2 * Digital Signature Standard implementation for PuTTY.
12 static void sha_mpint(SHA_State * s, Bignum b)
14 unsigned char lenbuf[4];
16 len = (bignum_bitcount(b) + 8) / 8;
17 PUT_32BIT(lenbuf, len);
18 SHA_Bytes(s, lenbuf, 4);
20 lenbuf[0] = bignum_byte(b, len);
21 SHA_Bytes(s, lenbuf, 1);
23 smemclr(lenbuf, sizeof(lenbuf));
26 static void sha512_mpint(SHA512_State * s, Bignum b)
28 unsigned char lenbuf[4];
30 len = (bignum_bitcount(b) + 8) / 8;
31 PUT_32BIT(lenbuf, len);
32 SHA512_Bytes(s, lenbuf, 4);
34 lenbuf[0] = bignum_byte(b, len);
35 SHA512_Bytes(s, lenbuf, 1);
37 smemclr(lenbuf, sizeof(lenbuf));
40 static void getstring(char **data, int *datalen, char **p, int *length)
45 *length = toint(GET_32BIT(*data));
50 if (*datalen < *length)
56 static Bignum getmp(char **data, int *datalen)
62 getstring(data, datalen, &p, &length);
66 return NULL; /* negative mp */
67 b = bignum_from_bytes((unsigned char *)p, length);
71 static Bignum get160(char **data, int *datalen)
75 b = bignum_from_bytes((unsigned char *)*data, 20);
82 static void *dss_newkey(char *data, int len)
88 dss = snew(struct dss_key);
91 getstring(&data, &len, &p, &slen);
97 for (i = 0; i < len; i++)
98 printf(" %02x", (unsigned char) (data[i]));
103 if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
107 dss->p = getmp(&data, &len);
108 dss->q = getmp(&data, &len);
109 dss->g = getmp(&data, &len);
110 dss->y = getmp(&data, &len);
115 static void dss_freekey(void *key)
117 struct dss_key *dss = (struct dss_key *) key;
125 static char *dss_fmtkey(void *key)
127 struct dss_key *dss = (struct dss_key *) key;
129 int len, i, pos, nibbles;
130 static const char hex[] = "0123456789abcdef";
133 len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */
134 len += 4 * (bignum_bitcount(dss->p) + 15) / 16;
135 len += 4 * (bignum_bitcount(dss->q) + 15) / 16;
136 len += 4 * (bignum_bitcount(dss->g) + 15) / 16;
137 len += 4 * (bignum_bitcount(dss->y) + 15) / 16;
138 p = snewn(len, char);
143 pos += sprintf(p + pos, "0x");
144 nibbles = (3 + bignum_bitcount(dss->p)) / 4;
147 for (i = nibbles; i--;)
149 hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF];
150 pos += sprintf(p + pos, ",0x");
151 nibbles = (3 + bignum_bitcount(dss->q)) / 4;
154 for (i = nibbles; i--;)
156 hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF];
157 pos += sprintf(p + pos, ",0x");
158 nibbles = (3 + bignum_bitcount(dss->g)) / 4;
161 for (i = nibbles; i--;)
163 hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF];
164 pos += sprintf(p + pos, ",0x");
165 nibbles = (3 + bignum_bitcount(dss->y)) / 4;
168 for (i = nibbles; i--;)
170 hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF];
175 static char *dss_fingerprint(void *key)
177 struct dss_key *dss = (struct dss_key *) key;
178 struct MD5Context md5c;
179 unsigned char digest[16], lenbuf[4];
180 char buffer[16 * 3 + 40];
185 MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11);
187 #define ADD_BIGNUM(bignum) \
188 numlen = (bignum_bitcount(bignum)+8)/8; \
189 PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \
190 for (i = numlen; i-- ;) { \
191 unsigned char c = bignum_byte(bignum, i); \
192 MD5Update(&md5c, &c, 1); \
200 MD5Final(digest, &md5c);
202 sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p));
203 for (i = 0; i < 16; i++)
204 sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
206 ret = snewn(strlen(buffer) + 1, char);
212 static int dss_verifysig(void *key, char *sig, int siglen,
213 char *data, int datalen)
215 struct dss_key *dss = (struct dss_key *) key;
219 Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v;
229 for (i = 0; i < siglen; i++)
230 printf(" %02x", (unsigned char) (sig[i]));
235 * Commercial SSH (2.0.13) and OpenSSH disagree over the format
236 * of a DSA signature. OpenSSH is in line with RFC 4253:
237 * it uses a string "ssh-dss", followed by a 40-byte string
238 * containing two 160-bit integers end-to-end. Commercial SSH
239 * can't be bothered with the header bit, and considers a DSA
240 * signature blob to be _just_ the 40-byte string containing
241 * the two 160-bit integers. We tell them apart by measuring
242 * the length: length 40 means the commercial-SSH bug, anything
243 * else is assumed to be RFC-compliant.
