diff --git a/library/asn1write.c b/library/asn1write.c
new file mode 100644
index 0000000..87c5ef6
--- /dev/null
+++ b/library/asn1write.c
@@ -0,0 +1,386 @@
+/*
+ * ASN.1 buffer writing functionality
+ *
+ *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  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.
+ *
+ *  This file is part of Mbed Crypto (https://tls.mbed.org)
+ */
+
+#if !defined(MBEDCRYPTO_CONFIG_FILE)
+#include "mbedcrypto/config.h"
+#else
+#include MBEDCRYPTO_CONFIG_FILE
+#endif
+
+#if defined(MBEDCRYPTO_ASN1_WRITE_C)
+
+#include "mbedcrypto/asn1write.h"
+
+#include <string.h>
+
+#if defined(MBEDCRYPTO_PLATFORM_C)
+#include "mbedcrypto/platform.h"
+#else
+#include <stdlib.h>
+#define mbedcrypto_calloc    calloc
+#define mbedcrypto_free       free
+#endif
+
+int mbedcrypto_asn1_write_len( unsigned char **p, unsigned char *start, size_t len )
+{
+    if( len < 0x80 )
+    {
+        if( *p - start < 1 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = (unsigned char) len;
+        return( 1 );
+    }
+
+    if( len <= 0xFF )
+    {
+        if( *p - start < 2 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = (unsigned char) len;
+        *--(*p) = 0x81;
+        return( 2 );
+    }
+
+    if( len <= 0xFFFF )
+    {
+        if( *p - start < 3 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = 0x82;
+        return( 3 );
+    }
+
+    if( len <= 0xFFFFFF )
+    {
+        if( *p - start < 4 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = ( len >> 16 ) & 0xFF;
+        *--(*p) = 0x83;
+        return( 4 );
+    }
+
+    if( len <= 0xFFFFFFFF )
+    {
+        if( *p - start < 5 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = ( len >> 16 ) & 0xFF;
+        *--(*p) = ( len >> 24 ) & 0xFF;
+        *--(*p) = 0x84;
+        return( 5 );
+    }
+
+    return( MBEDCRYPTO_ERR_ASN1_INVALID_LENGTH );
+}
+
+int mbedcrypto_asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag )
+{
+    if( *p - start < 1 )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    *--(*p) = tag;
+
+    return( 1 );
+}
+
+int mbedcrypto_asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
+                           const unsigned char *buf, size_t size )
+{
+    size_t len = 0;
+
+    if( *p < start || (size_t)( *p - start ) < size )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    len = size;
+    (*p) -= len;
+    memcpy( *p, buf, len );
+
+    return( (int) len );
+}
+
+#if defined(MBEDCRYPTO_BIGNUM_C)
+int mbedcrypto_asn1_write_mpi( unsigned char **p, unsigned char *start, const mbedcrypto_mpi *X )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write the MPI
+    //
+    len = mbedcrypto_mpi_size( X );
+
+    if( *p < start || (size_t)( *p - start ) < len )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    (*p) -= len;
+    MBEDCRYPTO_MPI_CHK( mbedcrypto_mpi_write_binary( X, *p, len ) );
+
+    // DER format assumes 2s complement for numbers, so the leftmost bit
+    // should be 0 for positive numbers and 1 for negative numbers.
