diff --git a/library/bignum_mod.h b/library/bignum_mod.h
new file mode 100644
index 0000000..378c58d
--- /dev/null
+++ b/library/bignum_mod.h
@@ -0,0 +1,68 @@
+/**
+ *  Internal bignum functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef MBEDTLS_BIGNUM_MOD_H
+#define MBEDTLS_BIGNUM_MOD_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_BIGNUM_C)
+#include "mbedtls/bignum.h"
+#endif
+
+
+typedef struct
+{
+    size_t n;
+    mbedtls_mpi_uint *p;
+} mbedtls_mpi_mod_residue;
+
+typedef void* mbedtls_mpi_mont_struct;
+typedef void* mbedtls_mpi_opt_red_struct;
+
+typedef struct {
+    mbedtls_mpi_uint *p;
+    size_t n;       // number of limbs
+    size_t plen;    // bitlen of p
+    int ext_rep;    // signals external representation (eg. byte order)
+    int int_rep;    // selector to signal the active member of the union
+    union rep
+    {
+        mbedtls_mpi_mont_struct mont;
+        mbedtls_mpi_opt_red_struct ored;
+    } rep;
+} mbedtls_mpi_mod_modulus;
+
+void mbedtls_mpi_mod_residue_release( mbedtls_mpi_mod_residue *r );
+
+int mbedtls_mpi_mod_residue_setup( mbedtls_mpi_mod_residue *r,
+                                   mbedtls_mpi_mod_modulus *m,
+                                   mbedtls_mpi_uint *p );
+
+void mbedtls_mpi_mod_modulus_init( mbedtls_mpi_mod_modulus *m );
+
+void mbedtls_mpi_mod_modulus_init( mbedtls_mpi_mod_modulus *m );
+
+int mbedtls_mpi_mod_modulus_setup( mbedtls_mpi_mod_modulus *m,
+                                   mbedtls_mpi_uint *p,
+                                   size_t n,
+                                   int ext_rep,
+                                   int int_rep );
+
+#endif /* MBEDTLS_BIGNUM_MOD_H */
diff --git a/library/bignum_new.c b/library/bignum_new.c
new file mode 100644
index 0000000..3292861
--- /dev/null
+++ b/library/bignum_new.c
@@ -0,0 +1,138 @@
+/**
+ *  Internal bignum functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#include "common.h"
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+#include "mbedtls/error.h"
+#include "mbedtls/bignum.h"
+#include "bignum_mod.h"
+
+#define ciL    (sizeof(mbedtls_mpi_uint))   /* chars in limb  */
+#define biL    (ciL << 3)                   /* bits  in limb  */
+#define biH    (ciL << 2)                   /* half limb size */
+
+/*
+ * Convert between bits/chars and number of limbs
+ * Divide first in order to avoid potential overflows
+ */
+#define BITS_TO_LIMBS(i)  ( (i) / biL + ( (i) % biL != 0 ) )
+#define CHARS_TO_LIMBS(i) ( (i) / ciL + ( (i) % ciL != 0 ) )
+
+/*
+ * Count leading zero bits in a given integer
+ */
+static size_t mpi_clz( const mbedtls_mpi_uint x )
+{
+    size_t j;
+    mbedtls_mpi_uint mask = (mbedtls_mpi_uint) 1 << (biL - 1);
+
+    for( j = 0; j < biL; j++ )
+    {
+        if( x & mask ) break;
+
+        mask >>= 1;
+    }
+
+    return j;
+}
+
+/*
+ * Return the number of bits
+ */
+static size_t mpi_bitlen( const mbedtls_mpi_uint *X, size_t nx )
+{
+    size_t i, j;
+
+    if( nx == 0 )
+        return( 0 );
+
+    for( i = nx - 1; i > 0; i-- )
+        if( X[i] != 0 )
+            break;
+
+    j = biL - mpi_clz( X[i] );
+
+    return( ( i * biL ) + j );
+}
+
+void mbedtls_mpi_mod_residue_release( mbedtls_mpi_mod_residue *r )
+{
+    if ( r == NULL )
+        return;
+
+    r->n = 0;
+    r->p = NULL;
+}
+
+int mbedtls_mpi_mod_residue_setup( mbedtls_mpi_mod_residue *r,
+                                   mbedtls_mpi_mod_modulus *m,
+                                   mbedtls_mpi_uint *X )
+{
+    if( X == NULL || m == NULL || r == NULL || X >= m->p)
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+    r->n = m->n;
+    r->p = X;
+
+    return( 0 );
+}
+
+void mbedtls_mpi_mod_modulus_init( mbedtls_mpi_mod_modulus *m )
+{
+    if ( m == NULL )
+        return;
+
+    m->rep.mont = 0;
+}
+
+void mbedtls_mpi_mod_modulus_free( mbedtls_mpi_mod_modulus *m )
+{
+    if ( m == NULL )
+        return;
+
+    m->p = NULL;
+    m->n = 0;
+    m->plen = 0;
+    m->ext_rep = 0;
+    m->int_rep = 0;
+    m->rep.mont = NULL;
+    m->rep.ored = NULL;
+}
+
+int mbedtls_mpi_mod_modulus_setup( mbedtls_mpi_mod_modulus *m,
+                                   mbedtls_mpi_uint *X,
+                                   size_t nx,
+                                   int ext_rep,
+                                   int int_rep )
+{
+    if ( X == NULL || m == NULL )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+    m->p = X;
+    m->n = nx;
+    m->ext_rep = ext_rep;
+    m->int_rep = int_rep;
+    m->plen = mpi_bitlen( X, nx );
+
+    return( 0 );
+}
+
+#endif /* MBEDTLS_BIGNUM_C */