X-Git-Url: https://git.distorted.org.uk/~mdw/sgt/putty/blobdiff_plain/164300003afeb59a3ab4e511f037e45ffee58311..25d3a4a3fd89d2814b9a361f2a8ac70125203761:/sshbn.c

diff --git a/sshbn.c b/sshbn.c
index ecdb90e8..a2067831 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -624,6 +624,7 @@ static void internal_mod(BignumInt *a, int alen,
     int i, k;
 
     m0 = m[0];
+    assert(m0 >> (BIGNUM_INT_BITS-1) == 1);
     if (mlen > 1)
 	m1 = m[1];
     else
@@ -868,6 +869,7 @@ Bignum modpow(Bignum base_in, Bignum exp, Bignum mod)
     len = mod[0];
     r = bn_power_2(BIGNUM_INT_BITS * len);
     inv = modinv(mod, r);
+    assert(inv); /* cannot fail, since mod is odd and r is a power of 2 */
 
     /*
      * Multiply the base by r mod n, to get it into Montgomery
@@ -988,6 +990,12 @@ Bignum modmul(Bignum p, Bignum q, Bignum mod)
     int pqlen, mlen, rlen, i, j;
     Bignum result;
 
+    /*
+     * The most significant word of mod needs to be non-zero. It
+     * should already be, but let's make sure.
+     */
+    assert(mod[mod[0]] != 0);
+
     /* Allocate m of size mlen, copy mod to m */
     /* We use big endian internally */
     mlen = mod[0];
@@ -1087,6 +1095,12 @@ static void bigdivmod(Bignum p, Bignum mod, Bignum result, Bignum quotient)
     int mshift;
     int plen, mlen, i, j;
 
+    /*
+     * The most significant word of mod needs to be non-zero. It
+     * should already be, but let's make sure.
+     */
+    assert(mod[mod[0]] != 0);
+
     /* Allocate m of size mlen, copy mod to m */
     /* We use big endian internally */
     mlen = mod[0];
@@ -1617,9 +1631,22 @@ Bignum modinv(Bignum number, Bignum modulus)
     Bignum x = copybn(One);
     int sign = +1;
 
+    assert(number[number[0]] != 0);
+    assert(modulus[modulus[0]] != 0);
+
     while (bignum_cmp(b, One) != 0) {
-	Bignum t = newbn(b[0]);
-	Bignum q = newbn(a[0]);
+	Bignum t, q;
+
+        if (bignum_cmp(b, Zero) == 0) {
+            /*
+             * Found a common factor between the inputs, so we cannot
+             * return a modular inverse at all.
+             */
+            return NULL;
+        }
+
+        t = newbn(b[0]);
+	q = newbn(a[0]);
 	bigdivmod(a, b, t, q);
 	while (t[0] > 1 && t[t[0]] == 0)
 	    t[0]--;