diff --git a/library/bignum.c b/library/bignum.c
index d15bc1a..bc2dbfa 100644
--- a/library/bignum.c
+++ b/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 @@
     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.
  *