+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,