library/x509write_crt.c

/*
 *  X.509 certificate writing
 *
 *  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.
 */
/*
 * References:
 * - certificates: RFC 5280, updated by RFC 6818
 * - CSRs: PKCS#10 v1.7 aka RFC 2986
 * - attributes: PKCS#9 v2.0 aka RFC 2985
 */

#include "common.h"

#if defined(MBEDTLS_X509_CRT_WRITE_C)

#include "mbedtls/x509_crt.h"
#include "mbedtls/asn1write.h"
#include "mbedtls/error.h"
#include "mbedtls/oid.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/sha1.h"

#include <string.h>

#if defined(MBEDTLS_PEM_WRITE_C)
#include "mbedtls/pem.h"
#endif /* MBEDTLS_PEM_WRITE_C */

#if defined(MBEDTLS_USE_PSA_CRYPTO)
#include "psa/crypto.h"
#include "mbedtls/psa_util.h"
#include "hash_info.h"
#endif /* MBEDTLS_USE_PSA_CRYPTO */

#include "legacy_or_psa.h"

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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    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 ),
                             is_ca, buf + sizeof(buf) - len, len ) );
}

#if defined(MBEDTLS_HAS_ALG_SHA_1_VIA_MD_OR_PSA_BASED_ON_USE_PSA)
static int mbedtls_x509write_crt_set_key_identifier( mbedtls_x509write_cert *ctx,
                                              int is_ca,
                                              unsigned char tag )
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char buf[MBEDTLS_MPI_MAX_SIZE * 2 + 20]; /* tag, length + 2xMPI */
    unsigned char *c = buf + sizeof(buf);
    size_t len = 0;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
    size_t hash_length;
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    memset( buf, 0, sizeof(buf) );
    MBEDTLS_ASN1_CHK_ADD( len,
                          mbedtls_pk_write_pubkey( &c,
                                                   buf,
                                                   is_ca ?
                                                   ctx->issuer_key :
                                                   ctx->subject_key ) );


#if defined(MBEDTLS_USE_PSA_CRYPTO)
    status = psa_hash_compute( PSA_ALG_SHA_1,
                               buf + sizeof(buf) - len,
                               len,
                               buf + sizeof(buf) - 20,
                               20,
                               &hash_length );
    if( status != PSA_SUCCESS )
    {
        return( MBEDTLS_ERR_PLATFORM_HW_ACCEL_FAILED );
    }
#else
    ret = mbedtls_sha1( buf + sizeof( buf ) - len, len,
                        buf + sizeof( buf ) - 20 );
    if( ret != 0 )
        return( ret );
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    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, tag ) );

    if( is_ca ) // writes AuthorityKeyIdentifier sequence
    {
        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 ) );
    }

    if( is_ca )
        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 ) );
    else
        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_subject_key_identifier( mbedtls_x509write_cert *ctx )
{
    return mbedtls_x509write_crt_set_key_identifier( ctx,
                                                     0,
                                                     MBEDTLS_ASN1_OCTET_STRING );
}

int mbedtls_x509write_crt_set_authority_key_identifier( mbedtls_x509write_cert *ctx )
{
    return mbedtls_x509write_crt_set_key_identifier( ctx,
                                                     1,
                                                     (MBEDTLS_ASN1_CONTEXT_SPECIFIC | 0) );
}
#endif /* MBEDTLS_HAS_ALG_SHA_1_VIA_MD_OR_PSA_BASED_ON_USE_PSA */

int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
                                         unsigned int key_usage )
{
    unsigned char buf[5] = {0}, ku[2] = {0};
    unsigned char *c;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    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;
    MBEDTLS_PUT_UINT16_LE( key_usage, ku, 0 );
    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_ext_key_usage( mbedtls_x509write_cert *ctx,
                                             const mbedtls_asn1_sequence *exts )
{
    unsigned char buf[256];
    unsigned char *c = buf + sizeof(buf);
    int ret;
    size_t len = 0;
    const mbedtls_asn1_sequence *last_ext = 0, *ext;

    /* We need at least one extension: SEQUENCE SIZE (1..MAX) OF KeyPurposeId */
    if( exts == NULL )
        return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );

    /* Iterate over exts backwards, so we write them out in the requested order */
    while( last_ext != exts )
    {
        for( ext = exts; ext->next != last_ext; ext = ext->next ) {}
        if( ext->buf.tag != MBEDTLS_ASN1_OID )
            return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_raw_buffer( &c, buf, ext->buf.p, ext->buf.len ) );
        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, ext->buf.len ) );
        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_OID ) );
        last_ext = ext;
    }

    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 ) );

    ret = mbedtls_x509write_crt_set_extension( ctx,
                         MBEDTLS_OID_EXTENDED_KEY_USAGE,
                         MBEDTLS_OID_SIZE( MBEDTLS_OID_EXTENDED_KEY_USAGE ),
                         1, c, len );
    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] = {0};
    unsigned char *c;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t len = 0;

