mirror of
https://git.tartarus.org/simon/putty.git
synced 2025-01-09 17:38:00 +00:00
ae3edcdfc0
Quite a few of the function pointers in the ssh_keyalg vtable now take ptrlen arguments in place of separate pointer and length pairs. Meanwhile, the various key types' implementations of those functions now work by initialising a BinarySource with the input ptrlen and using the new decode functions to walk along it. One exception is the openssh_createkey method which reads a private key in the wire format used by OpenSSH's SSH-2 agent protocol, which has to consume a prefix of a larger data stream, and tell the caller how much of that data was the private key. That function now takes an actual BinarySource, and passes that directly to the decode functions, so that on return the caller finds that the BinarySource's read pointer has been advanced exactly past the private key. This let me throw away _several_ reimplementations of mpint-reading functions, one in each of sshrsa, sshdss.c and sshecc.c. Worse still, they didn't all have exactly the SSH-2 semantics, because the thing in sshrsa.c whose name suggested it was an mpint-reading function actually tolerated the wrong number of leading zero bytes, which it had to be able to do to cope with the "ssh-rsa" signature format which contains a thing that isn't quite an SSH-2 mpint. Now that deviation is clearly commented!
504 lines
14 KiB
C
504 lines
14 KiB
C
/*
|
|
* Digital Signature Standard implementation for PuTTY.
|
|
*/
|
|
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <assert.h>
|
|
|
|
#include "ssh.h"
|
|
#include "misc.h"
|
|
|
|
static void dss_freekey(ssh_key *key); /* forward reference */
|
|
|
|
static ssh_key *dss_newkey(const ssh_keyalg *self, ptrlen data)
|
|
{
|
|
BinarySource src[1];
|
|
struct dss_key *dss;
|
|
|
|
BinarySource_BARE_INIT(src, data.ptr, data.len);
|
|
if (!ptrlen_eq_string(get_string(src), "ssh-dss"))
|
|
return NULL;
|
|
|
|
dss = snew(struct dss_key);
|
|
dss->p = get_mp_ssh2(src);
|
|
dss->q = get_mp_ssh2(src);
|
|
dss->g = get_mp_ssh2(src);
|
|
dss->y = get_mp_ssh2(src);
|
|
dss->x = NULL;
|
|
|
|
if (get_err(src) ||
|
|
!bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
|
|
/* Invalid key. */
|
|
dss_freekey(&dss->sshk);
|
|
return NULL;
|
|
}
|
|
|
|
return &dss->sshk;
|
|
}
|
|
|
|
static void dss_freekey(ssh_key *key)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
if (dss->p)
|
|
freebn(dss->p);
|
|
if (dss->q)
|
|
freebn(dss->q);
|
|
if (dss->g)
|
|
freebn(dss->g);
|
|
if (dss->y)
|
|
freebn(dss->y);
|
|
if (dss->x)
|
|
freebn(dss->x);
|
|
sfree(dss);
|
|
}
|
|
|
|
static char *dss_fmtkey(ssh_key *key)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
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 int dss_verifysig(ssh_key *key, ptrlen sig, ptrlen data)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
BinarySource src[1];
|
|
unsigned char hash[20];
|
|
Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v;
|
|
int ret;
|
|
|
|
if (!dss->p)
|
|
return 0;
|
|
|
|
BinarySource_BARE_INIT(src, sig.ptr, sig.len);
|
|
|
|
/*
|
|
* 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 (sig.len != 40) { /* bug not present; read admin fields */
|
|
ptrlen type = get_string(src);
|
|
sig = get_string(src);
|
|
|
|
if (get_err(src) || !ptrlen_eq_string(type, "ssh-dss") ||
|
|
sig.len != 40)
|
|
return 0;
|
|
}
|
|
|
|
/* Now we're sitting on a 40-byte string for sure. */
|
|
r = bignum_from_bytes(sig.ptr, 20);
|
|
s = bignum_from_bytes((const char *)sig.ptr + 20, 20);
|
|
if (!r || !s) {
|
|
if (r)
|
|
freebn(r);
|
|
if (s)
|
|
freebn(s);
|
|
return 0;
|
|
}
|
|
|
|
if (!bignum_cmp(s, Zero)) {
|
|
freebn(r);
|
|
freebn(s);
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* 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.
|
|
*/
|
|
SHA_Simple(data.ptr, data.len, hash);
|
|
sha = bignum_from_bytes(hash, 20);
|
|
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(u1);
|
|
freebn(u2);
|
|
freebn(gu1p);
|
|
freebn(yu2p);
|
|
freebn(gu1yu2p);
|
|
freebn(v);
|
|
freebn(r);
|
|
freebn(s);
|
|
|
|
return ret;
|
|
}
|
|
|
|
static void dss_public_blob(ssh_key *key, BinarySink *bs)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
|
|
put_stringz(bs, "ssh-dss");
|
|
put_mp_ssh2(bs, dss->p);
|
|
put_mp_ssh2(bs, dss->q);
|
|
put_mp_ssh2(bs, dss->g);
|
|
put_mp_ssh2(bs, dss->y);
|
|
}
|
|
|
|
static void dss_private_blob(ssh_key *key, BinarySink *bs)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
|
|
put_mp_ssh2(bs, dss->x);
|
|
}
|
|
|
|
static ssh_key *dss_createkey(const ssh_keyalg *self, ptrlen pub, ptrlen priv)
|
|
{
|
|
BinarySource src[1];
|
|
ssh_key *sshk;
|
|
struct dss_key *dss;
|
|
ptrlen hash;
|
|
SHA_State s;
|
|
unsigned char digest[20];
|
|
Bignum ytest;
|
|
|
|
sshk = dss_newkey(self, pub);
|
|
if (!sshk)
|
|
return NULL;
|
|
|
|
dss = FROMFIELD(sshk, struct dss_key, sshk);
|
|
BinarySource_BARE_INIT(src, priv.ptr, priv.len);
|
|
dss->x = get_mp_ssh2(src);
|
|
if (get_err(src)) {
|
|
dss_freekey(&dss->sshk);
|
|
return NULL;
|
|
}
|
|
|
|
/*
|
|
* Check the obsolete hash in the old DSS key format.
|
|
*/
|
|
hash = get_string(src);
|
|
if (hash.len == 20) {
|
|
SHA_Init(&s);
|
|
put_mp_ssh2(&s, dss->p);
|
|
put_mp_ssh2(&s, dss->q);
|
|
put_mp_ssh2(&s, dss->g);
|
|
SHA_Final(&s, digest);
|
|
if (0 != memcmp(hash.ptr, digest, 20)) {
|
|
dss_freekey(&dss->sshk);
|
|
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->sshk);
|
|
freebn(ytest);
|
|
return NULL;
|
|
}
|
|
freebn(ytest);
|
|
|
|
return &dss->sshk;
|
|
}
|
|
|
|
static ssh_key *dss_openssh_createkey(const ssh_keyalg *self,
|
|
BinarySource *src)
|
|
{
|
|
struct dss_key *dss;
|
|
|
|
dss = snew(struct dss_key);
|
|
|
|
dss->p = get_mp_ssh2(src);
|
|
dss->q = get_mp_ssh2(src);
|
|
dss->g = get_mp_ssh2(src);
|
|
dss->y = get_mp_ssh2(src);
|
|
dss->x = get_mp_ssh2(src);
|
|
|
|
if (get_err(src) ||
|
|
!bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
|
|
/* Invalid key. */
|
|
dss_freekey(&dss->sshk);
|
|
return NULL;
|
|
}
|
|
|
|
return &dss->sshk;
|
|
}
|
|
|
|
static void dss_openssh_fmtkey(ssh_key *key, BinarySink *bs)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
|
|
put_mp_ssh2(bs, dss->p);
|
|
put_mp_ssh2(bs, dss->q);
|
|
put_mp_ssh2(bs, dss->g);
|
|
put_mp_ssh2(bs, dss->y);
|
|
put_mp_ssh2(bs, dss->x);
|
|
}
|
|
|
|
static int dss_pubkey_bits(const ssh_keyalg *self, ptrlen pub)
|
|
{
|
|
ssh_key *sshk;
|
|
struct dss_key *dss;
|
|
int ret;
|
|
|
|
sshk = dss_newkey(self, pub);
|
|
if (!sshk)
|
|
return -1;
|
|
|
|
dss = FROMFIELD(sshk, struct dss_key, sshk);
|
|
ret = bignum_bitcount(dss->p);
|
|
dss_freekey(&dss->sshk);
|
|
|
|
return ret;
|
|
}
|
|
|
|
Bignum *dss_gen_k(const char *id_string, Bignum modulus, Bignum private_key,
|
|
unsigned char *digest, int digest_len)
|
|
{
|
|
/*
|
|
* 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.
