, dss->q); |
7cca0d81 |
316 | |
317 | /* |
318 | * Step 5. v should now be equal to r. |
319 | */ |
320 | |
c5fbd713 |
321 | ret = !bignum_cmp(v, r); |
7cca0d81 |
322 | |
323 | freebn(w); |
7cca0d81 |
324 | freebn(sha); |
038ec85e |
325 | freebn(u1); |
326 | freebn(u2); |
59600f67 |
327 | freebn(gu1p); |
328 | freebn(yu2p); |
329 | freebn(gu1yu2p); |
7cca0d81 |
330 | freebn(v); |
331 | freebn(r); |
332 | freebn(s); |
333 | |
334 | return ret; |
335 | } |
336 | |
32874aea |
337 | static unsigned char *dss_public_blob(void *key, int *len) |
338 | { |
339 | struct dss_key *dss = (struct dss_key *) key; |
65a22376 |
340 | int plen, qlen, glen, ylen, bloblen; |
341 | int i; |
342 | unsigned char *blob, *p; |
343 | |
32874aea |
344 | plen = (bignum_bitcount(dss->p) + 8) / 8; |
345 | qlen = (bignum_bitcount(dss->q) + 8) / 8; |
346 | glen = (bignum_bitcount(dss->g) + 8) / 8; |
347 | ylen = (bignum_bitcount(dss->y) + 8) / 8; |
65a22376 |
348 | |
349 | /* |
350 | * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total |
351 | * 27 + sum of lengths. (five length fields, 20+7=27). |
352 | */ |
32874aea |
353 | bloblen = 27 + plen + qlen + glen + ylen; |
3d88e64d |
354 | blob = snewn(bloblen, unsigned char); |
65a22376 |
355 | p = blob; |
32874aea |
356 | PUT_32BIT(p, 7); |
357 | p += 4; |
358 | memcpy(p, "ssh-dss", 7); |
359 | p += 7; |
360 | PUT_32BIT(p, plen); |
361 | p += 4; |
362 | for (i = plen; i--;) |
363 | *p++ = bignum_byte(dss->p, i); |
364 | PUT_32BIT(p, qlen); |
365 | p += 4; |
366 | for (i = qlen; i--;) |
367 | *p++ = bignum_byte(dss->q, i); |
368 | PUT_32BIT(p, glen); |
369 | p += 4; |
370 | for (i = glen; i--;) |
371 | *p++ = bignum_byte(dss->g, i); |
372 | PUT_32BIT(p, ylen); |
373 | p += 4; |
374 | for (i = ylen; i--;) |
375 | *p++ = bignum_byte(dss->y, i); |
65a22376 |
376 | assert(p == blob + bloblen); |
377 | *len = bloblen; |
378 | return blob; |
379 | } |
380 | |
32874aea |
381 | static unsigned char *dss_private_blob(void *key, int *len) |
382 | { |
5c72ca61 |
383 | struct dss_key *dss = (struct dss_key *) key; |
384 | int xlen, bloblen; |
385 | int i; |
386 | unsigned char *blob, *p; |
5c72ca61 |
387 | |
388 | xlen = (bignum_bitcount(dss->x) + 8) / 8; |
389 | |
390 | /* |
7bedb13c |
391 | * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen. |
5c72ca61 |
392 | */ |
7bedb13c |
393 | bloblen = 4 + xlen; |
3d88e64d |
394 | blob = snewn(bloblen, unsigned char); |
5c72ca61 |
395 | p = blob; |
396 | PUT_32BIT(p, xlen); |
397 | p += 4; |
398 | for (i = xlen; i--;) |
399 | *p++ = bignum_byte(dss->x, i); |
5c72ca61 |
400 | assert(p == blob + bloblen); |
401 | *len = bloblen; |
402 | return blob; |
65a22376 |
403 | } |
404 | |
405 | static void *dss_createkey(unsigned char *pub_blob, int pub_len, |
32874aea |
406 | unsigned char *priv_blob, int priv_len) |
407 | { |
5c72ca61 |
408 | struct dss_key *dss; |
409 | char *pb = (char *) priv_blob; |
410 | char *hash; |
411 | int hashlen; |
412 | SHA_State s; |
413 | unsigned char digest[20]; |
414 | Bignum ytest; |
415 | |
416 | dss = dss_newkey((char *) pub_blob, pub_len); |
36a70e5c |
417 | if (!dss) |
418 | return NULL; |
5c72ca61 |
419 | dss->x = getmp(&pb, &priv_len); |
36a70e5c |
420 | if (!dss->x) { |