    /*
     * write MBEDTLS_ASN1_UTC_TIME if year < 2050 (2 bytes shorter)
     */
    if( t[0] < '2' || ( 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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    const char *sig_oid;
    size_t sig_oid_len = 0;
    unsigned char *c, *c2;
    unsigned char sig[MBEDTLS_PK_SIGNATURE_MAX_SIZE];
    size_t hash_length = 0;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
    psa_algorithm_t psa_algorithm;
    unsigned char hash[PSA_HASH_MAX_SIZE];
#else
    unsigned char hash[64];
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    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 at the end of the target buffer
     */
    c = buf + size;

    /* 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,
                                                      buf, ctx->extensions ) );
        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 ) );
        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 |
                                               MBEDTLS_ASN1_CONSTRUCTED | 3 ) );
    }

    /*
     *  SubjectPublicKeyInfo
     */
    MBEDTLS_ASN1_CHK_ADD( pub_len,
                          mbedtls_pk_write_pubkey_der( ctx->subject_key,
                                                       buf, c - buf ) );
    c -= pub_len;
    len += pub_len;

    /*
     *  Subject  ::=  Name
     */
    MBEDTLS_ASN1_CHK_ADD( len,
                          mbedtls_x509_write_names( &c, buf,
                                                    ctx->subject ) );

    /*
     *  Validity ::= SEQUENCE {
     *       notBefore      Time,
     *       notAfter       Time }
     */
    sub_len = 0;

    MBEDTLS_ASN1_CHK_ADD( sub_len,
                          x509_write_time( &c, buf, ctx->not_after,
                                        MBEDTLS_X509_RFC5280_UTC_TIME_LEN ) );

    MBEDTLS_ASN1_CHK_ADD( sub_len,
                          x509_write_time( &c, buf, ctx->not_before,
                                        MBEDTLS_X509_RFC5280_UTC_TIME_LEN ) );

    len += sub_len;
    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, sub_len ) );
    MBEDTLS_ASN1_CHK_ADD( len,
                          mbedtls_asn1_write_tag( &c, buf,
                                                  MBEDTLS_ASN1_CONSTRUCTED |
                                                  MBEDTLS_ASN1_SEQUENCE ) );

    /*
     *  Issuer  ::=  Name
     */
    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_names( &c, buf,
                                                         ctx->issuer ) );

    /*
     *  Signature   ::=  AlgorithmIdentifier
     */
    MBEDTLS_ASN1_CHK_ADD( len,
                          mbedtls_asn1_write_algorithm_identifier( &c, buf,
                                              sig_oid, strlen( sig_oid ), 0 ) );

    /*
     *  Serial   ::=  INTEGER
     */
    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_mpi( &c, 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, buf, ctx->version ) );
        len += sub_len;
        MBEDTLS_ASN1_CHK_ADD( len,
                              mbedtls_asn1_write_len( &c, buf, sub_len ) );
        MBEDTLS_ASN1_CHK_ADD( len,
                              mbedtls_asn1_write_tag( &c, buf,
                                               MBEDTLS_ASN1_CONTEXT_SPECIFIC |
                                               MBEDTLS_ASN1_CONSTRUCTED | 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 ) );

    /*
     * Make signature
     */

    /* Compute hash of CRT. */
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    psa_algorithm = mbedtls_hash_info_psa_from_md( ctx->md_alg );

    status = psa_hash_compute( psa_algorithm,
                               c,
                               len,
                               hash,
                               sizeof( hash ),
                               &hash_length );
    if( status != PSA_SUCCESS )
    {
        return( MBEDTLS_ERR_PLATFORM_HW_ACCEL_FAILED );
    }
#else
    if( ( ret = mbedtls_md( mbedtls_md_info_from_type( ctx->md_alg ), c,
                            len, hash ) ) != 0 )
    {
        return( ret );
    }
#endif /* MBEDTLS_USE_PSA_CRYPTO */


    if( ( ret = mbedtls_pk_sign( ctx->issuer_key, ctx->md_alg,
                                 hash, hash_length, sig, sizeof( sig ), &sig_len,
                                 f_rng, p_rng ) ) != 0 )
    {
        return( ret );
    }

    /* Move CRT to the front of the buffer to have space
     * for the signature. */
    memmove( buf, c, len );
    c = buf + len;

    /* Add signature at the end of the buffer,
     * making sure that it doesn't underflow
     * into the CRT buffer. */
    c2 = buf + size;
    MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2, c,
                                        sig_oid, sig_oid_len, sig, sig_len ) );

    /*
     * Memory layout after this step:
     *
     * buf       c=buf+len                c2            buf+size
     * [CRT0,...,CRTn, UNUSED, ..., UNUSED, SIG0, ..., SIGm]
     */

    /* Move raw CRT to just before the signature. */
    c = c2 - len;
    memmove( c, buf, len );

    len += sig_and_oid_len;
    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( (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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t olen;

    if( ( ret = mbedtls_x509write_crt_der( crt, buf, size,
                                   f_rng, p_rng ) ) < 0 )
    {
        return( ret );
    }

    if( ( ret = mbedtls_pem_write_buffer( PEM_BEGIN_CRT, PEM_END_CRT,
                                          buf + size - ret, ret,
                                          buf, size, &olen ) ) != 0 )
    {
        return( ret );
    }

    return( 0 );
}
#endif /* MBEDTLS_PEM_WRITE_C */

#endif /* MBEDTLS_X509_CRT_WRITE_C */