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
= 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
);
302 static unsigned char *dss_public_blob(void *key
, int *len
)
304 struct dss_key
*dss
= (struct dss_key
*) key
;
305 int plen
, qlen
, glen
, ylen
, bloblen
;
307 unsigned char *blob
, *p
;
309 plen
= (bignum_bitcount(dss
->p
) + 8) / 8;
310 qlen
= (bignum_bitcount(dss
->q
) + 8) / 8;
311 glen
= (bignum_bitcount(dss
->g
) + 8) / 8;
312 ylen
= (bignum_bitcount(dss
->y
) + 8) / 8;
315 * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total
316 * 27 + sum of lengths. (five length fields, 20+7=27).
318 bloblen
= 27 + plen
+ qlen
+ glen
+ ylen
;
319 blob
= snewn(bloblen
, unsigned char);
323 memcpy(p
, "ssh-dss", 7);
328 *p
++ = bignum_byte(dss
->p
, i
);
332 *p
++ = bignum_byte(dss
->q
, i
);
336 *p
++ = bignum_byte(dss
->g
, i
);
340 *p
++ = bignum_byte(dss
->y
, i
);
341 assert(p
== blob
+ bloblen
);
346 static unsigned char *dss_private_blob(void *key
, int *len
)
348 struct dss_key
*dss
= (struct dss_key
*) key
;
351 unsigned char *blob
, *p
;
353 xlen
= (bignum_bitcount(dss
->x
) + 8) / 8;
356 * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
359 blob
= snewn(bloblen
, unsigned char);
364 *p
++ = bignum_byte(dss
->x
, i
);
365 assert(p
== blob
+ bloblen
);
370 static void *dss_createkey(unsigned char *pub_blob
, int pub_len
,
371 unsigned char *priv_blob
, int priv_len
)
374 char *pb
= (char *) priv_blob
;
378 unsigned char digest
[20];
381 dss
= dss_newkey((char *) pub_blob
, pub_len
);
382 dss
->x
= getmp(&pb
, &priv_len
);
385 * Check the obsolete hash in the old DSS key format.
388 getstring(&pb
, &priv_len
, &hash
, &hashlen
);
391 sha_mpint(&s
, dss
->p
);
392 sha_mpint(&s
, dss
->q
);
393 sha_mpint(&s
, dss
->g
);
394 SHA_Final(&s
, digest
);
395 if (0 != memcmp(hash
, digest
, 20)) {
402 * Now ensure g^x mod p really is y.
404 ytest
= modpow(dss
->g
, dss
->x
, dss
->p
);
405 if (0 != bignum_cmp(ytest
, dss
->y
)) {
414 static void *dss_openssh_createkey(unsigned char **blob
, int *len
)
416 char **b
= (char **) blob
;
419 dss
= snew(struct dss_key
);
423 dss
->p
= getmp(b
, len
);
424 dss
->q
= getmp(b
, len
);
425 dss
->g
= getmp(b
, len
);
426 dss
->y
= getmp(b
, len
);
427 dss
->x
= getmp(b
, len
);
429 if (!dss
->p
|| !dss
->q
|| !dss
->g
|| !dss
->y
|| !dss
->x
) {
442 static int dss_openssh_fmtkey(void *key
, unsigned char *blob
, int len
)
444 struct dss_key
*dss
= (struct dss_key
*) key
;
448 ssh2_bignum_length(dss
->p
) +
449 ssh2_bignum_length(dss
->q
) +
450 ssh2_bignum_length(dss
->g
) +
451 ssh2_bignum_length(dss
->y
) +
452 ssh2_bignum_length(dss
->x
);
459 PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
460 for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
470 static int dss_pubkey_bits(void *blob
, int len
)
475 dss
= dss_newkey((char *) blob
, len
);
476 ret
= bignum_bitcount(dss
->p
);
482 static unsigned char *dss_sign(void *key
, char *data
, int datalen
, int *siglen
)
485 * The basic DSS signing algorithm is:
487 * - invent a random k between 1 and q-1 (exclusive).
488 * - Compute r = (g^k mod p) mod q.
489 * - Compute s = k^-1 * (hash + x*r) mod q.
491 * This has the dangerous properties that:
493 * - if an attacker in possession of the public key _and_ the
494 * signature (for example, the host you just authenticated
495 * to) can guess your k, he can reverse the computation of s
496 * and work out x = r^-1 * (s*k - hash) mod q. That is, he
497 * can deduce the private half of your key, and masquerade
498 * as you for as long as the key is still valid.
