diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 7a5539d..7cf2fb2 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -179,7 +179,18 @@
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */
+int mbedtls_mpi_mod_sub( mbedtls_mpi_mod_residue *X,
+                         const mbedtls_mpi_mod_residue *A,
+                         const mbedtls_mpi_mod_residue *B,
+                         const mbedtls_mpi_mod_modulus *N )
+{
+    if( X->limbs != N->limbs || A->limbs != N->limbs || B->limbs != N->limbs )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 
+    mbedtls_mpi_mod_raw_sub( X->p, A->p, B->p, N );
+
+    return( 0 );
+}
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */