Update mbedtls_mpi_lt_mpi_ct to new interface
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index dd77bfc..9e215d8 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -61,9 +61,7 @@
const mbedtls_mpi *Y,
unsigned *ret)
{
- size_t i;
- /* The value of any of these variables is either 0 or 1 at all times. */
- unsigned cond, done, X_is_negative, Y_is_negative;
+ mbedtls_ct_condition_t cond, X_is_negative, Y_is_negative, result;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
@@ -77,46 +75,33 @@
* Set sign_N to 1 if N >= 0, 0 if N < 0.
* We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
*/
- X_is_negative = (X->s & 2) >> 1;
- Y_is_negative = (Y->s & 2) >> 1;
+ X_is_negative = mbedtls_ct_bool((X->s & 2) >> 1);
+ Y_is_negative = mbedtls_ct_bool((Y->s & 2) >> 1);
/*
* If the signs are different, then the positive operand is the bigger.
* That is if X is negative (X_is_negative == 1), then X < Y is true and it
* is false if X is positive (X_is_negative == 0).
*/
- cond = (X_is_negative ^ Y_is_negative);
- *ret = cond & X_is_negative;
+ cond = mbedtls_ct_bool_xor(X_is_negative, Y_is_negative); // non-zero if different sign
+ result = mbedtls_ct_bool_and(cond, X_is_negative);
- /*
- * This is a constant-time function. We might have the result, but we still
- * need to go through the loop. Record if we have the result already.
+ /* Assuming signs are the same, compare X and Y. We switch the comparison
+ * order if they are negative so that we get the right result, regardles of
+ * sign.
+ *
+ * Store in ret iff the signs are the same (i.e., iff cond == 0). If
+ * the signs differ, done has already been set.
*/
- done = cond;
- for (i = X->n; i > 0; i--) {
- /*
- * If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both
- * X and Y are negative.
- *
- * Again even if we can make a decision, we just mark the result and
- * the fact that we are done and continue looping.
- */
- cond = mbedtls_ct_mpi_uint_lt(Y->p[i - 1], X->p[i - 1]);
- *ret |= cond & (1 - done) & X_is_negative;
- done |= cond;
+ /* This is used to conditionally swap the pointers in const time */
+ void * const p[2] = { X->p, Y->p };
+ mbedtls_ct_condition_t lt = mbedtls_mpi_core_lt_ct(
+ p[X_is_negative & 1], p[(X_is_negative & 1) ^ 1], X->n);
- /*
- * If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both
- * X and Y are positive.
- *
- * Again even if we can make a decision, we just mark the result and
- * the fact that we are done and continue looping.
- */
- cond = mbedtls_ct_mpi_uint_lt(X->p[i - 1], Y->p[i - 1]);
- *ret |= cond & (1 - done) & (1 - X_is_negative);
- done |= cond;
- }
+ result = mbedtls_ct_bool_or(result, mbedtls_ct_bool_and(mbedtls_ct_bool_not(cond), lt));
+
+ *ret = mbedtls_ct_uint_if0(result, 1);
return 0;
}