diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index b43add7..9a8e3ee 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -108,6 +108,16 @@
 
 /* BEGIN MERGE SLOT 2 */
 
+void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N )
+{
+    mbedtls_mpi_uint c = mbedtls_mpi_core_sub( X, A, B, N->limbs );
+
+    (void) mbedtls_mpi_core_add_if( X, N->p, N->limbs, c );
+}
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index f738e91..0f1b0c0 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -144,6 +144,26 @@
 
 /* BEGIN MERGE SLOT 2 */
 
+/** \brief  Perform a subtraction of two MPIs and return the modulus
+ *          of the result.
+ *
+ * The size of the operation is determined by \p N.
+ *
+ * \param[out] X        The address of the result MPI.
+ *                      This must be initialized. Must have enough limbs to
+ *                      store the full value of the result.
+ * \param[in]  A        The address of the first MPI. This must be initialized.
+ * \param[in]  B        The address of the second MPI. This must be initialized.
+ * \param[in]  N        The address of the modulus. Use to perform a modulu
+ *                      operation on the result of the subtraction.
+ *
+ *  \note Both \p A and \p B must be smaller than the modulus \p N.
+ */
+void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N );
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */