diff --git a/library/x509write_crt.c b/library/x509write_crt.c
new file mode 100644
index 0000000..b6cb745
--- /dev/null
+++ b/library/x509write_crt.c
@@ -0,0 +1,495 @@
+/*
+ *  X.509 certificate writing
+ *
+ *  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 TLS (https://tls.mbed.org)
+ */
+/*
+ * References:
+ * - certificates: RFC 5280, updated by RFC 6818
+ * - CSRs: PKCS#10 v1.7 aka RFC 2986
+ * - attributes: PKCS#9 v2.0 aka RFC 2985
+ */
+
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#if defined(MBEDTLS_X509_CRT_WRITE_C)
+
+#include "mbedtls/x509_crt.h"
+#include "mbedtls/oid.h"
+#include "mbedtls/asn1write.h"
+#include "mbedtls/sha1.h"
+#include "mbedtls/platform_util.h"
+
+#include <string.h>
+
+#if defined(MBEDTLS_PEM_WRITE_C)
+#include "mbedtls/pem.h"
+#endif /* MBEDTLS_PEM_WRITE_C */
+
+void mbedtls_x509write_crt_init( mbedtls_x509write_cert *ctx )
+{
+    memset( ctx, 0, sizeof( mbedtls_x509write_cert ) );
+
+    mbedtls_mpi_init( &ctx->serial );
+    ctx->version = MBEDTLS_X509_CRT_VERSION_3;
+}
+
+void mbedtls_x509write_crt_free( mbedtls_x509write_cert *ctx )
+{
+    mbedtls_mpi_free( &ctx->serial );
+
+    mbedtls_asn1_free_named_data_list( &ctx->subject );
+    mbedtls_asn1_free_named_data_list( &ctx->issuer );
+    mbedtls_asn1_free_named_data_list( &ctx->extensions );
+
+    mbedtls_platform_zeroize( ctx, sizeof( mbedtls_x509write_cert ) );
+}
+
+void mbedtls_x509write_crt_set_version( mbedtls_x509write_cert *ctx, int version )
+{
+    ctx->version = version;
+}
+
+void mbedtls_x509write_crt_set_md_alg( mbedtls_x509write_cert *ctx, mbedtls_md_type_t md_alg )
+{
+    ctx->md_alg = md_alg;
+}
+
+void mbedtls_x509write_crt_set_subject_key( mbedtls_x509write_cert *ctx, mbedtls_pk_context *key )
+{
+    ctx->subject_key = key;
+}
+
+void mbedtls_x509write_crt_set_issuer_key( mbedtls_x509write_cert *ctx, mbedtls_pk_context *key )
+{
+    ctx->issuer_key = key;
+}
+
+int mbedtls_x509write_crt_set_subject_name( mbedtls_x509write_cert *ctx,
+                                    const char *subject_name )
+{
+    return mbedtls_x509_string_to_names( &ctx->subject, subject_name );
+}
+
+int mbedtls_x509write_crt_set_issuer_name( mbedtls_x509write_cert *ctx,
+                                   const char *issuer_name )
+{
+    return mbedtls_x509_string_to_names( &ctx->issuer, issuer_name );
+}
+
+int mbedtls_x509write_crt_set_serial( mbedtls_x509write_cert *ctx, const mbedtls_mpi *serial )
+{
+    int ret;
+
+    if( ( ret = mbedtls_mpi_copy( &ctx->serial, serial ) ) != 0 )
+        return( ret );
+
+    return( 0 );
+}
+
+int mbedtls_x509write_crt_set_validity( mbedtls_x509write_cert *ctx, const char *not_before,
+                                const char *not_after )
+{
+    if( strlen( not_before ) != MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1 ||
+        strlen( not_after )  != MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1 )
+    {
+        return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
+    }
+    strncpy( ctx->not_before, not_before, MBEDTLS_X509_RFC5280_UTC_TIME_LEN );
+    strncpy( ctx->not_after , not_after , MBEDTLS_X509_RFC5280_UTC_TIME_LEN );
+    ctx->not_before[MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1] = 'Z';
+    ctx->not_after[MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1] = 'Z';
+
+    return( 0 );
+}
+
+int mbedtls_x509write_crt_set_extension( mbedtls_x509write_cert *ctx,
+                                 const char *oid, size_t oid_len,
+                                 int critical,
+                                 const unsigned char *val, size_t val_len )
+{
+    return mbedtls_x509_set_extension( &ctx->extensions, oid, oid_len,
+                               critical, val, val_len );
+}
+
+int mbedtls_x509write_crt_set_basic_constraints( mbedtls_x509write_cert *ctx,
+                                         int is_ca, int max_pathlen )
+{
+    int ret;
+    unsigned char buf[9];
+    unsigned char *c = buf + sizeof(buf);
+    size_t len = 0;
+
+    memset( buf, 0, sizeof(buf) );
+
+    if( is_ca && max_pathlen > 127 )
+        return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
+
+    if( is_ca )
+    {
+        if( max_pathlen >= 0 )
+        {
+            MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, buf, max_pathlen ) );
+        }
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_bool( &c, buf, 1 ) );
+    }
+
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                MBEDTLS_ASN1_SEQUENCE ) );
+
+    return mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_BASIC_CONSTRAINTS,
+                                        MBEDTLS_OID_SIZE( MBEDTLS_OID_BASIC_CONSTRAINTS ),
+                                        0, buf + sizeof(buf) - len, len );
+}
+
+#if defined(MBEDTLS_SHA1_C)
+int mbedtls_x509write_crt_set_subject_key_identifier( mbedtls_x509write_cert *ctx )
+{
+    int ret;
+    unsigned char buf[MBEDTLS_MPI_MAX_SIZE * 2 + 20]; /* tag, length + 2xMPI */
+    unsigned char *c = buf + sizeof(buf);
+    size_t len = 0;
+
+    memset( buf, 0, sizeof(buf) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_pk_write_pubkey( &c, buf, ctx->subject_key ) );
+
+    ret = mbedtls_sha1_ret( buf + sizeof( buf ) - len, len,
+                            buf + sizeof( buf ) - 20 );
+    if( ret != 0 )
+        return( ret );
+    c = buf + sizeof( buf ) - 20;
+    len = 20;
+
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_OCTET_STRING ) );
+
+    return mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_SUBJECT_KEY_IDENTIFIER,
+                                        MBEDTLS_OID_SIZE( MBEDTLS_OID_SUBJECT_KEY_IDENTIFIER ),
+                                        0, buf + sizeof(buf) - len, len );
+}
+
+int mbedtls_x509write_crt_set_authority_key_identifier( mbedtls_x509write_cert *ctx )
+{
+    int ret;
+    unsigned char buf[MBEDTLS_MPI_MAX_SIZE * 2 + 20]; /* tag, length + 2xMPI */
+    unsigned char *c = buf + sizeof( buf );
+    size_t len = 0;
+
+    memset( buf, 0, sizeof(buf) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_pk_write_pubkey( &c, buf, ctx->issuer_key ) );
+
+    ret = mbedtls_sha1_ret( buf + sizeof( buf ) - len, len,
+                            buf + sizeof( buf ) - 20 );
+    if( ret != 0 )
+        return( ret );
+    c = buf + sizeof( buf ) - 20;
+    len = 20;
+
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_CONTEXT_SPECIFIC | 0 ) );
+
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                MBEDTLS_ASN1_SEQUENCE ) );
+
+    return mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_AUTHORITY_KEY_IDENTIFIER,
+                                   MBEDTLS_OID_SIZE( MBEDTLS_OID_AUTHORITY_KEY_IDENTIFIER ),
+                                   0, buf + sizeof( buf ) - len, len );
+}
+#endif /* MBEDTLS_SHA1_C */
+
+int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
+                                         unsigned int key_usage )
+{
+    unsigned char buf[5], ku[2];
+    unsigned char *c;
+    int ret;
+    const unsigned int allowed_bits = MBEDTLS_X509_KU_DIGITAL_SIGNATURE |
+        MBEDTLS_X509_KU_NON_REPUDIATION   |
+        MBEDTLS_X509_KU_KEY_ENCIPHERMENT  |
+        MBEDTLS_X509_KU_DATA_ENCIPHERMENT |
+        MBEDTLS_X509_KU_KEY_AGREEMENT     |
+        MBEDTLS_X509_KU_KEY_CERT_SIGN     |
+        MBEDTLS_X509_KU_CRL_SIGN          |
+        MBEDTLS_X509_KU_ENCIPHER_ONLY     |
+        MBEDTLS_X509_KU_DECIPHER_ONLY;
+
+    /* Check that nothing other than the allowed flags is set */
+    if( ( key_usage & ~allowed_bits ) != 0 )
+        return( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE );
+
+    c = buf + 5;
+    ku[0] = (unsigned char)( key_usage      );
+    ku[1] = (unsigned char)( key_usage >> 8 );
+    ret = mbedtls_asn1_write_named_bitstring( &c, buf, ku, 9 );
+
+    if( ret < 0 )
+        return( ret );
+    else if( ret < 3 || ret > 5 )
+        return( MBEDTLS_ERR_X509_INVALID_FORMAT );
+
+    ret = mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_KEY_USAGE,
