X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/5c72ca6161da0e7976245222c412d62ebae2e386..HEAD:/sshdss.c

diff --git a/sshdss.c b/sshdss.c
index b2179a83..84fcdac7 100644
--- a/sshdss.c
+++ b/sshdss.c
@@ -1,3 +1,7 @@
+/*
+ * Digital Signature Standard implementation for PuTTY.
+ */
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>
@@ -5,21 +9,8 @@
 #include "ssh.h"
 #include "misc.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); }
-
 static void sha_mpint(SHA_State * s, Bignum b)
 {
-    unsigned char *p;
     unsigned char lenbuf[4];
     int len;
     len = (bignum_bitcount(b) + 8) / 8;
@@ -29,12 +20,11 @@ static void sha_mpint(SHA_State * s, Bignum b)
 	lenbuf[0] = bignum_byte(b, len);
 	SHA_Bytes(s, lenbuf, 1);
     }
-    memset(lenbuf, 0, sizeof(lenbuf));
+    smemclr(lenbuf, 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;
@@ -44,7 +34,7 @@ static void sha512_mpint(SHA512_State * s, Bignum b)
 	lenbuf[0] = bignum_byte(b, len);
 	SHA512_Bytes(s, lenbuf, 1);
     }
-    memset(lenbuf, 0, sizeof(lenbuf));
+    smemclr(lenbuf, sizeof(lenbuf));
 }
 
 static void getstring(char **data, int *datalen, char **p, int *length)
@@ -52,7 +42,9 @@ static void getstring(char **data, int *datalen, char **p, int *length)
     *p = NULL;
     if (*datalen < 4)
 	return;
-    *length = GET_32BIT(*data);
+    *length = toint(GET_32BIT(*data));
+    if (*length < 0)
+        return;
     *datalen -= 4;
     *data += 4;
     if (*datalen < *length)
@@ -72,7 +64,7 @@ static Bignum getmp(char **data, int *datalen)
 	return NULL;
     if (p[0] & 0x80)
 	return NULL;		       /* negative mp */
-    b = bignum_from_bytes(p, length);
+    b = bignum_from_bytes((unsigned char *)p, length);
     return b;
 }
 
@@ -80,22 +72,25 @@ static Bignum get160(char **data, int *datalen)
 {
     Bignum b;
 
-    b = bignum_from_bytes(*data, 20);
+    if (*datalen < 20)
+        return NULL;
+
+    b = bignum_from_bytes((unsigned char *)*data, 20);
     *data += 20;
     *datalen -= 20;
 
     return b;
 }
 
+static void dss_freekey(void *key);    /* forward reference */
+
 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
@@ -108,7 +103,7 @@ static void *dss_newkey(char *data, int len)
     }
 #endif
 
-    if (!p || memcmp(p, "ssh-dss", 7)) {
+    if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
 	sfree(dss);
 	return NULL;
     }
@@ -116,6 +111,14 @@ 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;
 }
@@ -123,10 +126,16 @@ static void *dss_newkey(char *data, int len)
 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);
+    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);
 }
 
@@ -143,7 +152,7 @@ static char *dss_fmtkey(void *key)
     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 = smalloc(len);
+    p = snewn(len, char);
     if (!p)
 	return NULL;
 
@@ -190,7 +199,7 @@ static char *dss_fingerprint(void *key)
     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 = (bignum_bitcount(bignum)+8)/8; \
@@ -211,7 +220,7 @@ static char *dss_fingerprint(void *key)
     for (i = 0; i < 16; i++)
 	sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
 		digest[i]);
-    ret = smalloc(strlen(buffer) + 1);
+    ret = snewn(strlen(buffer) + 1, char);
     if (ret)
 	strcpy(ret, buffer);
     return ret;
@@ -241,14 +250,14 @@ static int dss_verifysig(void *key, char *sig, int siglen,
 #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);
@@ -259,18 +268,34 @@ static int dss_verifysig(void *key, char *sig, int siglen,
     }
     r = get160(&sig, &siglen);
     s = get160(&sig, &siglen);
-    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);
+    if (!w) {
+        freebn(r);
+        freebn(s);
+        return 0;
+    }
 
