X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/75374b2f8b6f78b865555ba9e5588f46003d5da7..62ef3d44b5dea02c9d1070c4f7da650d1df0e381:/sshbn.c

diff --git a/sshbn.c b/sshbn.c
index 8d4d703c..2a5d1af2 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -3,6 +3,7 @@
  */
 
 #include <stdio.h>
+#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -133,7 +134,7 @@ static void internal_add_shifted(BignumInt *number,
     int bshift = shift % BIGNUM_INT_BITS;
     BignumDblInt addend;
 
-    addend = n << bshift;
+    addend = (BignumDblInt)n << bshift;
 
     while (addend) {
 	addend += number[word];
@@ -184,17 +185,36 @@ static void internal_mod(BignumInt *a, int alen,
 	    ai1 = a[i + 1];
 
 	/* Find q = h:a[i] / m0 */
-	DIVMOD_WORD(q, r, h, a[i], m0);
-
-	/* Refine our estimate of q by looking at
-	   h:a[i]:a[i+1] / m0:m1 */
-	t = MUL_WORD(m1, q);
-	if (t > ((BignumDblInt) r << BIGNUM_INT_BITS) + ai1) {
-	    q--;
-	    t -= m1;
-	    r = (r + m0) & BIGNUM_INT_MASK;     /* overflow? */
-	    if (r >= (BignumDblInt) m0 &&
-		t > ((BignumDblInt) r << BIGNUM_INT_BITS) + ai1) q--;
+	if (h >= m0) {
+	    /*
+	     * Special case.
+	     * 
+	     * To illustrate it, suppose a BignumInt is 8 bits, and
+	     * we are dividing (say) A1:23:45:67 by A1:B2:C3. Then
+	     * our initial division will be 0xA123 / 0xA1, which
+	     * will give a quotient of 0x100 and a divide overflow.
+	     * However, the invariants in this division algorithm
+	     * are not violated, since the full number A1:23:... is
+	     * _less_ than the quotient prefix A1:B2:... and so the
+	     * following correction loop would have sorted it out.
+	     * 
+	     * In this situation we set q to be the largest
+	     * quotient we _can_ stomach (0xFF, of course).
+	     */
+	    q = BIGNUM_INT_MASK;
+	} else {
+	    DIVMOD_WORD(q, r, h, a[i], m0);
+
+	    /* Refine our estimate of q by looking at
+	     h:a[i]:a[i+1] / m0:m1 */
+	    t = MUL_WORD(m1, q);
+	    if (t > ((BignumDblInt) r << BIGNUM_INT_BITS) + ai1) {
+		q--;
+		t -= m1;
+		r = (r + m0) & BIGNUM_INT_MASK;     /* overflow? */
+		if (r >= (BignumDblInt) m0 &&
+		    t > ((BignumDblInt) r << BIGNUM_INT_BITS) + ai1) q--;
+	    }
 	}
 
 	/* Subtract q * m from a[i...] */
@@ -226,16 +246,25 @@ static void internal_mod(BignumInt *a, int alen,
 
 /*
  * Compute (base ^ exp) % mod.
- * The base MUST be smaller than the modulus.
- * The most significant word of mod MUST be non-zero.
- * We assume that the result array is the same size as the mod array.
  */
-Bignum modpow(Bignum base, Bignum exp, Bignum mod)
+Bignum modpow(Bignum base_in, Bignum exp, Bignum mod)
 {
     BignumInt *a, *b, *n, *m;
     int mshift;
     int mlen, i, j;
-    Bignum result;
+    Bignum base, 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);
+
+    /*
+     * Make sure the base is smaller than the modulus, by reducing
+     * it modulo the modulus if not.
+     */
+    base = bigmod(base_in, mod);
 
     /* Allocate m of size mlen, copy mod to m */
     /* We use big endian internally */
@@ -331,6 +360,8 @@ Bignum modpow(Bignum base, Bignum exp, Bignum mod)
 	n[i] = 0;
     sfree(n);
 
+    freebn(base);
+
     return result;
 }
 
@@ -528,19 +559,25 @@ Bignum bignum_from_bytes(const unsigned char *data, int nbytes)
 
 /*
  * Read an ssh1-format bignum from a data buffer. Return the number
- * of bytes consumed.
+ * of bytes consumed, or -1 if there wasn't enough data.
  */
-int ssh1_read_bignum(const unsigned char *data, Bignum * result)
+int ssh1_read_bignum(const unsigned char *data, int len, Bignum * result)
 {
     const unsigned char *p = data;
     int i;
     int w, b;
 
+    if (len < 2)
+	return -1;
+
     w = 0;
     for (i = 0; i < 2; i++)
 	w = (w << 8) + *p++;
     b = (w + 7) / 8;		       /* bits -> bytes */
 
+    if (len < b+2)
+	return -1;
+
     if (!result)		       /* just return length */
 	return b + 2;