245 if (siglen != 40) { /* bug not present; read admin fields */
246 getstring(&sig, &siglen, &p, &slen);
247 if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
250 sig += 4, siglen -= 4; /* skip yet another length field */
252 r = get160(&sig, &siglen);
253 s = get160(&sig, &siglen);
258 * Step 1. w <- s^-1 mod q.
260 w = modinv(s, dss->q);
263 * Step 2. u1 <- SHA(message) * w mod q.
265 SHA_Simple(data, datalen, (unsigned char *)hash);
268 sha = get160(&p, &slen);
269 u1 = modmul(sha, w, dss->q);
272 * Step 3. u2 <- r * w mod q.
274 u2 = modmul(r, w, dss->q);
277 * Step 4. v <- (g^u1 * y^u2 mod p) mod q.
279 gu1p = modpow(dss->g, u1, dss->p);
280 yu2p = modpow(dss->y, u2, dss->p);
281 gu1yu2p = modmul(gu1p, yu2p, dss->p);
282 v = modmul(gu1yu2p, One, dss->q);
285 * Step 5. v should now be equal to r.
288 ret = !bignum_cmp(v, r);
304 static unsigned char *dss_public_blob(void *key, int *len)
306 struct dss_key *dss = (struct dss_key *) key;
307 int plen, qlen, glen, ylen, bloblen;
309 unsigned char *blob, *p;
311 plen = (bignum_bitcount(dss->p) + 8) / 8;
312 qlen = (bignum_bitcount(dss->q) + 8) / 8;
313 glen = (bignum_bitcount(dss->g) + 8) / 8;
314 ylen = (bignum_bitcount(dss->y) + 8) / 8;
317 * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total
318 * 27 + sum of lengths. (five length fields, 20+7=27).
320 bloblen = 27 + plen + qlen + glen + ylen;
321 blob = snewn(bloblen, unsigned char);
325 memcpy(p, "ssh-dss", 7);
330 *p++ = bignum_byte(dss->p, i);
334 *p++ = bignum_byte(dss->q, i);
338 *p++ = bignum_byte(dss->g, i);
342 *p++ = bignum_byte(dss->y, i);
343 assert(p == blob + bloblen);
348 static unsigned char *dss_private_blob(void *key, int *len)
350 struct dss_key *dss = (struct dss_key *) key;
353 unsigned char *blob, *p;
355 xlen = (bignum_bitcount(dss->x) + 8) / 8;
358 * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
361 blob = snewn(bloblen, unsigned char);
366 *p++ = bignum_byte(dss->x, i);
367 assert(p == blob + bloblen);
372 static void *dss_createkey(unsigned char *pub_blob, int pub_len,
373 unsigned char *priv_blob, int priv_len)
376 char *pb = (char *) priv_blob;
380 unsigned char digest[20];
383 dss = dss_newkey((char *) pub_blob, pub_len);
384 dss->x = getmp(&pb, &priv_len);
387 * Check the obsolete hash in the old DSS key format.
390 getstring(&pb, &priv_len, &hash, &hashlen);
393 sha_mpint(&s, dss->p);
394 sha_mpint(&s, dss->q);
395 sha_mpint(&s, dss->g);
396 SHA_Final(&s, digest);
397 if (0 != memcmp(hash, digest, 20)) {
404 * Now ensure g^x mod p really is y.
406 ytest = modpow(dss->g, dss->x, dss->p);
407 if (0 != bignum_cmp(ytest, dss->y)) {
417 static void *dss_openssh_createkey(unsigned char **blob, int *len)
419 char **b = (char **) blob;
422 dss = snew(struct dss_key);
426 dss->p = getmp(b, len);
427 dss->q = getmp(b, len);
428 dss->g = getmp(b, len);
429 dss->y = getmp(b, len);
430 dss->x = getmp(b, len);
432 if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x) {
445 static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len)
447 struct dss_key *dss = (struct dss_key *) key;
451 ssh2_bignum_length(dss->p) +
452 ssh2_bignum_length(dss->q) +
453 ssh2_bignum_length(dss->g) +
454 ssh2_bignum_length(dss->y) +
455 ssh2_bignum_length(dss->x);
462 PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
463 for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
473 static int dss_pubkey_bits(void *blob, int len)
478 dss = dss_newkey((char *) blob, len);
479 ret = bignum_bitcount(dss->p);
485 static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
488 * The basic DSS signing algorithm is:
490 * - invent a random k between 1 and q-1 (exclusive).