+                                       MBEDTLS_OID_SIZE( MBEDTLS_OID_KEY_USAGE ),
+                                       1, c, (size_t)ret );
+    if( ret != 0 )
+        return( ret );
+
+    return( 0 );
+}
+
+int mbedtls_x509write_crt_set_ns_cert_type( mbedtls_x509write_cert *ctx,
+                                    unsigned char ns_cert_type )
+{
+    unsigned char buf[4];
+    unsigned char *c;
+    int ret;
+
+    c = buf + 4;
+
+    ret = mbedtls_asn1_write_named_bitstring( &c, buf, &ns_cert_type, 8 );
+    if( ret < 3 || ret > 4 )
+        return( ret );
+
+    ret = mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_NS_CERT_TYPE,
+                                       MBEDTLS_OID_SIZE( MBEDTLS_OID_NS_CERT_TYPE ),
+                                       0, c, (size_t)ret );
+    if( ret != 0 )
+        return( ret );
+
+    return( 0 );
+}
+
+static int x509_write_time( unsigned char **p, unsigned char *start,
+                            const char *t, size_t size )
+{
+    int ret;
+    size_t len = 0;
+
+    /*
+     * write MBEDTLS_ASN1_UTC_TIME if year < 2050 (2 bytes shorter)
+     */
+    if( t[0] == '2' && t[1] == '0' && t[2] < '5' )
+    {
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_raw_buffer( p, start,
+                                             (const unsigned char *) t + 2,
+                                             size - 2 ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( p, start, len ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( p, start, MBEDTLS_ASN1_UTC_TIME ) );
+    }
+    else
+    {
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_raw_buffer( p, start,
+                                                  (const unsigned char *) t,
+                                                  size ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( p, start, len ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( p, start, MBEDTLS_ASN1_GENERALIZED_TIME ) );
+    }
+
+    return( (int) len );
+}
+
+int mbedtls_x509write_crt_der( mbedtls_x509write_cert *ctx, unsigned char *buf, size_t size,
+                       int (*f_rng)(void *, unsigned char *, size_t),
+                       void *p_rng )
+{
+    int ret;
+    const char *sig_oid;
+    size_t sig_oid_len = 0;
+    unsigned char *c, *c2;
+    unsigned char hash[64];
+    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char tmp_buf[2048];
+    size_t sub_len = 0, pub_len = 0, sig_and_oid_len = 0, sig_len;
+    size_t len = 0;
+    mbedtls_pk_type_t pk_alg;
+
+    /*
+     * Prepare data to be signed in tmp_buf
+     */
+    c = tmp_buf + sizeof( tmp_buf );
+
+    /* Signature algorithm needed in TBS, and later for actual signature */
+
+    /* There's no direct way of extracting a signature algorithm
+     * (represented as an element of mbedtls_pk_type_t) from a PK instance. */
+    if( mbedtls_pk_can_do( ctx->issuer_key, MBEDTLS_PK_RSA ) )
+        pk_alg = MBEDTLS_PK_RSA;
+    else if( mbedtls_pk_can_do( ctx->issuer_key, MBEDTLS_PK_ECDSA ) )
+        pk_alg = MBEDTLS_PK_ECDSA;
+    else
+        return( MBEDTLS_ERR_X509_INVALID_ALG );
+
+    if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
+                                          &sig_oid, &sig_oid_len ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    /*
+     *  Extensions  ::=  SEQUENCE SIZE (1..MAX) OF Extension
+     */
+
+    /* Only for v3 */
+    if( ctx->version == MBEDTLS_X509_CRT_VERSION_3 )
+    {
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_extensions( &c, tmp_buf, ctx->extensions ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                           MBEDTLS_ASN1_SEQUENCE ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONTEXT_SPECIFIC |
+                                                           MBEDTLS_ASN1_CONSTRUCTED | 3 ) );
+    }
+
+    /*
+     *  SubjectPublicKeyInfo
+     */
+    MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_pk_write_pubkey_der( ctx->subject_key,
+                                                tmp_buf, c - tmp_buf ) );
