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Fix padding in ssh-dss signature blob encoding

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DSA signatures consist of two 160-bit integers called r and s. In ssh-dss
signature blobs r and s are stored directly after each other in binary
representation, making up a 320-bit (40 byte) string. (See RFC4253 p14.)

The crypto wrappers in libssh2 would either pack r and s incorrectly, or
fail, when at least one integer was small enough to be stored in 19 bytes
or less.

The patch ensures that r and s are always stored as two 160 bit numbers.
This commit is contained in:
Peter Stuge
2009-12-06 07:20:58 +01:00
parent 7317edab61
commit 1aba38cd7d
2 changed files with 16 additions and 29 deletions
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15
src/openssl.c
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@@ -420,7 +420,7 @@ _libssh2_dsa_sha1_sign(libssh2_dsa_ctx * dsactx,
unsigned long hash_len, unsigned char *signature)
{
DSA_SIG *sig;
int r_len, s_len, rs_pad;
int r_len, s_len;
(void) hash_len;
sig = DSA_do_sign(hash, SHA_DIGEST_LENGTH, dsactx);
@@ -429,15 +429,20 @@ _libssh2_dsa_sha1_sign(libssh2_dsa_ctx * dsactx,
}
r_len = BN_num_bytes(sig->r);
if (r_len < 1 || r_len > 20) {
DSA_SIG_free(sig);
return -1;
}
s_len = BN_num_bytes(sig->s);
rs_pad = (2 * SHA_DIGEST_LENGTH) - (r_len + s_len);
if (rs_pad < 0) {
if (s_len < 1 || s_len > 20) {
DSA_SIG_free(sig);
return -1;
}
BN_bn2bin(sig->r, signature + rs_pad);
BN_bn2bin(sig->s, signature + rs_pad + r_len);
memset(signature, 0, 40);
BN_bn2bin(sig->r, signature + (20 - r_len));
BN_bn2bin(sig->s, signature + 20 + (20 - s_len));
DSA_SIG_free(sig);