491 * - Compute r = (g^k mod p) mod q.
492 * - Compute s = k^-1 * (hash + x*r) mod q.
494 * This has the dangerous properties that:
496 * - if an attacker in possession of the public key _and_ the
497 * signature (for example, the host you just authenticated
498 * to) can guess your k, he can reverse the computation of s
499 * and work out x = r^-1 * (s*k - hash) mod q. That is, he
500 * can deduce the private half of your key, and masquerade
501 * as you for as long as the key is still valid.
503 * - since r is a function purely of k and the public key, if
504 * the attacker only has a _range of possibilities_ for k
505 * it's easy for him to work through them all and check each
506 * one against r; he'll never be unsure of whether he's got
509 * - if you ever sign two different hashes with the same k, it
510 * will be immediately obvious because the two signatures
511 * will have the same r, and moreover an attacker in
512 * possession of both signatures (and the public key of
513 * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
514 * and from there deduce x as before.
516 * - the Bleichenbacher attack on DSA makes use of methods of
517 * generating k which are significantly non-uniformly
518 * distributed; in particular, generating a 160-bit random
519 * number and reducing it mod q is right out.
521 * For this reason we must be pretty careful about how we
522 * generate our k. Since this code runs on Windows, with no
523 * particularly good system entropy sources, we can't trust our
524 * RNG itself to produce properly unpredictable data. Hence, we
525 * use a totally different scheme instead.
527 * What we do is to take a SHA-512 (_big_) hash of the private
528 * key x, and then feed this into another SHA-512 hash that
529 * also includes the message hash being signed. That is:
531 * proto_k = SHA512 ( SHA512(x) || SHA160(message) )
533 * This number is 512 bits long, so reducing it mod q won't be
534 * noticeably non-uniform. So
538 * This has the interesting property that it's _deterministic_:
539 * signing the same hash twice with the same key yields the
542 * Despite this determinism, it's still not predictable to an
543 * attacker, because in order to repeat the SHA-512
544 * construction that created it, the attacker would have to
545 * know the private key value x - and by assumption he doesn't,
546 * because if he knew that he wouldn't be attacking k!
548 * (This trick doesn't, _per se_, protect against reuse of k.
549 * Reuse of k is left to chance; all it does is prevent
550 * _excessively high_ chances of reuse of k due to entropy
553 * Thanks to Colin Plumb for the general idea of using x to
554 * ensure k is hard to guess, and to the Cambridge University
555 * Computer Security Group for helping to argue out all the
558 struct dss_key *dss = (struct dss_key *) key;
560 unsigned char digest[20], digest512[64];
561 Bignum proto_k, k, gkp, hash, kinv, hxr, r, s;
562 unsigned char *bytes;
565 SHA_Simple(data, datalen, digest);
568 * Hash some identifying text plus x.
571 SHA512_Bytes(&ss, "DSA deterministic k generator", 30);
572 sha512_mpint(&ss, dss->x);
573 SHA512_Final(&ss, digest512);
576 * Now hash that digest plus the message hash.
579 SHA512_Bytes(&ss, digest512, sizeof(digest512));
580 SHA512_Bytes(&ss, digest, sizeof(digest));
581 SHA512_Final(&ss, digest512);
583 smemclr(&ss, sizeof(ss));
586 * Now convert the result into a bignum, and reduce it mod q.
588 proto_k = bignum_from_bytes(digest512, 64);
589 k = bigmod(proto_k, dss->q);
592 smemclr(digest512, sizeof(digest512));
595 * Now we have k, so just go ahead and compute the signature.
597 gkp = modpow(dss->g, k, dss->p); /* g^k mod p */
598 r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */
601 hash = bignum_from_bytes(digest, 20);
602 kinv = modinv(k, dss->q); /* k^-1 mod q */
603 hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */
604 s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */
613 * string two 20-byte numbers r and s, end to end
615 * i.e. 4+7 + 4+40 bytes.
617 nbytes = 4 + 7 + 4 + 40;
618 bytes = snewn(nbytes, unsigned char);
620 memcpy(bytes + 4, "ssh-dss", 7);
621 PUT_32BIT(bytes + 4 + 7, 40);
622 for (i = 0; i < 20; i++) {
623 bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i);
624 bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i);
633 const struct ssh_signkey ssh_dss = {
640 dss_openssh_createkey,