8c9f93e3956a32f7871b63fe7872cf5ea5d7eded
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
)
78 b
= bignum_from_bytes((unsigned char *)*data
, 20);
85 static void *dss_newkey(char *data
, int len
)
91 dss
= snew(struct dss_key
);
94 getstring(&data
, &len
, &p
, &slen
);
100 for (i
= 0; i
< len
; i
++)
101 printf(" %02x", (unsigned char) (data
[i
]));
106 if (!p
|| slen
!= 7 || memcmp(p
, "ssh-dss", 7)) {
110 dss
->p
= getmp(&data
, &len
);
111 dss
->q
= getmp(&data
, &len
);
112 dss
->g
= getmp(&data
, &len
);
113 dss
->y
= getmp(&data
, &len
);
119 static void dss_freekey(void *key
)
121 struct dss_key
*dss
= (struct dss_key
*) key
;
135 static char *dss_fmtkey(void *key
)
137 struct dss_key
*dss
= (struct dss_key
*) key
;
139 int len
, i
, pos
, nibbles
;
140 static const char hex
[] = "0123456789abcdef";
143 len
= 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */
144 len
+= 4 * (bignum_bitcount(dss
->p
) + 15) / 16;
145 len
+= 4 * (bignum_bitcount(dss
->q
) + 15) / 16;
146 len
+= 4 * (bignum_bitcount(dss
->g
) + 15) / 16;
147 len
+= 4 * (bignum_bitcount(dss
->y
) + 15) / 16;
148 p
= snewn(len
, char);
153 pos
+= sprintf(p
+ pos
, "0x");
154 nibbles
= (3 + bignum_bitcount(dss
->p
)) / 4;
157 for (i
= nibbles
; i
--;)
159 hex
[(bignum_byte(dss
->p
, i
/ 2) >> (4 * (i
% 2))) & 0xF];
160 pos
+= sprintf(p
+ pos
, ",0x");
161 nibbles
= (3 + bignum_bitcount(dss
->q
)) / 4;
164 for (i
= nibbles
; i
--;)
166 hex
[(bignum_byte(dss
->q
, i
/ 2) >> (4 * (i
% 2))) & 0xF];
167 pos
+= sprintf(p
+ pos
, ",0x");
168 nibbles
= (3 + bignum_bitcount(dss
->g
)) / 4;
171 for (i
= nibbles
; i
--;)
173 hex
[(bignum_byte(dss
->g
, i
/ 2) >> (4 * (i
% 2))) & 0xF];
174 pos
+= sprintf(p
+ pos
, ",0x");
175 nibbles
= (3 + bignum_bitcount(dss
->y
)) / 4;
178 for (i
= nibbles
; i
--;)
180 hex
[(bignum_byte(dss
->y
, i
/ 2) >> (4 * (i
% 2))) & 0xF];
185 static char *dss_fingerprint(void *key
)
187 struct dss_key
*dss
= (struct dss_key
*) key
;
188 struct MD5Context md5c
;
189 unsigned char digest
[16], lenbuf
[4];
190 char buffer
[16 * 3 + 40];
195 MD5Update(&md5c
, (unsigned char *)"\0\0\0\7ssh-dss", 11);
197 #define ADD_BIGNUM(bignum) \
198 numlen = (bignum_bitcount(bignum)+8)/8; \
199 PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \
200 for (i = numlen; i-- ;) { \
201 unsigned char c = bignum_byte(bignum, i); \
202 MD5Update(&md5c, &c, 1); \
210 MD5Final(digest
, &md5c
);
212 sprintf(buffer
, "ssh-dss %d ", bignum_bitcount(dss
->p
));
213 for (i
= 0; i
< 16; i
++)
214 sprintf(buffer
+ strlen(buffer
), "%s%02x", i ?
":" : "",
216 ret
= snewn(strlen(buffer
) + 1, char);
222 static int dss_verifysig(void *key
, char *sig
, int siglen
,
223 char *data
, int datalen
)
225 struct dss_key
*dss
= (struct dss_key
*) key
;
229 Bignum r
, s
, w
, gu1p
, yu2p
, gu1yu2p
, u1
, u2
, sha
, v
;
239 for (i
= 0; i
< siglen
; i
++)
240 printf(" %02x", (unsigned char) (sig
[i
]));
245 * Commercial SSH (2.0.13) and OpenSSH disagree over the format
246 * of a DSA signature. OpenSSH is in line with RFC 4253:
247 * it uses a string "ssh-dss", followed by a 40-byte string
248 * containing two 160-bit integers end-to-end. Commercial SSH
249 * can't be bothered with the header bit, and considers a DSA
250 * signature blob to be _just_ the 40-byte string containing
251 * the two 160-bit integers. We tell them apart by measuring
252 * the length: length 40 means the commercial-SSH bug, anything
253 * else is assumed to be RFC-compliant.
