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Make modinv able to return NULL if its inputs are not coprime, and

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check for that return value everywhere it is used.

[originally from svn r9990]
This commit is contained in:
Simon Tatham
2013-08-04 19:34:07 +00:00
parent 9c054cf467
commit cb1df53360
4 changed files with 59 additions and 14 deletions
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38
sshdss.c
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@ -286,6 +286,11 @@ static int dss_verifysig(void *key, char *sig, int siglen,
* Step 1. w <- s^-1 mod q.
*/
w = modinv(s, dss->q);
if (!w) {
freebn(r);
freebn(s);
return 0;
}
/*
* Step 2. u1 <- SHA(message) * w mod q.
@ -609,17 +614,33 @@ static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
SHA512_Init(&ss);
SHA512_Bytes(&ss, digest512, sizeof(digest512));
SHA512_Bytes(&ss, digest, sizeof(digest));
SHA512_Final(&ss, digest512);
while (1) {
SHA512_State ss2 = ss; /* structure copy */
SHA512_Final(&ss2, digest512);
smemclr(&ss2, sizeof(ss2));
/*
* Now convert the result into a bignum, and reduce it mod q.
*/
proto_k = bignum_from_bytes(digest512, 64);
k = bigmod(proto_k, dss->q);
freebn(proto_k);
kinv = modinv(k, dss->q); /* k^-1 mod q */
if (!kinv) { /* very unlikely */
freebn(k);
/* Perturb the hash to think of a different k. */
SHA512_Bytes(&ss, "x", 1);
/* Go round and try again. */
continue;
}
break;
}
smemclr(&ss, sizeof(ss));
/*
* Now convert the result into a bignum, and reduce it mod q.
*/
proto_k = bignum_from_bytes(digest512, 64);
k = bigmod(proto_k, dss->q);
freebn(proto_k);
smemclr(digest512, sizeof(digest512));
/*
@ -630,7 +651,6 @@ static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
freebn(gkp);
hash = bignum_from_bytes(digest, 20);
kinv = modinv(k, dss->q); /* k^-1 mod q */
hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */
s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */
freebn(hxr);