Move mbedtls_cf_cond_select_sign function to the constant-time module

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>

library/bignum.c

@@ -269,36 +269,6 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
     memcpy(  Y, &T, sizeof( mbedtls_mpi ) );
 }
 
-/**
- * Select between two sign values in constant-time.
- *
- * This is functionally equivalent to second ? a : b but uses only bit
- * operations in order to avoid branches.
- *
- * \param[in] a         The first sign; must be either +1 or -1.
- * \param[in] b         The second sign; must be either +1 or -1.
- * \param[in] second    Must be either 1 (return b) or 0 (return a).
- *
- * \return The selected sign value.
- */
-static int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second )
-{
-    /* In order to avoid questions about what we can reasonnably assume about
-     * the representations of signed integers, move everything to unsigned
-     * by taking advantage of the fact that a and b are either +1 or -1. */
-    unsigned ua = a + 1;
-    unsigned ub = b + 1;
-
-    /* second was 0 or 1, mask is 0 or 2 as are ua and ub */
-    const unsigned mask = second << 1;
-
-    /* select ua or ub */
-    unsigned ur = ( ua & ~mask ) | ( ub & mask );
-
-    /* ur is now 0 or 2, convert back to -1 or +1 */
-    return( (int) ur - 1 );
-}
-
 /*
  * Conditionally assign dest = src, without leaking information
  * about whether the assignment was made or not.

library/constant_time.c

@@ -289,3 +289,33 @@ unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 )
     unsigned mask = mbedtls_cf_uint_mask( cond );
     return( ( mask & if1 ) | (~mask & if0 ) );
 }
+
+/**
+ * Select between two sign values in constant-time.
+ *
+ * This is functionally equivalent to second ? a : b but uses only bit
+ * operations in order to avoid branches.
+ *
+ * \param[in] a         The first sign; must be either +1 or -1.
+ * \param[in] b         The second sign; must be either +1 or -1.
+ * \param[in] second    Must be either 1 (return b) or 0 (return a).
+ *
+ * \return The selected sign value.
+ */
+int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second )
+{
+    /* In order to avoid questions about what we can reasonnably assume about
+     * the representations of signed integers, move everything to unsigned
+     * by taking advantage of the fact that a and b are either +1 or -1. */
+    unsigned ua = a + 1;
+    unsigned ub = b + 1;
+
+    /* second was 0 or 1, mask is 0 or 2 as are ua and ub */
+    const unsigned mask = second << 1;
+
+    /* select ua or ub */
+    unsigned ur = ( ua & ~mask ) | ( ub & mask );
+
+    /* ur is now 0 or 2, convert back to -1 or +1 */
+    return( (int) ur - 1 );
+}

library/constant_time.h

@@ -55,3 +55,4 @@ unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
 
 unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 );
 
+int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second );