Move mbedtls_cf_size_bool_eq function to the constant-time module

There were multiple functions called mbedtls_cf_size_bool_eq. They had exactly
the same behavior, so move the one in bignum.c and remove the other.

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

library/bignum.c

@@ -41,6 +41,7 @@
 #include "mbedtls/bn_mul.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
+#include "constant_time.h"
 
 #include <string.h>
 
@@ -2227,42 +2228,6 @@ static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N,
     mpi_montmul( A, &U, N, mm, T );
 }
 
-/*
- * Constant-flow boolean "equal" comparison:
- * return x == y
- *
- * This function can be used to write constant-time code by replacing branches
- * with bit operations - it can be used in conjunction with
- * mbedtls_ssl_cf_mask_from_bit().
- *
- * This function is implemented without using comparison operators, as those
- * might be translated to branches by some compilers on some platforms.
- */
-static size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
-{
-    /* diff = 0 if x == y, non-zero otherwise */
-    const size_t diff = x ^ y;
-
-    /* MSVC has a warning about unary minus on unsigned integer types,
-     * but this is well-defined and precisely what we want to do here. */
-#if defined(_MSC_VER)
-#pragma warning( push )
-#pragma warning( disable : 4146 )
-#endif
-
-    /* diff_msb's most significant bit is equal to x != y */
-    const size_t diff_msb = ( diff | (size_t) -diff );
-
-#if defined(_MSC_VER)
-#pragma warning( pop )
-#endif
-
-    /* diff1 = (x != y) ? 1 : 0 */
-    const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
-
-    return( 1 ^ diff1 );
-}
-
 /**
  * Select an MPI from a table without leaking the index.
  *

library/constant_time.c

@@ -177,3 +177,39 @@ size_t mbedtls_cf_size_mask_ge( size_t x, size_t y )
 {
     return( ~mbedtls_cf_size_mask_lt( x, y ) );
 }
+
+/*
+ * Constant-flow boolean "equal" comparison:
+ * return x == y
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations - it can be used in conjunction with
+ * mbedtls_ssl_cf_mask_from_bit().
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
+{
+    /* diff = 0 if x == y, non-zero otherwise */
+    const size_t diff = x ^ y;
+
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    /* diff_msb's most significant bit is equal to x != y */
+    const size_t diff_msb = ( diff | (size_t) -diff );
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    /* diff1 = (x != y) ? 1 : 0 */
+    const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
+
+    return( 1 ^ diff1 );
+}

library/constant_time.h

@@ -37,3 +37,5 @@ size_t mbedtls_cf_size_mask( size_t bit );
 size_t mbedtls_cf_size_mask_lt( size_t x, size_t y );
 
 size_t mbedtls_cf_size_mask_ge( size_t x, size_t y );
+
+size_t mbedtls_cf_size_bool_eq( size_t x, size_t y );

library/ssl_msg.c

@@ -1045,41 +1045,6 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
 }
 
 #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
-/*
- * Constant-flow boolean "equal" comparison:
- * return x == y
- *
- * This function can be used to write constant-time code by replacing branches
- * with bit operations - it can be used in conjunction with
- * mbedtls_cf_size_mask().
- *
- * This function is implemented without using comparison operators, as those
- * might be translated to branches by some compilers on some platforms.
- */
-static size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
-{
-    /* diff = 0 if x == y, non-zero otherwise */
-    const size_t diff = x ^ y;
-
-    /* MSVC has a warning about unary minus on unsigned integer types,
-     * but this is well-defined and precisely what we want to do here. */
-#if defined(_MSC_VER)
-#pragma warning( push )
-#pragma warning( disable : 4146 )
-#endif
-
-    /* diff_msb's most significant bit is equal to x != y */
-    const size_t diff_msb = ( diff | -diff );
-
-#if defined(_MSC_VER)
-#pragma warning( pop )
-#endif
-
-    /* diff1 = (x != y) ? 1 : 0 */
-    const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
-
-    return( 1 ^ diff1 );
-}
 
 /*
  * Constant-flow conditional memcpy: