Add support for using Diffie-Hellman with short exponents (sshdh.c

contains a reference to a paper on the subject). Reduces time taken
for DH group exchange to the point where it's viable to enable it
all the time, so I have. :-)

[originally from svn r991]

ssh.c

@@ -179,9 +179,7 @@ const static struct ssh2_ciphers *ciphers[] = {
 };
 
 const static struct ssh_kex *kex_algs[] = {
-#ifdef DO_DIFFIE_HELLMAN_GEX
     &ssh_diffiehellman_gex,
-#endif
     &ssh_diffiehellman };
 
 const static struct ssh_signkey *hostkey_algs[] = { &ssh_rsa, &ssh_dss };
@@ -2331,7 +2329,7 @@ static void ssh2_mkkey(Bignum K, char *H, char *sessid, char chr, char *keyspace
  */
 static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
 {
-    static int i, j, len, nbits;
+    static int i, j, len, nbits, pbits;
     static char *str;
     static Bignum p, g, e, f, K;
     static int kex_init_value, kex_reply_value;
@@ -2553,31 +2551,35 @@ static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
         }
     }
 
+    /*
+     * Work out the number of bits of key we will need from the key
+     * exchange. We start with the maximum key length of either
+     * cipher...
+     */
+    {
+        int csbits, scbits;
+
+	csbits = cscipher_tobe->keylen;
+	scbits = sccipher_tobe->keylen;
+	nbits = (csbits > scbits ? csbits : scbits);
+    }
+    /* The keys only have 160-bit entropy, since they're based on
+     * a SHA-1 hash. So cap the key size at 160 bits. */
+    if (nbits > 160) nbits = 160;
+
     /*
      * If we're doing Diffie-Hellman group exchange, start by
      * requesting a group.
      */
     if (kex == &ssh_diffiehellman_gex) {
-        int csbits, scbits;
-
         logevent("Doing Diffie-Hellman group exchange");
         /*
-         * Work out number of bits. We start with the maximum key
-         * length of either cipher...
-         */
-        csbits = cscipher_tobe->keylen;
-        scbits = sccipher_tobe->keylen;
-        nbits = (csbits > scbits ? csbits : scbits);
-        /* The keys only have 160-bit entropy, since they're based on
-         * a SHA-1 hash. So cap the key size at 160 bits. */
-        if (nbits > 160) nbits = 160;
-        /*
-         * ... and then work out how big a DH group we will need to
-         * allow that much data.
-         */
-        nbits = 512 << ((nbits-1) / 64);
+         * Work out how big a DH group we will need to allow that
+         * much data.
+	 */
+        pbits = 512 << ((nbits-1) / 64);
         ssh2_pkt_init(SSH2_MSG_KEX_DH_GEX_REQUEST);
-        ssh2_pkt_adduint32(nbits);
+        ssh2_pkt_adduint32(pbits);
         ssh2_pkt_send();
 
         crWaitUntil(ispkt);
@@ -2600,7 +2602,7 @@ static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
     /*
      * Now generate and send e for Diffie-Hellman.
      */
-    e = dh_create_e();
+    e = dh_create_e(nbits*2);
     ssh2_pkt_init(kex_init_value);
     ssh2_pkt_addmp(e);
     ssh2_pkt_send();
@@ -2618,7 +2620,7 @@ static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
 
     sha_string(&exhash, hostkeydata, hostkeylen);
     if (kex == &ssh_diffiehellman_gex) {
-        sha_uint32(&exhash, nbits);
+        sha_uint32(&exhash, pbits);
         sha_mpint(&exhash, p);
         sha_mpint(&exhash, g);
     }

ssh.h

@@ -222,7 +222,7 @@ char *bignum_decimal(Bignum x);
 void dh_setup_group1(void);
 void dh_setup_group(Bignum pval, Bignum gval);
 void dh_cleanup(void);
-Bignum dh_create_e(void);
+Bignum dh_create_e(int nbits);
 Bignum dh_find_K(Bignum f);
 
 int loadrsakey(char *filename, struct RSAKey *key, char *passphrase);

sshdh.c

@@ -75,8 +75,19 @@ void dh_cleanup(void) {
 /*
  * DH stage 1: invent a number x between 1 and q, and compute e =
  * g^x mod p. Return e.
+ * 
+ * If `nbits' is greater than zero, it is used as an upper limit
+ * for the number of bits in x. This is safe provided that (a) you
+ * use twice as many bits in x as the number of bits you expect to
+ * use in your session key, and (b) the DH group is a safe prime
+ * (which SSH demands that it must be).
+ * 
+ * P. C. van Oorschot, M. J. Wiener
+ * "On Diffie-Hellman Key Agreement with Short Exponents".
+ * Advances in Cryptology: Proceedings of Eurocrypt '96
+ * Springer-Verlag, May 1996.
  */
-Bignum dh_create_e(void) {
+Bignum dh_create_e(int nbits) {
     int i;
 
     int nbytes;
@@ -91,10 +102,25 @@ Bignum dh_create_e(void) {
 	 * with qmask.
 	 */
         if (x) freebn(x);
-        ssh1_write_bignum(buf, qmask);
-	for (i = 2; i < nbytes; i++)
-            buf[i] &= random_byte();
-        ssh1_read_bignum(buf, &x);
+	if (nbits == 0 || nbits > ssh1_bignum_bitcount(qmask)) {
+	    ssh1_write_bignum(buf, qmask);
+	    for (i = 2; i < nbytes; i++)
+		buf[i] &= random_byte();
+	    ssh1_read_bignum(buf, &x);
+	} else {
+	    int b, nb;
+	    x = bn_power_2(nbits);
+	    nb = 0;
+	    for (i = 0; i < nbits; i++) {
+		if (nb == 0) {
+		    nb = 8;
+		    b = random_byte();
+		}
+		bignum_set_bit(x, i, b & 1);
+		b >>= 1;
+		nb--;
+	    }
+	}
     } while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, q) >= 0);
 
     /*
@@ -109,5 +135,7 @@ Bignum dh_create_e(void) {
  * DH stage 2: given a number f, compute K = f^x mod p.
  */
 Bignum dh_find_K(Bignum f) {
-    return modpow(f, x, p);
+    Bignum ret;
+    ret = modpow(f, x, p);
+    return ret;
 }