421 | dss_freekey(dss); |
422 | return NULL; |
423 | } |
5c72ca61 |
424 | |
425 | /* |
7bedb13c |
426 | * Check the obsolete hash in the old DSS key format. |
5c72ca61 |
427 | */ |
7bedb13c |
428 | hashlen = -1; |
429 | getstring(&pb, &priv_len, &hash, &hashlen); |
430 | if (hashlen == 20) { |
431 | SHA_Init(&s); |
432 | sha_mpint(&s, dss->p); |
433 | sha_mpint(&s, dss->q); |
434 | sha_mpint(&s, dss->g); |
435 | SHA_Final(&s, digest); |
436 | if (0 != memcmp(hash, digest, 20)) { |
437 | dss_freekey(dss); |
438 | return NULL; |
439 | } |
5c72ca61 |
440 | } |
441 | |
442 | /* |
443 | * Now ensure g^x mod p really is y. |
444 | */ |
445 | ytest = modpow(dss->g, dss->x, dss->p); |
446 | if (0 != bignum_cmp(ytest, dss->y)) { |
447 | dss_freekey(dss); |
038ec85e |
448 | freebn(ytest); |
5c72ca61 |
449 | return NULL; |
450 | } |
451 | freebn(ytest); |
452 | |
453 | return dss; |
65a22376 |
454 | } |
455 | |
32874aea |
456 | static void *dss_openssh_createkey(unsigned char **blob, int *len) |
457 | { |
5c72ca61 |
458 | char **b = (char **) blob; |
459 | struct dss_key *dss; |
460 | |
3d88e64d |
461 | dss = snew(struct dss_key); |
5c72ca61 |
462 | |
463 | dss->p = getmp(b, len); |
464 | dss->q = getmp(b, len); |
465 | dss->g = getmp(b, len); |
466 | dss->y = getmp(b, len); |
467 | dss->x = getmp(b, len); |
468 | |
36a70e5c |
469 | if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x || |
470 | !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) { |
471 | /* Invalid key. */ |
472 | dss_freekey(dss); |
473 | return NULL; |
5c72ca61 |
474 | } |
475 | |
476 | return dss; |
45cebe79 |
477 | } |
478 | |
32874aea |
479 | static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len) |
480 | { |
5c72ca61 |
481 | struct dss_key *dss = (struct dss_key *) key; |
482 | int bloblen, i; |
483 | |
484 | bloblen = |
485 | ssh2_bignum_length(dss->p) + |
486 | ssh2_bignum_length(dss->q) + |
487 | ssh2_bignum_length(dss->g) + |
488 | ssh2_bignum_length(dss->y) + |
489 | ssh2_bignum_length(dss->x); |
490 | |
491 | if (bloblen > len) |
492 | return bloblen; |
493 | |
494 | bloblen = 0; |
495 | #define ENC(x) \ |
496 | PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \ |
497 | for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i); |
498 | ENC(dss->p); |
499 | ENC(dss->q); |
500 | ENC(dss->g); |
501 | ENC(dss->y); |
502 | ENC(dss->x); |
503 | |
504 | return bloblen; |
ddecd643 |
505 | } |
506 | |
47a6b94c |
507 | static int dss_pubkey_bits(void *blob, int len) |
508 | { |
509 | struct dss_key *dss; |
510 | int ret; |
511 | |
512 | dss = dss_newkey((char *) blob, len); |
36a70e5c |
513 | if (!dss) |
514 | return -1; |
47a6b94c |
515 | ret = bignum_bitcount(dss->p); |
516 | dss_freekey(dss); |
517 | |
518 | return ret; |
519 | } |
520 | |
900852ab |
521 | static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen) |
32874aea |
522 | { |
5c72ca61 |
523 | /* |
524 | * The basic DSS signing algorithm is: |
525 | * |
526 | * - invent a random k between 1 and q-1 (exclusive). |
527 | * - Compute r = (g^k mod p) mod q. |
528 | * - Compute s = k^-1 * (hash + x*r) mod q. |
529 | * |
530 | * This has the dangerous properties that: |
531 | * |