|
|
*/
|
|
SHA512_State ss;
|
|
unsigned char digest512[64];
|
|
Bignum proto_k, k;
|
|
|
|
/*
|
|
* Hash some identifying text plus x.
|
|
*/
|
|
SHA512_Init(&ss);
|
|
put_asciz(&ss, id_string);
|
|
put_mp_ssh2(&ss, private_key);
|
|
SHA512_Final(&ss, digest512);
|
|
|
|
/*
|
|
* Now hash that digest plus the message hash.
|
|
*/
|
|
SHA512_Init(&ss);
|
|
put_data(&ss, digest512, sizeof(digest512));
|
|
put_data(&ss, digest, digest_len);
|
|
|
|
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, modulus);
|
|
freebn(proto_k);
|
|
|
|
if (bignum_cmp(k, One) != 0 && bignum_cmp(k, Zero) != 0) {
|
|
smemclr(&ss, sizeof(ss));
|
|
smemclr(digest512, sizeof(digest512));
|
|
return k;
|
|
}
|
|
|
|
/* Very unlikely we get here, but if so, k was unsuitable. */
|
|
freebn(k);
|
|
/* Perturb the hash to think of a different k. */
|
|
put_byte(&ss, 'x');
|
|
/* Go round and try again. */
|
|
}
|
|
}
|
|
|
|
static void dss_sign(ssh_key *key, const void *data, int datalen,
|
|
BinarySink *bs)
|
|
{
|
|
struct dss_key *dss = FROMFIELD(key, struct dss_key, sshk);
|
|
Bignum k, gkp, hash, kinv, hxr, r, s;
|
|
unsigned char digest[20];
|
|
int i;
|
|
|
|
SHA_Simple(data, datalen, digest);
|
|
|
|
k = dss_gen_k("DSA deterministic k generator", dss->q, dss->x,
|
|
digest, sizeof(digest));
|
|
kinv = modinv(k, dss->q); /* k^-1 mod q */
|
|
assert(kinv);
|
|
|
|
/*
|
|
* 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);
|
|
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(k);
|
|
freebn(hash);
|
|
|
|
put_stringz(bs, "ssh-dss");
|
|
put_uint32(bs, 40);
|
|
for (i = 0; i < 20; i++)
|
|
put_byte(bs, bignum_byte(r, 19 - i));
|
|
for (i = 0; i < 20; i++)
|
|
put_byte(bs, bignum_byte(s, 19 - i));
|
|
freebn(r);
|
|
freebn(s);
|
|
}
|
|
|
|
const ssh_keyalg ssh_dss = {
|
|
dss_newkey,
|
|
dss_freekey,
|
|
dss_fmtkey,
|
|
dss_public_blob,
|
|
dss_private_blob,
|
|
dss_createkey,
|
|
dss_openssh_createkey,
|
|
dss_openssh_fmtkey,
|
|
5 /* p,q,g,y,x */,
|
|
dss_pubkey_bits,
|
|
dss_verifysig,
|
|
dss_sign,
|
|
"ssh-dss",
|
|
"dss",
|
|
NULL,
|
|
};
|