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/*
* Digital Signature Standard implementation for PuTTY.
*/
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include "ssh.h"
#include "misc.h"
static void sha_mpint(SHA_State * s, Bignum b)
{
unsigned char lenbuf[4];
int len;
len = (bignum_bitcount(b) + 8) / 8;
PUT_32BIT(lenbuf, len);
SHA_Bytes(s, lenbuf, 4);
while (len-- > 0) {
lenbuf[0] = bignum_byte(b, len);
SHA_Bytes(s, lenbuf, 1);
}
memset(lenbuf, 0, sizeof(lenbuf));
}
static void sha512_mpint(SHA512_State * s, Bignum b)
{
unsigned char lenbuf[4];
int len;
len = (bignum_bitcount(b) + 8) / 8;
PUT_32BIT(lenbuf, len);
SHA512_Bytes(s, lenbuf, 4);
while (len-- > 0) {
lenbuf[0] = bignum_byte(b, len);
SHA512_Bytes(s, lenbuf, 1);
}
memset(lenbuf, 0, sizeof(lenbuf));
}
static void getstring(char **data, int *datalen, char **p, int *length)
{
*p = NULL;
if (*datalen < 4)
return;
*length = GET_32BIT(*data);
*datalen -= 4;
*data += 4;
if (*datalen < *length)
return;
*p = *data;
*data += *length;
*datalen -= *length;
}
static Bignum getmp(char **data, int *datalen)
{
char *p;
int length;
Bignum b;
getstring(data, datalen, &p, &length);
if (!p)
return NULL;
if (p[0] & 0x80)
return NULL; /* negative mp */
b = bignum_from_bytes((unsigned char *)p, length);
return b;
}
static Bignum get160(char **data, int *datalen)
{
Bignum b;
b = bignum_from_bytes((unsigned char *)*data, 20);
*data += 20;
*datalen -= 20;
return b;
}
static void *dss_newkey(char *data, int len)
{
char *p;
int slen;
struct dss_key *dss;
dss = snew(struct dss_key);
if (!dss)
return NULL;
getstring(&data, &len, &p, &slen);
#ifdef DEBUG_DSS
{
int i;
printf("key:");
for (i = 0; i < len; i++)
printf(" %02x", (unsigned char) (data[i]));
printf("\n");
}
#endif
if (!p || memcmp(p, "ssh-dss", 7)) {
sfree(dss);
return NULL;
}
dss->p = getmp(&data, &len);
dss->q = getmp(&data, &len);
dss->g = getmp(&data, &len);
dss->y = getmp(&data, &len);
return dss;
}
static void dss_freekey(void *key)
{
struct dss_key *dss = (struct dss_key *) key;
freebn(dss->p);
freebn(dss->q);
freebn(dss->g);
freebn(dss->y);
sfree(dss);
}
static char *dss_fmtkey(void *key)
{
struct dss_key *dss = (struct dss_key *) key;
char *p;
int len, i, pos, nibbles;
static const char hex[] = "0123456789abcdef";
if (!dss->p)
return NULL;
len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */
len += 4 * (bignum_bitcount(dss->p) + 15) / 16;
len += 4 * (bignum_bitcount(dss->q) + 15) / 16;
len += 4 * (bignum_bitcount(dss->g) + 15) / 16;
len += 4 * (bignum_bitcount(dss->y) + 15) / 16;
p = snewn(len, char);
if (!p)
return NULL;
pos = 0;
pos += sprintf(p + pos, "0x");
nibbles = (3 + bignum_bitcount(dss->p)) / 4;
if (nibbles < 1)
nibbles = 1;
for (i = nibbles; i--;)
p[pos++] =
hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF];
pos += sprintf(p + pos, ",0x");
nibbles = (3 + bignum_bitcount(dss->q)) / 4;
if (nibbles < 1)
nibbles = 1;
for (i = nibbles; i--;)
p[pos++] =
hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF];
pos += sprintf(p + pos, ",0x");
nibbles = (3 + bignum_bitcount(dss->g)) / 4;
if (nibbles < 1)
nibbles = 1;
for (i = nibbles; i--;)
p[pos++] =
hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF];
pos += sprintf(p + pos, ",0x");
nibbles = (3 + bignum_bitcount(dss->y)) / 4;
if (nibbles < 1)
nibbles = 1;
for (i = nibbles; i--;)