532 | * - if an attacker in possession of the public key _and_ the |
533 | * signature (for example, the host you just authenticated |
534 | * to) can guess your k, he can reverse the computation of s |
535 | * and work out x = r^-1 * (s*k - hash) mod q. That is, he |
536 | * can deduce the private half of your key, and masquerade |
537 | * as you for as long as the key is still valid. |
538 | * |
539 | * - since r is a function purely of k and the public key, if |
540 | * the attacker only has a _range of possibilities_ for k |
541 | * it's easy for him to work through them all and check each |
542 | * one against r; he'll never be unsure of whether he's got |
543 | * the right one. |
544 | * |
545 | * - if you ever sign two different hashes with the same k, it |
546 | * will be immediately obvious because the two signatures |
547 | * will have the same r, and moreover an attacker in |
548 | * possession of both signatures (and the public key of |
549 | * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q, |
550 | * and from there deduce x as before. |
551 | * |
552 | * - the Bleichenbacher attack on DSA makes use of methods of |
553 | * generating k which are significantly non-uniformly |
554 | * distributed; in particular, generating a 160-bit random |
555 | * number and reducing it mod q is right out. |
556 | * |
557 | * For this reason we must be pretty careful about how we |
558 | * generate our k. Since this code runs on Windows, with no |
559 | * particularly good system entropy sources, we can't trust our |
560 | * RNG itself to produce properly unpredictable data. Hence, we |
561 | * use a totally different scheme instead. |
562 | * |
563 | * What we do is to take a SHA-512 (_big_) hash of the private |
564 | * key x, and then feed this into another SHA-512 hash that |
565 | * also includes the message hash being signed. That is: |
566 | * |
567 | * proto_k = SHA512 ( SHA512(x) || SHA160(message) ) |
568 | * |
569 | * This number is 512 bits long, so reducing it mod q won't be |
570 | * noticeably non-uniform. So |
571 | * |
572 | * k = proto_k mod q |
573 | * |
574 | * This has the interesting property that it's _deterministic_: |
575 | * signing the same hash twice with the same key yields the |
576 | * same signature. |
577 | * |
5ced2a02 |
578 | * Despite this determinism, it's still not predictable to an |
579 | * attacker, because in order to repeat the SHA-512 |
580 | * construction that created it, the attacker would have to |
581 | * know the private key value x - and by assumption he doesn't, |
582 | * because if he knew that he wouldn't be attacking k! |
583 | * |
584 | * (This trick doesn't, _per se_, protect against reuse of k. |
585 | * Reuse of k is left to chance; all it does is prevent |
586 | * _excessively high_ chances of reuse of k due to entropy |
587 | * problems.) |
5c72ca61 |
588 | * |
589 | * Thanks to Colin Plumb for the general idea of using x to |
590 | * ensure k is hard to guess, and to the Cambridge University |
591 | * Computer Security Group for helping to argue out all the |
592 | * fine details. |
593 | */ |
594 | struct dss_key *dss = (struct dss_key *) key; |
595 | SHA512_State ss; |
596 | unsigned char digest[20], digest512[64]; |
