X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/dcbde23605bb045df116a1dd522d4ddb4d9288fa..HEAD:/sshdss.c

diff --git a/sshdss.c b/sshdss.c
index bacf68d4..84fcdac7 100644
--- a/sshdss.c
+++ b/sshdss.c
@@ -1,122 +1,109 @@
-#include <stdio.h>
-#include <stdlib.h>
-
-#include "ssh.h"
-
-#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
 /*
- * Condition this section in for debugging of DSS.
+ * Digital Signature Standard implementation for PuTTY.
  */
-static void diagbn(char *prefix, Bignum md) {
-    int i, nibbles, morenibbles;
-    static const char hex[] = "0123456789ABCDEF";
 
-    printf("%s0x", prefix ? prefix : "");
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
 
-    nibbles = (3 + ssh1_bignum_bitcount(md))/4; if (nibbles<1) nibbles=1;
-    morenibbles = 4*md[0] - nibbles;
-    for (i=0; i<morenibbles; i++) putchar('-');
-    for (i=nibbles; i-- ;)
-        putchar(hex[(bignum_byte(md, i/2) >> (4*(i%2))) & 0xF]);
+#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));
+}
 
-    if (prefix) putchar('\n');
+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));
 }
-#define DEBUG_DSS
-#else
-#define diagbn(x,y)
-#endif
 
-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 i, j, length;
+    int length;
     Bignum b;
 
     getstring(data, datalen, &p, &length);
     if (!p)
-        return NULL;
+	return NULL;
     if (p[0] & 0x80)
-        return NULL;                   /* negative mp */
-    b = newbn((length+1)/2);
-    for (i = 0; i < length; i++) {
-        j = length - 1 - i;
-        if (j & 1)
-            b[j/2+1] |= ((unsigned char)p[i]) << 8;
-        else
-            b[j/2+1] |= ((unsigned char)p[i]);
-    }
-    while (b[0] > 1 && b[b[0]] == 0) b[0]--;
+	return NULL;		       /* negative mp */
+    b = bignum_from_bytes((unsigned char *)p, length);
     return b;
 }
 
-static Bignum get160(char **data, int *datalen) {
-    char *p;
-    int i, j, length;
+static Bignum get160(char **data, int *datalen)
+{
     Bignum b;
 
-    p = *data;
-    *data += 20; *datalen -= 20;
-
-    length = 20;
-    while (length > 0 && !p[0])
-        p++, length--;
-    b = newbn((length+1)/2);
-    for (i = 0; i < length; i++) {
-        j = length - 1 - i;
-        if (j & 1)
-            b[j/2+1] |= ((unsigned char)p[i]) << 8;
-        else
-            b[j/2+1] |= ((unsigned char)p[i]);
-    }
+    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;
     }
@@ -124,65 +111,98 @@ static void *dss_newkey(char *data, int len) {
     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 * (dss->p[0] + dss->q[0] + dss->g[0] + dss->y[0]);   /* digits */
-    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); \
@@ -196,18 +216,20 @@ static char *dss_fingerprint(void *key) {
 
     MD5Final(digest, &md5c);
 
-    sprintf(buffer, "%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];
@@ -215,79 +237,82 @@ static int dss_verifysig(void *key, char *sig, int siglen,
     int ret;
 
     if (!dss->p)
-        return 0;
+	return 0;
 
 #ifdef DEBUG_DSS
     {
-        int i;
-        printf("sig:");
-        for (i=0;i<siglen;i++)
-            printf("  %02xf", (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 || 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.
@@ -297,6 +322,8 @@ static int dss_verifysig(void *key, char *sig, int siglen,
 
     freebn(w);
     freebn(sha);
+    freebn(u1);
+    freebn(u2);
     freebn(gu1p);
     freebn(yu2p);
     freebn(gu1yu2p);
@@ -307,15 +334,364 @@ static int dss_verifysig(void *key, char *sig, int siglen,
     return ret;
 }
 
-int dss_sign(void *key, char *sig, int siglen,
-	     char *data, int datalen) {
-    return 0;			       /* do nothing */
+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);
+    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;
+}
+
+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;
 }
 
-struct ssh_signkey ssh_dss = {
+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,