500 * - since r is a function purely of k and the public key, if
501 * the attacker only has a _range of possibilities_ for k
502 * it's easy for him to work through them all and check each
503 * one against r; he'll never be unsure of whether he's got
506 * - if you ever sign two different hashes with the same k, it
507 * will be immediately obvious because the two signatures
508 * will have the same r, and moreover an attacker in
509 * possession of both signatures (and the public key of
510 * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
511 * and from there deduce x as before.
513 * - the Bleichenbacher attack on DSA makes use of methods of
514 * generating k which are significantly non-uniformly
515 * distributed; in particular, generating a 160-bit random
516 * number and reducing it mod q is right out.
518 * For this reason we must be pretty careful about how we
519 * generate our k. Since this code runs on Windows, with no
520 * particularly good system entropy sources, we can't trust our
521 * RNG itself to produce properly unpredictable data. Hence, we
522 * use a totally different scheme instead.
524 * What we do is to take a SHA-512 (_big_) hash of the private
525 * key x, and then feed this into another SHA-512 hash that
526 * also includes the message hash being signed. That is:
528 * proto_k = SHA512 ( SHA512(x) || SHA160(message) )
530 * This number is 512 bits long, so reducing it mod q won't be
531 * noticeably non-uniform. So
535 * This has the interesting property that it's _deterministic_:
536 * signing the same hash twice with the same key yields the
539 * Despite this determinism, it's still not predictable to an
540 * attacker, because in order to repeat the SHA-512
541 * construction that created it, the attacker would have to
542 * know the private key value x - and by assumption he doesn't,
543 * because if he knew that he wouldn't be attacking k!
545 * (This trick doesn't, _per se_, protect against reuse of k.
546 * Reuse of k is left to chance; all it does is prevent
547 * _excessively high_ chances of reuse of k due to entropy
550 * Thanks to Colin Plumb for the general idea of using x to
551 * ensure k is hard to guess, and to the Cambridge University
552 * Computer Security Group for helping to argue out all the
555 struct dss_key
*dss
= (struct dss_key
*) key
;
557 unsigned char digest
[20], digest512
[64];
558 Bignum proto_k
, k
, gkp
, hash
, kinv
, hxr
, r
, s
;
559 unsigned char *bytes
;
562 SHA_Simple(data
, datalen
, digest
);
565 * Hash some identifying text plus x.
568 SHA512_Bytes(&ss
, "DSA deterministic k generator", 30);
569 sha512_mpint(&ss
, dss
->x
);
570 SHA512_Final(&ss
, digest512
);
573 * Now hash that digest plus the message hash.
576 SHA512_Bytes(&ss
, digest512
, sizeof(digest512
));
577 SHA512_Bytes(&ss
, digest
, sizeof(digest
));
578 SHA512_Final(&ss
, digest512
);
580 smemclr(&ss
, sizeof(ss
));
583 * Now convert the result into a bignum, and reduce it mod q.
585 proto_k
= bignum_from_bytes(digest512
, 64);
586 k
= bigmod(proto_k
, dss
->q
);
589 smemclr(digest512
, sizeof(digest512
));
592 * Now we have k, so just go ahead and compute the signature.
594 gkp
= modpow(dss
->g
, k
, dss
->p
); /* g^k mod p */
595 r
= bigmod(gkp
, dss
->q
); /* r = (g^k mod p) mod q */
598 hash
= bignum_from_bytes(digest
, 20);
599 kinv
= modinv(k
, dss
->q
); /* k^-1 mod q */
600 hxr
= bigmuladd(dss
->x
, r
, hash
); /* hash + x*r */
601 s
= modmul(kinv
, hxr
, dss
->q
); /* s = k^-1 * (hash + x*r) mod q */
610 * string two 20-byte numbers r and s, end to end
612 * i.e. 4+7 + 4+40 bytes.
614 nbytes
= 4 + 7 + 4 + 40;
615 bytes
= snewn(nbytes
, unsigned char);
617 memcpy(bytes
+ 4, "ssh-dss", 7);
618 PUT_32BIT(bytes
+ 4 + 7, 40);
619 for (i
= 0; i
< 20; i
++) {
620 bytes
[4 + 7 + 4 + i
] = bignum_byte(r
, 19 - i
);
621 bytes
[4 + 7 + 4 + 20 + i
] = bignum_byte(s
, 19 - i
);
630 const struct ssh_signkey ssh_dss
= {
637 dss_openssh_createkey
,