+    c -= pub_len;
+    len += pub_len;
+
+    /*
+     *  Subject  ::=  Name
+     */
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_names( &c, tmp_buf, ctx->subject ) );
+
+    /*
+     *  Validity ::= SEQUENCE {
+     *       notBefore      Time,
+     *       notAfter       Time }
+     */
+    sub_len = 0;
+
+    MBEDTLS_ASN1_CHK_ADD( sub_len, x509_write_time( &c, tmp_buf, ctx->not_after,
+                                            MBEDTLS_X509_RFC5280_UTC_TIME_LEN ) );
+
+    MBEDTLS_ASN1_CHK_ADD( sub_len, x509_write_time( &c, tmp_buf, ctx->not_before,
+                                            MBEDTLS_X509_RFC5280_UTC_TIME_LEN ) );
+
+    len += sub_len;
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, sub_len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                    MBEDTLS_ASN1_SEQUENCE ) );
+
+    /*
+     *  Issuer  ::=  Name
+     */
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_names( &c, tmp_buf, ctx->issuer ) );
+
+    /*
+     *  Signature   ::=  AlgorithmIdentifier
+     */
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_algorithm_identifier( &c, tmp_buf,
+                       sig_oid, strlen( sig_oid ), 0 ) );
+
+    /*
+     *  Serial   ::=  INTEGER
+     */
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_mpi( &c, tmp_buf, &ctx->serial ) );
+
+    /*
+     *  Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }
+     */
+
+    /* Can be omitted for v1 */
+    if( ctx->version != MBEDTLS_X509_CRT_VERSION_1 )
+    {
+        sub_len = 0;
+        MBEDTLS_ASN1_CHK_ADD( sub_len, mbedtls_asn1_write_int( &c, tmp_buf, ctx->version ) );
+        len += sub_len;
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, sub_len ) );
+        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONTEXT_SPECIFIC |
+                                                           MBEDTLS_ASN1_CONSTRUCTED | 0 ) );
+    }
+
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                       MBEDTLS_ASN1_SEQUENCE ) );
+
+    /*
+     * Make signature
+     */
+    if( ( ret = mbedtls_md( mbedtls_md_info_from_type( ctx->md_alg ), c,
+                            len, hash ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    if( ( ret = mbedtls_pk_sign( ctx->issuer_key, ctx->md_alg, hash, 0, sig, &sig_len,
+                         f_rng, p_rng ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    /*
+     * Write data to output buffer
+     */
+    c2 = buf + size;
+    MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2, buf,
+                                        sig_oid, sig_oid_len, sig, sig_len ) );
+
+    if( len > (size_t)( c2 - buf ) )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+    c2 -= len;
+    memcpy( c2, c, len );
+
+    len += sig_and_oid_len;
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c2, buf, len ) );
+    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c2, buf, MBEDTLS_ASN1_CONSTRUCTED |
+                                                 MBEDTLS_ASN1_SEQUENCE ) );
+
+    return( (int) len );
+}
+
+#define PEM_BEGIN_CRT           "-----BEGIN CERTIFICATE-----\n"
+#define PEM_END_CRT             "-----END CERTIFICATE-----\n"
+
+#if defined(MBEDTLS_PEM_WRITE_C)
+int mbedtls_x509write_crt_pem( mbedtls_x509write_cert *crt, unsigned char *buf, size_t size,
+                       int (*f_rng)(void *, unsigned char *, size_t),
+                       void *p_rng )
+{
+    int ret;
+    unsigned char output_buf[4096];
+    size_t olen = 0;
+
+    if( ( ret = mbedtls_x509write_crt_der( crt, output_buf, sizeof(output_buf),
+                                   f_rng, p_rng ) ) < 0 )
+    {
+        return( ret );
+    }
+
+    if( ( ret = mbedtls_pem_write_buffer( PEM_BEGIN_CRT, PEM_END_CRT,
+                                  output_buf + sizeof(output_buf) - ret,
+                                  ret, buf, size, &olen ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    return( 0 );
+}
+#endif /* MBEDTLS_PEM_WRITE_C */
+
+#endif /* MBEDTLS_X509_CRT_WRITE_C */