Backport 2.28: Use mbedtls_ct_uint_if() rather than mbedtls_ct_cond_select_sign()
Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/constant_time.c b/library/constant_time.c
index 5279301..c0f53bb 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -263,40 +263,6 @@
#if defined(MBEDTLS_BIGNUM_C)
-/** Select between two sign values without branches.
- *
- * This is functionally equivalent to `condition ? if1 : if0` but uses only bit
- * operations in order to avoid branches.
- *
- * \note if1 and if0 must be either 1 or -1, otherwise the result
- * is undefined.
- *
- * \param condition Condition to test; must be either 0 or 1.
- * \param if1 The first sign; must be either +1 or -1.
- * \param if0 The second sign; must be either +1 or -1.
- *
- * \return \c if1 if \p condition is nonzero, otherwise \c if0.
- * */
-static int mbedtls_ct_cond_select_sign(unsigned char condition,
- int if1,
- int if0)
-{
- /* In order to avoid questions about what we can reasonably assume about
- * the representations of signed integers, move everything to unsigned
- * by taking advantage of the fact that if1 and if0 are either +1 or -1. */
- unsigned uif1 = if1 + 1;
- unsigned uif0 = if0 + 1;
-
- /* condition was 0 or 1, mask is 0 or 2 as are uif1 and uif0 */
- const unsigned mask = condition << 1;
-
- /* select uif1 or uif0 */
- unsigned ur = (uif0 & ~mask) | (uif1 & mask);
-
- /* ur is now 0 or 2, convert back to -1 or +1 */
- return (int) ur - 1;
-}
-
void mbedtls_ct_mpi_uint_cond_assign(size_t n,
mbedtls_mpi_uint *dest,
const mbedtls_mpi_uint *src,
@@ -559,7 +525,7 @@
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
- X->s = mbedtls_ct_cond_select_sign(assign, Y->s, X->s);
+ X->s = (int) mbedtls_ct_uint_if(assign, Y->s, X->s);
mbedtls_ct_mpi_uint_cond_assign(Y->n, X->p, Y->p, assign);
@@ -599,8 +565,8 @@
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Y, X->n));
s = X->s;
- X->s = mbedtls_ct_cond_select_sign(swap, Y->s, X->s);
- Y->s = mbedtls_ct_cond_select_sign(swap, s, Y->s);
+ X->s = (int) mbedtls_ct_uint_if(swap, Y->s, X->s);
+ Y->s = (int) mbedtls_ct_uint_if(swap, s, Y->s);
for (i = 0; i < X->n; i++) {