diff --git a/library/bignum.c b/library/bignum.c
index 7caace776..51a64974b 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -269,37 +269,6 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
     memcpy(  Y, &T, sizeof( mbedtls_mpi ) );
 }
 
-/*
- * Conditionally assign dest = src, without leaking information
- * about whether the assignment was made or not.
- * dest and src must be arrays of limbs of size n.
- * assign must be 0 or 1.
- */
-void mbedtls_cf_mpi_uint_cond_assign( size_t n,
-                                      mbedtls_mpi_uint *dest,
-                                      const mbedtls_mpi_uint *src,
-                                      unsigned char assign )
-{
-    size_t i;
-
-    /* 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
-
-    /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
-    const mbedtls_mpi_uint mask = -assign;
-
-#if defined(_MSC_VER)
-#pragma warning( pop )
-#endif
-
-    for( i = 0; i < n; i++ )
-        dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
-}
-
 /*
  * Conditionally assign X = Y, without leaking information
  * about whether the assignment was made or not.
diff --git a/library/constant_time.c b/library/constant_time.c
index 6f59884ef..b48305a1f 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -319,3 +319,38 @@ int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second )
     /* ur is now 0 or 2, convert back to -1 or +1 */
     return( (int) ur - 1 );
 }
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+/*
+ * Conditionally assign dest = src, without leaking information
+ * about whether the assignment was made or not.
+ * dest and src must be arrays of limbs of size n.
+ * assign must be 0 or 1.
+ */
+void mbedtls_cf_mpi_uint_cond_assign( size_t n,
+                                      mbedtls_mpi_uint *dest,
+                                      const mbedtls_mpi_uint *src,
+                                      unsigned char assign )
+{
+    size_t i;
+
+    /* 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
+
+    /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
+    const mbedtls_mpi_uint mask = -assign;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    for( i = 0; i < n; i++ )
+        dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
+}
+
+#endif /* MBEDTLS_BIGNUM_C */
diff --git a/library/constant_time.h b/library/constant_time.h
index f97c57e7c..588181ec9 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -56,3 +56,12 @@ 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 );
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+void mbedtls_cf_mpi_uint_cond_assign( size_t n,
+                                      mbedtls_mpi_uint *dest,
+                                      const mbedtls_mpi_uint *src,
+                                      unsigned char assign );
+
+#endif /* MBEDTLS_BIGNUM_C */