     /*
      * Step 2. u1 <- SHA(message) * w mod q.
      */
-    SHA_Simple(data, datalen, hash);
+    SHA_Simple(data, datalen, (unsigned char *)hash);
     p = hash;
     slen = 20;
     sha = get160(&p, &slen);
@@ -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);
@@ -324,7 +351,7 @@ static unsigned char *dss_public_blob(void *key, int *len)
      * 27 + sum of lengths. (five length fields, 20+7=27).
      */
     bloblen = 27 + plen + qlen + glen + ylen;
-    blob = smalloc(bloblen);
+    blob = snewn(bloblen, unsigned char);
     p = blob;
     PUT_32BIT(p, 7);
     p += 4;
@@ -357,31 +384,19 @@ static unsigned char *dss_private_blob(void *key, int *len)
     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).
+     * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
      */
-    bloblen = 28 + xlen;
-    blob = smalloc(bloblen);
+    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);
-    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;
@@ -399,25 +414,29 @@ static void *dss_createkey(unsigned char *pub_blob, int pub_len,
     Bignum ytest;
 
     dss = dss_newkey((char *) pub_blob, pub_len);
+    if (!dss)
+        return NULL;
     dss->x = getmp(&pb, &priv_len);
-    getstring(&pb, &priv_len, &hash, &hashlen);
+    if (!dss->x) {
+        dss_freekey(dss);
+        return NULL;
+    }
 
     /*
-     * Verify details of the key. First check that the hash is
-     * indeed a hash of p||q||g.
+     * Check the obsolete hash in the old DSS key format.
      */
-    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;
+    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;
+	}
     }
 
     /*
@@ -426,6 +445,7 @@ static void *dss_createkey(unsigned char *pub_blob, int pub_len,
     ytest = modpow(dss->g, dss->x, dss->p);
     if (0 != bignum_cmp(ytest, dss->y)) {
 	dss_freekey(dss);
+        freebn(ytest);
 	return NULL;
     }
     freebn(ytest);
@@ -438,9 +458,7 @@ static void *dss_openssh_createkey(unsigned char **blob, int *len)
     char **b = (char **) blob;
     struct dss_key *dss;
 
-    dss = smalloc(sizeof(struct dss_key));
-    if (!dss)
-	return NULL;
+    dss = snew(struct dss_key);
 
     dss->p = getmp(b, len);
     dss->q = getmp(b, len);
@@ -448,14 +466,11 @@ static void *dss_openssh_createkey(unsigned char **blob, int *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;
+    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;
@@ -489,7 +504,21 @@ static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len)
     return bloblen;
 }
 
-unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
+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:
@@ -546,9 +575,16 @@ unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
      * 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.)
+     * 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
@@ -578,18 +614,34 @@ unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
     SHA512_Init(&ss);
     SHA512_Bytes(&ss, digest512, sizeof(digest512));
     SHA512_Bytes(&ss, digest, sizeof(digest));
-    SHA512_Final(&ss, digest512);
 
-    memset(&ss, 0, sizeof(ss));
+    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;
+    }
 
-    /*
-     * 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);
+    smemclr(&ss, sizeof(ss));
 
-    memset(digest512, 0, sizeof(digest512));
+    smemclr(digest512, sizeof(digest512));
 
     /*
      * Now we have k, so just go ahead and compute the signature.
@@ -599,11 +651,11 @@ unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
     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(k);
     freebn(hash);
 
     /*
@@ -615,7 +667,7 @@ unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
      * i.e. 4+7 + 4+40 bytes.
      */
     nbytes = 4 + 7 + 4 + 40;
-    bytes = smalloc(nbytes);
+    bytes = snewn(nbytes, unsigned char);
     PUT_32BIT(bytes, 7);
     memcpy(bytes + 4, "ssh-dss", 7);
     PUT_32BIT(bytes + 4 + 7, 40);
@@ -639,6 +691,7 @@ const struct ssh_signkey ssh_dss = {
     dss_createkey,
     dss_openssh_createkey,
     dss_openssh_fmtkey,
+    dss_pubkey_bits,
     dss_fingerprint,
     dss_verifysig,
     dss_sign,