597 | Bignum proto_k, k, gkp, hash, kinv, hxr, r, s; |
598 | unsigned char *bytes; |
599 | int nbytes, i; |
600 | |
601 | SHA_Simple(data, datalen, digest); |
602 | |
603 | /* |
604 | * Hash some identifying text plus x. |
605 | */ |
606 | SHA512_Init(&ss); |
607 | SHA512_Bytes(&ss, "DSA deterministic k generator", 30); |
608 | sha512_mpint(&ss, dss->x); |
609 | SHA512_Final(&ss, digest512); |
610 | |
611 | /* |
612 | * Now hash that digest plus the message hash. |
613 | */ |
614 | SHA512_Init(&ss); |
615 | SHA512_Bytes(&ss, digest512, sizeof(digest512)); |
616 | SHA512_Bytes(&ss, digest, sizeof(digest)); |
5c72ca61 |
617 | |
de81309d |
618 | while (1) { |
619 | SHA512_State ss2 = ss; /* structure copy */ |
620 | SHA512_Final(&ss2, digest512); |
621 | |
622 | smemclr(&ss2, sizeof(ss2)); |
623 | |
624 | /* |
625 | * Now convert the result into a bignum, and reduce it mod q. |
626 | */ |
627 | proto_k = bignum_from_bytes(digest512, 64); |
628 | k = bigmod(proto_k, dss->q); |
629 | freebn(proto_k); |
630 | kinv = modinv(k, dss->q); /* k^-1 mod q */ |
631 | if (!kinv) { /* very unlikely */ |
632 | freebn(k); |
633 | /* Perturb the hash to think of a different k. */ |
634 | SHA512_Bytes(&ss, "x", 1); |
635 | /* Go round and try again. */ |
636 | continue; |
637 | } |
638 | |
639 | break; |
640 | } |
5c72ca61 |
641 | |
de81309d |
642 | smemclr(&ss, sizeof(ss)); |
5c72ca61 |
643 | |
dfb88efd |
644 | smemclr(digest512, sizeof(digest512)); |
5c72ca61 |
645 | |
646 | /* |
647 | * Now we have k, so just go ahead and compute the signature. |
648 | */ |
649 | gkp = modpow(dss->g, k, dss->p); /* g^k mod p */ |
650 | r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */ |
651 | freebn(gkp); |
652 | |
653 | hash = bignum_from_bytes(digest, 20); |
5c72ca61 |
654 | hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */ |
655 | s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */ |
656 | freebn(hxr); |
657 | freebn(kinv); |
43472509 |
658 | freebn(k); |
5c72ca61 |
659 | freebn(hash); |
660 | |
661 | /* |
662 | * Signature blob is |
663 | * |
664 | * string "ssh-dss" |
665 | * string two 20-byte numbers r and s, end to end |
666 | * |
667 | * i.e. 4+7 + 4+40 bytes. |
668 | */ |
669 | nbytes = 4 + 7 + 4 + 40; |
3d88e64d |
670 | bytes = snewn(nbytes, unsigned char); |
5c72ca61 |
671 | PUT_32BIT(bytes, 7); |
672 | memcpy(bytes + 4, "ssh-dss", 7); |
673 | PUT_32BIT(bytes + 4 + 7, 40); |
674 | for (i = 0; i < 20; i++) { |
675 | bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i); |
676 | bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i); |
677 | } |
678 | freebn(r); |
679 | freebn(s); |
680 | |
681 | *siglen = nbytes; |
682 | return bytes; |
e055a386 |
683 | } |
684 | |
65a22376 |
685 | const struct ssh_signkey ssh_dss = { |
e055a386 |
686 | dss_newkey, |
687 | dss_freekey, |
7cca0d81 |
688 | dss_fmtkey, |
65a22376 |
689 | dss_public_blob, |
690 | dss_private_blob, |
691 | dss_createkey, |
45cebe79 |
692 | dss_openssh_createkey, |
ddecd643 |
693 | dss_openssh_fmtkey, |
47a6b94c |
694 | dss_pubkey_bits, |
d5859615 |
695 | dss_fingerprint, |
7cca0d81 |
696 | dss_verifysig, |
e055a386 |
697 | dss_sign, |
d5859615 |
698 | "ssh-dss", |
699 | "dss" |
e5574168 |
700 | }; |