#include <assert.h>
#include "ssh.h"
+#include "misc.h"
#define GET_32BIT(cp) \
(((unsigned long)(unsigned char)(cp)[0] << 24) | \
(cp)[2] = (unsigned char)((value) >> 8); \
(cp)[3] = (unsigned char)(value); }
-#if 0
-#define DEBUG_DSS
-#else
-#define diagbn(x,y)
-#endif
+static void sha_mpint(SHA_State * s, Bignum b)
+{
+ unsigned char *p;
+ 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 *p;
+ 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)
{
return b;
}
-struct dss_key {
- Bignum p, q, g, y;
-};
-
static void *dss_newkey(char *data, int len)
{
char *p;
}
sig += 4, siglen -= 4; /* skip yet another length field */
}
- diagbn("p=", dss->p);
- diagbn("q=", dss->q);
- diagbn("g=", dss->g);
- diagbn("y=", dss->y);
r = get160(&sig, &siglen);
- diagbn("r=", r);
s = get160(&sig, &siglen);
- diagbn("s=", s);
if (!r || !s)
return 0;
* Step 1. w <- s^-1 mod q.
*/
w = modinv(s, dss->q);
- diagbn("w=", w);
/*
* Step 2. u1 <- SHA(message) * w mod q.
p = hash;
slen = 20;
sha = get160(&p, &slen);
- diagbn("sha=", sha);
u1 = modmul(sha, w, dss->q);
- diagbn("u1=", u1);
/*
* Step 3. u2 <- r * w mod q.
*/
u2 = modmul(r, w, dss->q);
- diagbn("u2=", u2);
/*
* Step 4. v <- (g^u1 * y^u2 mod p) mod q.
*/
gu1p = modpow(dss->g, u1, dss->p);
- diagbn("gu1p=", gu1p);
yu2p = modpow(dss->y, u2, dss->p);
- diagbn("yu2p=", yu2p);
gu1yu2p = modmul(gu1p, yu2p, dss->p);
- diagbn("gu1yu2p=", gu1yu2p);
v = modmul(gu1yu2p, One, dss->q);
- diagbn("gu1yu2q=v=", v);
- diagbn("r=", r);
/*
* Step 5. v should now be equal to r.
static unsigned char *dss_private_blob(void *key, int *len)
{
- return NULL; /* can't handle DSS private keys */
+ struct dss_key *dss = (struct dss_key *) key;
+ int xlen, bloblen;
+ int i;
+ unsigned char *blob, *p;
+ SHA_State s;
+ unsigned char digest[20];
+
+ xlen = (bignum_bitcount(dss->x) + 8) / 8;
+
+ /*
+ * mpint x, string[20] the SHA of p||q||g. Total 28 + xlen.
+ * (two length fields and twenty bytes, 20+8=28).
+ */
+ bloblen = 28 + xlen;
+ blob = smalloc(bloblen);
+ p = blob;
+ PUT_32BIT(p, xlen);
+ p += 4;
+ for (i = xlen; i--;)
+ *p++ = bignum_byte(dss->x, i);
+ PUT_32BIT(p, 20);
+ SHA_Init(&s);
+ sha_mpint(&s, dss->p);
+ sha_mpint(&s, dss->q);
+ sha_mpint(&s, dss->g);
+ SHA_Final(&s, digest);
+ p += 4;
+ for (i = 0; i < 20; i++)
+ *p++ = digest[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)
{
- return NULL; /* can't handle DSS private keys */
+ 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);
+ getstring(&pb, &priv_len, &hash, &hashlen);
+
+ /*
+ * Verify details of the key. First check that the hash is
+ * indeed a hash of p||q||g.
+ */
+ if (hashlen != 20) {
+ dss_freekey(dss);
+ return NULL;
+ }
+ 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)
{
- return NULL; /* can't handle DSS private keys */
+ char **b = (char **) blob;
+ struct dss_key *dss;
+
+ dss = smalloc(sizeof(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)
{
- return -1; /* can't handle DSS private keys */
+ 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;
}
unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
{
- return NULL; /* can't handle DSS private keys */
+ /*
+ * 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.
+ *
+ * (It 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 = smalloc(nbytes);
+ 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 = {
projects / u / mdw / putty / blobdiff