+/*
+ * 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);
+ }
+ smemclr(lenbuf, sizeof(lenbuf));
+}
-#define GET_32BIT(cp) \
- (((unsigned long)(unsigned char)(cp)[0] << 24) | \
- ((unsigned long)(unsigned char)(cp)[1] << 16) | \
- ((unsigned long)(unsigned char)(cp)[2] << 8) | \
- ((unsigned long)(unsigned char)(cp)[3]))
-
-#define PUT_32BIT(cp, value) { \
- (cp)[0] = (unsigned char)((value) >> 24); \
- (cp)[1] = (unsigned char)((value) >> 16); \
- (cp)[2] = (unsigned char)((value) >> 8); \
- (cp)[3] = (unsigned char)(value); }
-
-#if 0
-#define DEBUG_DSS
-#else
-#define diagbn(x,y)
-#endif
+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);
+ }
+ smemclr(lenbuf, sizeof(lenbuf));
+}
-static void getstring(char **data, int *datalen, char **p, int *length) {
+static void getstring(char **data, int *datalen, char **p, int *length)
+{
*p = NULL;
if (*datalen < 4)
+ return;
+ *length = toint(GET_32BIT(*data));
+ if (*length < 0)
return;
- *length = GET_32BIT(*data);
- *datalen -= 4; *data += 4;
+ *datalen -= 4;
+ *data += 4;
if (*datalen < *length)
- return;
+ return;
*p = *data;
- *data += *length; *datalen -= *length;
+ *data += *length;
+ *datalen -= *length;
}
-static Bignum getmp(char **data, int *datalen) {
+static Bignum getmp(char **data, int *datalen)
+{
char *p;
int length;
Bignum b;
getstring(data, datalen, &p, &length);
if (!p)
- return NULL;
+ return NULL;
if (p[0] & 0x80)
- return NULL; /* negative mp */
- b = bignum_from_bytes(p, length);
+ return NULL; /* negative mp */
+ b = bignum_from_bytes((unsigned char *)p, length);
return b;
}
-static Bignum get160(char **data, int *datalen) {
+static Bignum get160(char **data, int *datalen)
+{
Bignum b;
- b = bignum_from_bytes(*data, 20);
- *data += 20; *datalen -= 20;
+ if (*datalen < 20)
+ return NULL;
+
+ b = bignum_from_bytes((unsigned char *)*data, 20);
+ *data += 20;
+ *datalen -= 20;
return b;
}
-struct dss_key {
- Bignum p, q, g, y;
-};
+static void dss_freekey(void *key); /* forward reference */
-static void *dss_newkey(char *data, int len) {
+static void *dss_newkey(char *data, int len)
+{
char *p;
int slen;
struct dss_key *dss;
- dss = smalloc(sizeof(struct dss_key));
- if (!dss) return NULL;
+ dss = snew(struct dss_key);
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");
+ 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)) {
+ if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
sfree(dss);
return NULL;
}
dss->q = getmp(&data, &len);
dss->g = getmp(&data, &len);
dss->y = getmp(&data, &len);
+ dss->x = NULL;
+
+ if (!dss->p || !dss->q || !dss->g || !dss->y ||
+ !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
+ /* Invalid key. */
+ dss_freekey(dss);
+ return NULL;
+ }
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);
+static void dss_freekey(void *key)
+{
+ struct dss_key *dss = (struct dss_key *) key;
+ 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(void *key) {
- struct dss_key *dss = (struct dss_key *)key;
+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 * (ssh1_bignum_bitcount(dss->p)+15)/16;
- len += 4 * (ssh1_bignum_bitcount(dss->q)+15)/16;
- len += 4 * (ssh1_bignum_bitcount(dss->g)+15)/16;
- len += 4 * (ssh1_bignum_bitcount(dss->y)+15)/16;
- p = smalloc(len);
- if (!p) return NULL;
+ 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 + ssh1_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 + ssh1_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 + ssh1_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 + ssh1_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];
+ 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;
+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 buffer[16 * 3 + 40];
char *ret;
int numlen, i;
MD5Init(&md5c);
- MD5Update(&md5c, "\0\0\0\7ssh-dss", 11);
+ MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11);
#define ADD_BIGNUM(bignum) \
- numlen = (ssh1_bignum_bitcount(bignum)+8)/8; \
+ 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); \
MD5Final(digest, &md5c);
- sprintf(buffer, "ssh-dss %d ", ssh1_bignum_bitcount(dss->p));
+ 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 = smalloc(strlen(buffer)+1);
+ sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
+ digest[i]);
+ ret = snewn(strlen(buffer) + 1, char);
if (ret)
- strcpy(ret, buffer);
+ 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 *data, int datalen)
+{
+ struct dss_key *dss = (struct dss_key *) key;
char *p;
int slen;
char hash[20];
int ret;
if (!dss->p)
- return 0;
+ return 0;
#ifdef DEBUG_DSS
{
- int i;
- printf("sig:");
- for (i=0;i<siglen;i++)
- printf(" %02x", (unsigned char)(sig[i]));
- printf("\n");
+ 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 the IETF drafts:
+ * 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 IETF-compliant.
