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More consistently defend against division by zero with assertions. We

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now check that all the modular functions (modpow, modinv, modmul,
bigdivmod) have nonzero moduli, and that modinv also has a nonzero
thing to try to invert.

[originally from svn r9987]
This commit is contained in:
Simon Tatham 2013-08-04 19:33:53 +00:00
parent 76dc7c49a2
commit 5bcb8d6aac
1 changed files with 16 additions and 0 deletions
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16
sshbn.c
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@ -624,6 +624,7 @@ static void internal_mod(BignumInt *a, int alen,
int i, k; int i, k;
m0 = m[0]; m0 = m[0];
assert(m0 >> (BIGNUM_INT_BITS-1) == 1);
if (mlen > 1) if (mlen > 1)
m1 = m[1]; m1 = m[1];
else else
@ -988,6 +989,12 @@ Bignum modmul(Bignum p, Bignum q, Bignum mod)
int pqlen, mlen, rlen, i, j; int pqlen, mlen, rlen, i, j;
Bignum result; 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 */ /* Allocate m of size mlen, copy mod to m */
/* We use big endian internally */ /* We use big endian internally */
mlen = mod[0]; mlen = mod[0];
@ -1087,6 +1094,12 @@ static void bigdivmod(Bignum p, Bignum mod, Bignum result, Bignum quotient)
int mshift; int mshift;
int plen, mlen, i, j; 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 */ /* Allocate m of size mlen, copy mod to m */
/* We use big endian internally */ /* We use big endian internally */
mlen = mod[0]; mlen = mod[0];
@ -1617,6 +1630,9 @@ Bignum modinv(Bignum number, Bignum modulus)
Bignum x = copybn(One); Bignum x = copybn(One);
int sign = +1; int sign = +1;
assert(number[number[0]] != 0);
assert(modulus[modulus[0]] != 0);
while (bignum_cmp(b, One) != 0) { while (bignum_cmp(b, One) != 0) {
Bignum t = newbn(b[0]); Bignum t = newbn(b[0]);
Bignum q = newbn(a[0]); Bignum q = newbn(a[0]);