+    //
+    if( X->s ==1 && **p & 0x80 )
+    {
+        if( *p - start < 1 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = 0x00;
+        len += 1;
+    }
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_INTEGER ) );
+
+    ret = (int) len;
+
+cleanup:
+    return( ret );
+}
+#endif /* MBEDCRYPTO_BIGNUM_C */
+
+int mbedcrypto_asn1_write_null( unsigned char **p, unsigned char *start )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write NULL
+    //
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, 0) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_NULL ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_oid( unsigned char **p, unsigned char *start,
+                    const char *oid, size_t oid_len )
+{
+    int ret;
+    size_t len = 0;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_raw_buffer( p, start,
+                                  (const unsigned char *) oid, oid_len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len , mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len , mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_OID ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start,
+                                     const char *oid, size_t oid_len,
+                                     size_t par_len )
+{
+    int ret;
+    size_t len = 0;
+
+    if( par_len == 0 )
+        MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_null( p, start ) );
+    else
+        len += par_len;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_oid( p, start, oid, oid_len ) );
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start,
+                                       MBEDCRYPTO_ASN1_CONSTRUCTED | MBEDCRYPTO_ASN1_SEQUENCE ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_bool( unsigned char **p, unsigned char *start, int boolean )
+{
+    int ret;
+    size_t len = 0;
+
+    if( *p - start < 1 )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    *--(*p) = (boolean) ? 255 : 0;
+    len++;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_BOOLEAN ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_int( unsigned char **p, unsigned char *start, int val )
+{
+    int ret;
+    size_t len = 0;
+
+    if( *p - start < 1 )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    len += 1;
+    *--(*p) = val;
+
+    if( val > 0 && **p & 0x80 )
+    {
+        if( *p - start < 1 )
+            return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = 0x00;
+        len += 1;
+    }
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_INTEGER ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_printable_string( unsigned char **p, unsigned char *start,
+                                 const char *text, size_t text_len )
+{
+    int ret;
+    size_t len = 0;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_raw_buffer( p, start,
+                  (const unsigned char *) text, text_len ) );
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_PRINTABLE_STRING ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
+                           const char *text, size_t text_len )
+{
+    int ret;
+    size_t len = 0;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_raw_buffer( p, start,
+                  (const unsigned char *) text, text_len ) );
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_IA5_STRING ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_bitstring( unsigned char **p, unsigned char *start,
+                          const unsigned char *buf, size_t bits )
+{
+    int ret;
+    size_t len = 0, size;
+
+    size = ( bits / 8 ) + ( ( bits % 8 ) ? 1 : 0 );
+
+    // Calculate byte length
+    //
+    if( *p < start || (size_t)( *p - start ) < size + 1 )
+        return( MBEDCRYPTO_ERR_ASN1_BUF_TOO_SMALL );
+
+    len = size + 1;
+    (*p) -= size;
+    memcpy( *p, buf, size );
+
+    // Write unused bits
+    //
+    *--(*p) = (unsigned char) (size * 8 - bits);
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_BIT_STRING ) );
+
+    return( (int) len );
+}
+
+int mbedcrypto_asn1_write_octet_string( unsigned char **p, unsigned char *start,
+                             const unsigned char *buf, size_t size )
+{
+    int ret;
+    size_t len = 0;
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_raw_buffer( p, start, buf, size ) );
+
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_len( p, start, len ) );
+    MBEDCRYPTO_ASN1_CHK_ADD( len, mbedcrypto_asn1_write_tag( p, start, MBEDCRYPTO_ASN1_OCTET_STRING ) );
+
+    return( (int) len );
+}
+
+mbedcrypto_asn1_named_data *mbedcrypto_asn1_store_named_data( mbedcrypto_asn1_named_data **head,
+                                        const char *oid, size_t oid_len,
+                                        const unsigned char *val,
+                                        size_t val_len )
+{
+    mbedcrypto_asn1_named_data *cur;
+
+    if( ( cur = mbedcrypto_asn1_find_named_data( *head, oid, oid_len ) ) == NULL )
+    {
+        // Add new entry if not present yet based on OID
+        //
+        cur = (mbedcrypto_asn1_named_data*)mbedcrypto_calloc( 1,
+                                            sizeof(mbedcrypto_asn1_named_data) );
+        if( cur == NULL )
+            return( NULL );
+
+        cur->oid.len = oid_len;
+        cur->oid.p = mbedcrypto_calloc( 1, oid_len );
+        if( cur->oid.p == NULL )
+        {
+            mbedcrypto_free( cur );
+            return( NULL );
+        }
+
+        memcpy( cur->oid.p, oid, oid_len );
+
+        cur->val.len = val_len;
+        cur->val.p = mbedcrypto_calloc( 1, val_len );
+        if( cur->val.p == NULL )
+        {
+            mbedcrypto_free( cur->oid.p );
+            mbedcrypto_free( cur );
+            return( NULL );
+        }
+
+        cur->next = *head;
+        *head = cur;
+    }
+    else if( cur->val.len < val_len )
+    {
+        /*
+         * Enlarge existing value buffer if needed
+         * Preserve old data until the allocation succeeded, to leave list in
+         * a consistent state in case allocation fails.
+         */
+        void *p = mbedcrypto_calloc( 1, val_len );
+        if( p == NULL )
+            return( NULL );
+
+        mbedcrypto_free( cur->val.p );
+        cur->val.p = p;
+        cur->val.len = val_len;
+    }
+
+    if( val != NULL )
+        memcpy( cur->val.p, val, val_len );
+
+    return( cur );
+}
+#endif /* MBEDCRYPTO_ASN1_WRITE_C */