255 if (siglen
!= 40) { /* bug not present; read admin fields */
256 getstring(&sig
, &siglen
, &p
, &slen
);
257 if (!p
|| slen
!= 7 || memcmp(p
, "ssh-dss", 7)) {
260 sig
+= 4, siglen
-= 4; /* skip yet another length field */
262 r
= get160(&sig
, &siglen
);
263 s
= get160(&sig
, &siglen
);
268 * Step 1. w <- s^-1 mod q.
270 w
= modinv(s
, dss
->q
);
273 * Step 2. u1 <- SHA(message) * w mod q.
275 SHA_Simple(data
, datalen
, (unsigned char *)hash
);
278 sha
= get160(&p
, &slen
);
279 u1
= modmul(sha
, w
, dss
->q
);
282 * Step 3. u2 <- r * w mod q.
284 u2
= modmul(r
, w
, dss
->q
);
287 * Step 4. v <- (g^u1 * y^u2 mod p) mod q.
289 gu1p
= modpow(dss
->g
, u1
, dss
->p
);
290 yu2p
= modpow(dss
->y
, u2
, dss
->p
);
291 gu1yu2p
= modmul(gu1p
, yu2p
, dss
->p
);
292 v
= modmul(gu1yu2p
, One
, dss
->q
);
295 * Step 5. v should now be equal to r.
298 ret
= !bignum_cmp(v
, r
);
314 static unsigned char *dss_public_blob(void *key
, int *len
)
316 struct dss_key
*dss
= (struct dss_key
*) key
;
317 int plen
, qlen
, glen
, ylen
, bloblen
;
319 unsigned char *blob
, *p
;
321 plen
= (bignum_bitcount(dss
->p
) + 8) / 8;
322 qlen
= (bignum_bitcount(dss
->q
) + 8) / 8;
323 glen
= (bignum_bitcount(dss
->g
) + 8) / 8;
324 ylen
= (bignum_bitcount(dss
->y
) + 8) / 8;
327 * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total
328 * 27 + sum of lengths. (five length fields, 20+7=27).
330 bloblen
= 27 + plen
+ qlen
+ glen
+ ylen
;
331 blob
= snewn(bloblen
, unsigned char);
335 memcpy(p
, "ssh-dss", 7);
340 *p
++ = bignum_byte(dss
->p
, i
);
344 *p
++ = bignum_byte(dss
->q
, i
);
348 *p
++ = bignum_byte(dss
->g
, i
);
352 *p
++ = bignum_byte(dss
->y
, i
);
353 assert(p
== blob
+ bloblen
);
358 static unsigned char *dss_private_blob(void *key
, int *len
)
360 struct dss_key
*dss
= (struct dss_key
*) key
;
363 unsigned char *blob
, *p
;
365 xlen
= (bignum_bitcount(dss
->x
) + 8) / 8;
368 * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
371 blob
= snewn(bloblen
, unsigned char);
376 *p
++ = bignum_byte(dss
->x
, i
);
377 assert(p
== blob
+ bloblen
);
382 static void *dss_createkey(unsigned char *pub_blob
, int pub_len
,
383 unsigned char *priv_blob
, int priv_len
)
386 char *pb
= (char *) priv_blob
;
390 unsigned char digest
[20];
393 dss
= dss_newkey((char *) pub_blob
, pub_len
);
394 dss
->x
= getmp(&pb
, &priv_len
);
397 * Check the obsolete hash in the old DSS key format.
400 getstring(&pb
, &priv_len
, &hash
, &hashlen
);
403 sha_mpint(&s
, dss
->p
);
404 sha_mpint(&s
, dss
->q
);
405 sha_mpint(&s
, dss
->g
);
406 SHA_Final(&s
, digest
);
407 if (0 != memcmp(hash
, digest
, 20)) {
414 * Now ensure g^x mod p really is y.
416 ytest
= modpow(dss
->g
, dss
->x
, dss
->p
);
417 if (0 != bignum_cmp(ytest
, dss
->y
)) {
427 static void *dss_openssh_createkey(unsigned char **blob
, int *len
)
429 char **b
= (char **) blob
;
432 dss
= snew(struct dss_key
);
436 dss
->p
= getmp(b
, len
);
437 dss
->q
= getmp(b
, len
);
438 dss
->g
= getmp(b
, len
);
439 dss
->y
= getmp(b
, len
);
440 dss
->x
= getmp(b
, len
);
442 if (!dss
->p
|| !dss
->q
|| !dss
->g
|| !dss
->y
|| !dss
->x
) {
455 static int dss_openssh_fmtkey(void *key
, unsigned char *blob
, int len
)
457 struct dss_key
*dss
= (struct dss_key
*) key
;
461 ssh2_bignum_length(dss
->p
) +
462 ssh2_bignum_length(dss
->q
) +
463 ssh2_bignum_length(dss
->g
) +
464 ssh2_bignum_length(dss
->y
) +
465 ssh2_bignum_length(dss
->x
);
472 PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
473 for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
483 static int dss_pubkey_bits(void *blob
, int len
)
488 dss
= dss_newkey((char *) blob
, len
);
489 ret
= bignum_bitcount(dss
->p
);
495 static unsigned char *dss_sign(void *key
, char *data
, int datalen
, int *siglen
)
498 * The basic DSS signing algorithm is:
500 * - invent a random k between 1 and q-1 (exclusive).
501 * - Compute r = (g^k mod p) mod q.
502 * - Compute s = k^-1 * (hash + x*r) mod q.
504 * This has the dangerous properties that:
506 * - if an attacker in possession of the public key _and_ the
507 * signature (for example, the host you just authenticated
508 * to) can guess your k, he can reverse the computation of s
509 * and work out x = r^-1 * (s*k - hash) mod q. That is, he
510 * can deduce the private half of your key, and masquerade
511 * as you for as long as the key is still valid.
513 * - since r is a function purely of k and the public key, if
514 * the attacker only has a _range of possibilities_ for k
515 * it's easy for him to work through them all and check each
516 * one against r; he'll never be unsure of whether he's got
519 * - if you ever sign two different hashes with the same k, it
520 * will be immediately obvious because the two signatures
521 * will have the same r, and moreover an attacker in
522 * possession of both signatures (and the public key of
523 * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
524 * and from there deduce x as before.
526 * - the Bleichenbacher attack on DSA makes use of methods of
527 * generating k which are significantly non-uniformly
528 * distributed; in particular, generating a 160-bit random
529 * number and reducing it mod q is right out.
531 * For this reason we must be pretty careful about how we
532 * generate our k. Since this code runs on Windows, with no
533 * particularly good system entropy sources, we can't trust our
534 * RNG itself to produce properly unpredictable data. Hence, we
535 * use a totally different scheme instead.
537 * What we do is to take a SHA-512 (_big_) hash of the private
538 * key x, and then feed this into another SHA-512 hash that
539 * also includes the message hash being signed. That is:
541 * proto_k = SHA512 ( SHA512(x) || SHA160(message) )
543 * This number is 512 bits long, so reducing it mod q won't be
544 * noticeably non-uniform. So
548 * This has the interesting property that it's _deterministic_:
549 * signing the same hash twice with the same key yields the
552 * Despite this determinism, it's still not predictable to an
553 * attacker, because in order to repeat the SHA-512
554 * construction that created it, the attacker would have to
555 * know the private key value x - and by assumption he doesn't,
556 * because if he knew that he wouldn't be attacking k!
558 * (This trick doesn't, _per se_, protect against reuse of k.
559 * Reuse of k is left to chance; all it does is prevent
560 * _excessively high_ chances of reuse of k due to entropy
563 * Thanks to Colin Plumb for the general idea of using x to
564 * ensure k is hard to guess, and to the Cambridge University
565 * Computer Security Group for helping to argue out all the
568 struct dss_key
*dss
= (struct dss_key
*) key
;
570 unsigned char digest
[20], digest512
[64];
571 Bignum proto_k
, k
, gkp
, hash
, kinv
, hxr
, r
, s
;
572 unsigned char *bytes
;
575 SHA_Simple(data
, datalen
, digest
);
578 * Hash some identifying text plus x.
581 SHA512_Bytes(&ss
, "DSA deterministic k generator", 30);
582 sha512_mpint(&ss
, dss
->x
);
583 SHA512_Final(&ss
, digest512
);
586 * Now hash that digest plus the message hash.
589 SHA512_Bytes(&ss
, digest512
, sizeof(digest512
));
590 SHA512_Bytes(&ss
, digest
, sizeof(digest
));
591 SHA512_Final(&ss
, digest512
);
593 smemclr(&ss
, sizeof(ss
));
596 * Now convert the result into a bignum, and reduce it mod q.
598 proto_k
= bignum_from_bytes(digest512
, 64);
599 k
= bigmod(proto_k
, dss
->q
);
602 smemclr(digest512
, sizeof(digest512
));
605 * Now we have k, so just go ahead and compute the signature.
607 gkp
= modpow(dss
->g
, k
, dss
->p
); /* g^k mod p */
608 r
= bigmod(gkp
, dss
->q
); /* r = (g^k mod p) mod q */
611 hash
= bignum_from_bytes(digest
, 20);
612 kinv
= modinv(k
, dss
->q
); /* k^-1 mod q */
613 hxr
= bigmuladd(dss
->x
, r
, hash
); /* hash + x*r */
614 s
= modmul(kinv
, hxr
, dss
->q
); /* s = k^-1 * (hash + x*r) mod q */
623 * string two 20-byte numbers r and s, end to end
625 * i.e. 4+7 + 4+40 bytes.
627 nbytes
= 4 + 7 + 4 + 40;
628 bytes
= snewn(nbytes
, unsigned char);
630 memcpy(bytes
+ 4, "ssh-dss", 7);
631 PUT_32BIT(bytes
+ 4 + 7, 40);
632 for (i
= 0; i
< 20; i
++) {
633 bytes
[4 + 7 + 4 + i
] = bignum_byte(r
, 19 - i
);
634 bytes
[4 + 7 + 4 + 20 + i
] = bignum_byte(s
, 19 - i
);
643 const struct ssh_signkey ssh_dss
= {
650 dss_openssh_createkey
,