Merge pull request #7545 from tom-cosgrove-arm/use-mbedtls_ct_uint_if…

…-rather-than-mbedtls_ct_cond_select_sign-2.28

Backport 2.28: Use mbedtls_ct_uint_if() rather than mbedtls_ct_cond_select_sign()

library/constant_time.c

@@ -263,40 +263,6 @@ unsigned mbedtls_ct_uint_if(unsigned condition,
 
 #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 @@ int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
 
     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 @@ int mbedtls_mpi_safe_cond_swap(mbedtls_mpi *X,
     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++) {