diff --git a/include/mbedcrypto/asn1write.h b/include/mbedcrypto/asn1write.h
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+/**
+ * \file asn1write.h
+ *
+ * \brief 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)
+ */
+#ifndef MBEDCRYPTO_ASN1_WRITE_H
+#define MBEDCRYPTO_ASN1_WRITE_H
+
+#include "asn1.h"
+
+#define MBEDCRYPTO_ASN1_CHK_ADD(g, f) do { if( ( ret = f ) < 0 ) return( ret ); else   \
+                                g += ret; } while( 0 )
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \brief           Write a length field in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param len       the length to write
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_len( unsigned char **p, unsigned char *start, size_t len );
+
+/**
+ * \brief           Write a ASN.1 tag in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param tag       the tag to write
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_tag( unsigned char **p, unsigned char *start,
+                    unsigned char tag );
+
+/**
+ * \brief           Write raw buffer data
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param buf       data buffer to write
+ * \param size      length of the data buffer
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
+                           const unsigned char *buf, size_t size );
+
+#if defined(MBEDCRYPTO_BIGNUM_C)
+/**
+ * \brief           Write a big number (MBEDCRYPTO_ASN1_INTEGER) in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param X         the MPI to write
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_mpi( unsigned char **p, unsigned char *start, const mbedcrypto_mpi *X );
+#endif /* MBEDCRYPTO_BIGNUM_C */
+
+/**
+ * \brief           Write a NULL tag (MBEDCRYPTO_ASN1_NULL) with zero data in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_null( unsigned char **p, unsigned char *start );
+
+/**
+ * \brief           Write an OID tag (MBEDCRYPTO_ASN1_OID) and data in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param oid       the OID to write
+ * \param oid_len   length of the OID
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_oid( unsigned char **p, unsigned char *start,
+                    const char *oid, size_t oid_len );
+
+/**
+ * \brief           Write an AlgorithmIdentifier sequence in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param oid       the OID of the algorithm
+ * \param oid_len   length of the OID
+ * \param par_len   length of parameters, which must be already written.
+ *                  If 0, NULL parameters are added
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start,
+                                     const char *oid, size_t oid_len,
+                                     size_t par_len );
+
+/**
+ * \brief           Write a boolean tag (MBEDCRYPTO_ASN1_BOOLEAN) and value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param boolean   0 or 1
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_bool( unsigned char **p, unsigned char *start, int boolean );
+
+/**
+ * \brief           Write an int tag (MBEDCRYPTO_ASN1_INTEGER) and value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param val       the integer value
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_int( unsigned char **p, unsigned char *start, int val );
+
+/**
+ * \brief           Write a printable string tag (MBEDCRYPTO_ASN1_PRINTABLE_STRING) and
+ *                  value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param text      the text to write
+ * \param text_len  length of the text
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_printable_string( unsigned char **p, unsigned char *start,
+                                 const char *text, size_t text_len );
+
+/**
+ * \brief           Write an IA5 string tag (MBEDCRYPTO_ASN1_IA5_STRING) and
+ *                  value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param text      the text to write
+ * \param text_len  length of the text
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
+                           const char *text, size_t text_len );
+
+/**
+ * \brief           Write a bitstring tag (MBEDCRYPTO_ASN1_BIT_STRING) and
+ *                  value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param buf       the bitstring
+ * \param bits      the total number of bits in the bitstring
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_bitstring( unsigned char **p, unsigned char *start,
+                          const unsigned char *buf, size_t bits );
+
+/**
+ * \brief           Write an octet string tag (MBEDCRYPTO_ASN1_OCTET_STRING) and
+ *                  value in ASN.1 format
+ *                  Note: function works backwards in data buffer
+ *
+ * \param p         reference to current position pointer
+ * \param start     start of the buffer (for bounds-checking)
+ * \param buf       data buffer to write
+ * \param size      length of the data buffer
+ *
+ * \return          the length written or a negative error code
+ */
+int mbedcrypto_asn1_write_octet_string( unsigned char **p, unsigned char *start,
+                             const unsigned char *buf, size_t size );
+
+/**
+ * \brief           Create or find a specific named_data entry for writing in a
+ *                  sequence or list based on the OID. If not already in there,
+ *                  a new entry is added to the head of the list.
+ *                  Warning: Destructive behaviour for the val data!
+ *
+ * \param list      Pointer to the location of the head of the list to seek
+ *                  through (will be updated in case of a new entry)
+ * \param oid       The OID to look for
+ * \param oid_len   Size of the OID
+ * \param val       Data to store (can be NULL if you want to fill it by hand)
+ * \param val_len   Minimum length of the data buffer needed
+ *
+ * \return      NULL if if there was a memory allocation error, or a pointer
+ *              to the new / existing entry.
+ */
+mbedcrypto_asn1_named_data *mbedcrypto_asn1_store_named_data( mbedcrypto_asn1_named_data **list,
+                                        const char *oid, size_t oid_len,
+                                        const unsigned char *val,
+                                        size_t val_len );
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* MBEDCRYPTO_ASN1_WRITE_H */