X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/8bd9144b754ef3b1f7fbb88a701fed37c7c064ad..HEAD:/sshbn.c

diff --git a/sshbn.c b/sshbn.c
index 76529cfa..a5e0552f 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -6,6 +6,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <limits.h>
 
 #include "misc.h"
 
@@ -120,7 +121,11 @@ Bignum Zero = bnZero, One = bnOne;
 
 static Bignum newbn(int length)
 {
-    Bignum b = snewn(length + 1, BignumInt);
+    Bignum b;
+
+    assert(length >= 0 && length < INT_MAX / BIGNUM_INT_BITS);
+
+    b = snewn(length + 1, BignumInt);
     if (!b)
 	abort();		       /* FIXME */
     memset(b, 0, (length + 1) * sizeof(*b));
@@ -154,7 +159,11 @@ void freebn(Bignum b)
 
 Bignum bn_power_2(int n)
 {
-    Bignum ret = newbn(n / BIGNUM_INT_BITS + 1);
+    Bignum ret;
+
+    assert(n >= 0);
+
+    ret = newbn(n / BIGNUM_INT_BITS + 1);
     bignum_set_bit(ret, n, 1);
     return ret;
 }
@@ -598,6 +607,7 @@ static void internal_add_shifted(BignumInt *number,
     addend = (BignumDblInt)n << bshift;
 
     while (addend) {
+        assert(word <= number[0]);
 	addend += number[word];
 	number[word] = (BignumInt) addend & BIGNUM_INT_MASK;
 	addend >>= BIGNUM_INT_BITS;
@@ -869,6 +879,7 @@ 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
@@ -1173,6 +1184,8 @@ Bignum bignum_from_bytes(const unsigned char *data, int nbytes)
     Bignum result;
     int w, i;
 
+    assert(nbytes >= 0 && nbytes < INT_MAX/8);
+
     w = (nbytes + BIGNUM_INT_BYTES - 1) / BIGNUM_INT_BYTES; /* bytes->words */
 
     result = newbn(w);
@@ -1249,7 +1262,7 @@ int ssh2_bignum_length(Bignum bn)
  */
 int bignum_byte(Bignum bn, int i)
 {
-    if (i >= (int)(BIGNUM_INT_BYTES * bn[0]))
+    if (i < 0 || i >= (int)(BIGNUM_INT_BYTES * bn[0]))
 	return 0;		       /* beyond the end */
     else
 	return (bn[i / BIGNUM_INT_BYTES + 1] >>
@@ -1261,7 +1274,7 @@ int bignum_byte(Bignum bn, int i)
  */
 int bignum_bit(Bignum bn, int i)
 {
-    if (i >= (int)(BIGNUM_INT_BITS * bn[0]))
+    if (i < 0 || i >= (int)(BIGNUM_INT_BITS * bn[0]))
 	return 0;		       /* beyond the end */
     else
 	return (bn[i / BIGNUM_INT_BITS + 1] >> (i % BIGNUM_INT_BITS)) & 1;
@@ -1272,7 +1285,7 @@ int bignum_bit(Bignum bn, int i)
  */
 void bignum_set_bit(Bignum bn, int bitnum, int value)
 {
-    if (bitnum >= (int)(BIGNUM_INT_BITS * bn[0]))
+    if (bitnum < 0 || bitnum >= (int)(BIGNUM_INT_BITS * bn[0]))
 	abort();		       /* beyond the end */
     else {
 	int v = bitnum / BIGNUM_INT_BITS + 1;
@@ -1308,7 +1321,18 @@ int ssh1_write_bignum(void *data, Bignum bn)
 int bignum_cmp(Bignum a, Bignum b)
 {
     int amax = a[0], bmax = b[0];
-    int i = (amax > bmax ? amax : bmax);
+    int i;
+
+    /* Annoyingly we have two representations of zero */
+    if (amax == 1 && a[amax] == 0)
+        amax = 0;
+    if (bmax == 1 && b[bmax] == 0)
+        bmax = 0;
+
+    assert(amax == 0 || a[amax] != 0);
+    assert(bmax == 0 || b[bmax] != 0);
+
+    i = (amax > bmax ? amax : bmax);
     while (i) {
 	BignumInt aval = (i > amax ? 0 : a[i]);
 	BignumInt bval = (i > bmax ? 0 : b[i]);
@@ -1330,6 +1354,8 @@ Bignum bignum_rshift(Bignum a, int shift)
     int i, shiftw, shiftb, shiftbb, bits;
     BignumInt ai, ai1;
 
+    assert(shift >= 0);
+
     bits = bignum_bitcount(a) - shift;
     ret = newbn((bits + BIGNUM_INT_BITS - 1) / BIGNUM_INT_BITS);
 
@@ -1634,8 +1660,22 @@ Bignum modinv(Bignum number, Bignum modulus)
     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.
+             */
+            freebn(b);
+            freebn(a);
+            freebn(xp);
+            freebn(x);
+            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]--;