p[pos++] =
hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF];
p[pos] = '\0';
return p;
}
static char *dss_fingerprint(void *key)
{
struct dss_key *dss = (struct dss_key *) key;
struct MD5Context md5c;
unsigned char digest[16], lenbuf[4];
char buffer[16 * 3 + 40];
char *ret;
int numlen, i;
MD5Init(&md5c);
MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11);
#define ADD_BIGNUM(bignum) \
numlen = (bignum_bitcount(bignum)+8)/8; \
PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \
for (i = numlen; i-- ;) { \
unsigned char c = bignum_byte(bignum, i); \
MD5Update(&md5c, &c, 1); \
}
ADD_BIGNUM(dss->p);
ADD_BIGNUM(dss->q);
ADD_BIGNUM(dss->g);
ADD_BIGNUM(dss->y);
#undef ADD_BIGNUM
MD5Final(digest, &md5c);
sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p));
for (i = 0; i < 16; i++)
sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
digest[i]);
ret = snewn(strlen(buffer) + 1, char);
if (ret)
strcpy(ret, buffer);
return ret;
}
static int dss_verifysig(void *key, char *sig, int siglen,
char *data, int datalen)
{
struct dss_key *dss = (struct dss_key *) key;
char *p;
int slen;
char hash[20];
Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v;
int ret;
if (!dss->p)
return 0;
#ifdef DEBUG_DSS
{
int i;
printf("sig:");
for (i = 0; i < siglen; i++)
printf(" %02x", (unsigned char) (sig[i]));
printf("\n");
}
#endif
/*
* Commercial SSH (2.0.13) and OpenSSH disagree over the format
* of a DSA signature. OpenSSH is in line with RFC 4253:
* it uses a string "ssh-dss", followed by a 40-byte string
* containing two 160-bit integers end-to-end. Commercial SSH
* can't be bothered with the header bit, and considers a DSA
* signature blob to be _just_ the 40-byte string containing
* the two 160-bit integers. We tell them apart by measuring
* the length: length 40 means the commercial-SSH bug, anything
* else is assumed to be RFC-compliant.
*/
if (siglen != 40) { /* bug not present; read admin fields */
getstring(&sig, &siglen, &p, &slen);
if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
return 0;
}
sig += 4, siglen -= 4; /* skip yet another length field */
}
r = get160(&sig, &siglen);
s = get160(&sig, &siglen);
if (!r || !s)
return 0;
/*
* Step 1. w <- s^-1 mod q.
*/
w = modinv(s, dss->q);
/*
* Step 2. u1 <- SHA(message) * w mod q.
*/
SHA_Simple(data, datalen, (unsigned char *)hash);
p = hash;
slen = 20;
sha = get160(&p, &slen);
u1 = modmul(sha, w, dss->q);
/*
* Step 3. u2 <- r * w mod q.
*/
u2 = modmul(r, w, dss->q);
/*
* Step 4. v <- (g^u1 * y^u2 mod p) mod q.
*/
gu1p = modpow(dss->g, u1, dss->p);
yu2p = modpow(dss->y, u2, dss->p);
gu1yu2p = modmul(gu1p, yu2p, dss->p);
v = modmul(gu1yu2p, One, dss->q);
/*
* Step 5. v should now be equal to r.
*/
ret = !bignum_cmp(v, r);
freebn(w);
freebn(sha);
freebn(gu1p);
freebn(yu2p);
freebn(gu1yu2p);
freebn(v);
freebn(r);
freebn(s);
return ret;
}
static unsigned char *dss_public_blob(void *key, int *len)
{
struct dss_key *dss = (struct dss_key *) key;
int plen, qlen, glen, ylen, bloblen;
int i;
unsigned char *blob, *p;
plen = (bignum_bitcount(dss->p) + 8) / 8;
qlen = (bignum_bitcount(dss->q) + 8) / 8;
glen = (bignum_bitcount(dss->g) + 8) / 8;
ylen = (bignum_bitcount(dss->y) + 8) / 8;
/*
* string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total
* 27 + sum of lengths. (five length fields, 20+7=27).