+ * 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 */
+ 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 */
}
- 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)
+ 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);
- diagbn("w=", w);
+ if (!w) {
+ freebn(r);
+ freebn(s);
+ return 0;
+ }
/*
* Step 2. u1 <- SHA(message) * w mod q.
*/
- SHA_Simple(data, datalen, hash);
- p = hash; slen = 20; sha = get160(&p, &slen);
- diagbn("sha=", sha);
+ SHA_Simple(data, datalen, (unsigned char *)hash);
+ p = hash;
+ slen = 20;
+ sha = get160(&p, &slen);
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.
freebn(w);
freebn(sha);
+ freebn(u1);
+ freebn(u2);
freebn(gu1p);
freebn(yu2p);
freebn(gu1yu2p);
return ret;
}
-static unsigned char *dss_public_blob(void *key, int *len) {
- struct dss_key *dss = (struct dss_key *)key;
+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 = (ssh1_bignum_bitcount(dss->p)+8)/8;
- qlen = (ssh1_bignum_bitcount(dss->q)+8)/8;
- glen = (ssh1_bignum_bitcount(dss->g)+8)/8;
- ylen = (ssh1_bignum_bitcount(dss->y)+8)/8;
+ 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 = smalloc(bloblen);
+ 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);
+ 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) {
- return NULL; /* can't handle DSS private keys */
+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) {
- return NULL; /* can't handle DSS private keys */
+ 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);
+ if (!dss)
+ return NULL;
+ dss->x = getmp(&pb, &priv_len);
+ if (!dss->x) {
+ dss_freekey(dss);
+ return NULL;
+ }
+
+ /*
+ * 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);
+ freebn(ytest);
+ 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);
+
+ 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 ||
+ !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
+ /* Invalid key. */
+ dss_freekey(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);
+ if (!dss)
+ return -1;
+ ret = bignum_bitcount(dss->p);
+ dss_freekey(dss);
+
+ return ret;
}
-unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen) {
- return NULL; /* can't handle DSS private keys */
+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));
+
+ 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, dss->q);
+ freebn(proto_k);
+ kinv = modinv(k, dss->q); /* k^-1 mod q */
+ if (!kinv) { /* very unlikely */
+ freebn(k);
+ /* Perturb the hash to think of a different k. */
+ SHA512_Bytes(&ss, "x", 1);
+ /* Go round and try again. */
+ continue;
+ }
+
+ break;
+ }
+
+ smemclr(&ss, sizeof(ss));
+
+ smemclr(digest512, 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);
+ 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);
+
+ /*
+ * 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_public_blob,
dss_private_blob,
dss_createkey,
+ dss_openssh_createkey,
+ dss_openssh_fmtkey,
+ dss_pubkey_bits,
dss_fingerprint,
dss_verifysig,
dss_sign,
projects / u / mdw / putty / blobdiff