*/
bloblen = 27 + plen + qlen + glen + ylen;
blob = snewn(bloblen, unsigned char);
p = blob;
PUT_32BIT(p, 7);
p += 4;
memcpy(p, "ssh-dss", 7);
p += 7;
PUT_32BIT(p, plen);
p += 4;
for (i = plen; i--;)
*p++ = bignum_byte(dss->p, i);
PUT_32BIT(p, qlen);
p += 4;
for (i = qlen; i--;)
*p++ = bignum_byte(dss->q, i);
PUT_32BIT(p, glen);
p += 4;
for (i = glen; i--;)
*p++ = bignum_byte(dss->g, i);
PUT_32BIT(p, ylen);
p += 4;
for (i = ylen; i--;)
*p++ = bignum_byte(dss->y, i);
assert(p == blob + bloblen);
*len = bloblen;
return blob;
}
static unsigned char *dss_private_blob(void *key, int *len)
{
struct dss_key *dss = (struct dss_key *) key;
int xlen, bloblen;
int i;
unsigned char *blob, *p;
xlen = (bignum_bitcount(dss->x) + 8) / 8;
/*
* mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
*/
bloblen = 4 + xlen;
blob = snewn(bloblen, unsigned char);
p = blob;
PUT_32BIT(p, xlen);
p += 4;
for (i = xlen; i--;)
*p++ = bignum_byte(dss->x, i);
assert(p == blob + bloblen);
*len = bloblen;
return blob;
}
static void *dss_createkey(unsigned char *pub_blob, int pub_len,
unsigned char *priv_blob, int priv_len)
{
struct dss_key *dss;
char *pb = (char *) priv_blob;
char *hash;
int hashlen;
SHA_State s;
unsigned char digest[20];
Bignum ytest;
dss = dss_newkey((char *) pub_blob, pub_len);
dss->x = getmp(&pb, &priv_len);
/*
* Check the obsolete hash in the old DSS key format.
*/
hashlen = -1;
getstring(&pb, &priv_len, &hash, &hashlen);
if (hashlen == 20) {
SHA_Init(&s);
sha_mpint(&s, dss->p);
sha_mpint(&s, dss->q);
sha_mpint(&s, dss->g);
SHA_Final(&s, digest);
if (0 != memcmp(hash, digest, 20)) {
dss_freekey(dss);
return NULL;
}
}
/*
* Now ensure g^x mod p really is y.
*/
ytest = modpow(dss->g, dss->x, dss->p);
if (0 != bignum_cmp(ytest, dss->y)) {
dss_freekey(dss);
return NULL;
}
freebn(ytest);
return dss;
}
static void *dss_openssh_createkey(unsigned char **blob, int *len)
{
char **b = (char **) blob;
struct dss_key *dss;
dss = snew(struct dss_key);
if (!dss)
return NULL;
dss->p = getmp(b, len);
dss->q = getmp(b, len);
dss->g = getmp(b, len);
dss->y = getmp(b, len);
dss->x = getmp(b, len);
if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x) {
sfree(dss->p);
sfree(dss->q);
sfree(dss->g);
sfree(dss->y);
sfree(dss->x);
sfree(dss);
return NULL;
}
return dss;
}
static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len)
{
struct dss_key *dss = (struct dss_key *) key;
int bloblen, i;
bloblen =
ssh2_bignum_length(dss->p) +
ssh2_bignum_length(dss->q) +
ssh2_bignum_length(dss->g) +
ssh2_bignum_length(dss->y) +
ssh2_bignum_length(dss->x);
if (bloblen > len)
return bloblen;
bloblen = 0;
#define ENC(x) \
PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
ENC(dss->p);
ENC(dss->q);
ENC(dss->g);
ENC(dss->y);
ENC(dss->x);
return bloblen;
}
static int dss_pubkey_bits(void *blob, int len)
{
struct dss_key *dss;
int ret;
dss = dss_newkey((char *) blob, len);
ret = bignum_bitcount(dss->p);
dss_freekey(dss);
return ret;
}
static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
{
/*
* The basic DSS signing algorithm is:
*
* - invent a random k between 1 and q-1 (exclusive).
* - Compute r = (g^k mod p) mod q.
* - Compute s = k^-1 * (hash + x*r) mod q.
*
* This has the dangerous properties that:
*
* - if an attacker in possession of the public key _and_ the
* signature (for example, the host you just authenticated
* to) can guess your k, he can reverse the computation of s
* and work out x = r^-1 * (s*k - hash) mod q. That is, he
* can deduce the private half of your key, and masquerade
* as you for as long as the key is still valid.
*
* - since r is a function purely of k and the public key, if
* the attacker only has a _range of possibilities_ for k
* it's easy for him to work through them all and check each
* one against r; he'll never be unsure of whether he's got
* the right one.
*
* - if you ever sign two different hashes with the same k, it
* will be immediately obvious because the two signatures
* will have the same r, and moreover an attacker in
* possession of both signatures (and the public key of
* course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
* and from there deduce x as before.
*
* - the Bleichenbacher attack on DSA makes use of methods of
* generating k which are significantly non-uniformly
* distributed; in particular, generating a 160-bit random
* number and reducing it mod q is right out.
*
* For this reason we must be pretty careful about how we
* generate our k. Since this code runs on Windows, with no
* particularly good system entropy sources, we can't trust our
* RNG itself to produce properly unpredictable data. Hence, we
* use a totally different scheme instead.
*
* What we do is to take a SHA-512 (_big_) hash of the private
* key x, and then feed this into another SHA-512 hash that
* also includes the message hash being signed. That is:
*
* proto_k = SHA512 ( SHA512(x) || SHA160(message) )
*
* This number is 512 bits long, so reducing it mod q won't be
* noticeably non-uniform. So
*
* k = proto_k mod q
*
* This has the interesting property that it's _deterministic_:
* signing the same hash twice with the same key yields the
* same signature.
*
* Despite this determinism, it's still not predictable to an
* attacker, because in order to repeat the SHA-512
* construction that created it, the attacker would have to
* know the private key value x - and by assumption he doesn't,
* because if he knew that he wouldn't be attacking k!
*
* (This trick doesn't, _per se_, protect against reuse of k.
* Reuse of k is left to chance; all it does is prevent
* _excessively high_ chances of reuse of k due to entropy
* problems.)
*
* Thanks to Colin Plumb for the general idea of using x to
* ensure k is hard to guess, and to the Cambridge University
* Computer Security Group for helping to argue out all the
* fine details.
*/
struct dss_key *dss = (struct dss_key *) key;
SHA512_State ss;
unsigned char digest[20], digest512[64];
Bignum proto_k, k, gkp, hash, kinv, hxr, r, s;
unsigned char *bytes;
int nbytes, i;
SHA_Simple(data, datalen, digest);
/*
* Hash some identifying text plus x.
*/
SHA512_Init(&ss);
SHA512_Bytes(&ss, "DSA deterministic k generator", 30);
sha512_mpint(&ss, dss->x);
SHA512_Final(&ss, digest512);
/*
* Now hash that digest plus the message hash.
*/
SHA512_Init(&ss);
SHA512_Bytes(&ss, digest512, sizeof(digest512));
SHA512_Bytes(&ss, digest, sizeof(digest));
SHA512_Final(&ss, digest512);
memset(&ss, 0, 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);
memset(digest512, 0, sizeof(digest512));
/*
* Now we have k, so just go ahead and compute the signature.
*/
gkp = modpow(dss->g, k, dss->p); /* g^k mod p */
r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */
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);
freebn(kinv);
freebn(hash);
/*
* Signature blob is
*
* string "ssh-dss"
* string two 20-byte numbers r and s, end to end
*
* i.e. 4+7 + 4+40 bytes.
*/
nbytes = 4 + 7 + 4 + 40;
bytes = snewn(nbytes, unsigned char);
PUT_32BIT(bytes, 7);
memcpy(bytes + 4, "ssh-dss", 7);
PUT_32BIT(bytes + 4 + 7, 40);
for (i = 0; i < 20; i++) {
bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i);
bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i);
}
freebn(r);
freebn(s);
*siglen = nbytes;
return bytes;
}
const struct ssh_signkey ssh_dss = {
dss_newkey,
dss_freekey,
dss_fmtkey,
dss_public_blob,
dss_private_blob,
dss_createkey,
dss_openssh_createkey,
dss_openssh_fmtkey,
dss_pubkey_bits,
dss_fingerprint,
dss_verifysig,
dss_sign,
"ssh-dss",
"dss"
};
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