cosmopolitan/third_party/mbedtls/pkwrite.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:4;tab-width:4;coding:utf-8 -*-│
│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright The Mbed TLS Contributors                                          │
│                                                                              │
│ 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 "third_party/mbedtls/asn1write.h"
#include "third_party/mbedtls/bignum.h"
#include "third_party/mbedtls/common.h"
#include "third_party/mbedtls/ecdsa.h"
#include "third_party/mbedtls/ecp.h"
#include "third_party/mbedtls/error.h"
#include "third_party/mbedtls/oid.h"
#include "third_party/mbedtls/pem.h"
#include "third_party/mbedtls/pk.h"
#include "third_party/mbedtls/platform.h"
#include "third_party/mbedtls/rsa.h"
__static_yoink("mbedtls_notice");

/*
 *  Public Key layer for writing key files and structures
 *
 *  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.
 */

#if defined(MBEDTLS_PK_WRITE_C)

/* Parameter validation macros based on platform_util.h */
#define PK_VALIDATE_RET( cond )    \
    MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_PK_BAD_INPUT_DATA )
#define PK_VALIDATE( cond )        \
    MBEDTLS_INTERNAL_VALIDATE( cond )

#if defined(MBEDTLS_RSA_C)
/*
 *  RSAPublicKey ::= SEQUENCE {
 *      modulus           INTEGER,  -- n
 *      publicExponent    INTEGER   -- e
 *  }
 */
static int pk_write_rsa_pubkey( unsigned char **p, unsigned char *start,
                                mbedtls_rsa_context *rsa )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    size_t len = 0;
    mbedtls_mpi T;

    mbedtls_mpi_init( &T );

    /* Export E */
    if ( ( ret = mbedtls_rsa_export( rsa, NULL, NULL, NULL, NULL, &T ) ) != 0 ||
         ( ret = mbedtls_asn1_write_mpi( p, start, &T ) ) < 0 )
        goto end_of_export;
    len += ret;

    /* Export N */
    if ( ( ret = mbedtls_rsa_export( rsa, &T, NULL, NULL, NULL, NULL ) ) != 0 ||
         ( ret = mbedtls_asn1_write_mpi( p, start, &T ) ) < 0 )
        goto end_of_export;
    len += ret;

end_of_export:

    mbedtls_mpi_free( &T );
    if( ret < 0 )
        return( ret );

    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_CONSTRUCTED |
                                                 MBEDTLS_ASN1_SEQUENCE ) );

    return( (int) len );
}
#endif /* MBEDTLS_RSA_C */

#if defined(MBEDTLS_ECP_C)
/*
 * EC public key is an EC point
 */
static int pk_write_ec_pubkey( unsigned char **p, unsigned char *start,
                               mbedtls_ecp_keypair *ec )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    size_t len = 0;
    unsigned char buf[MBEDTLS_ECP_MAX_PT_LEN];

    if( ( ret = mbedtls_ecp_point_write_binary( &ec->grp, &ec->Q,
                                        MBEDTLS_ECP_PF_UNCOMPRESSED,
                                        &len, buf, sizeof( buf ) ) ) != 0 )
    {
        return( ret );
    }

    if( *p < start || (size_t)( *p - start ) < len )
        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );

    *p -= len;
    memcpy( *p, buf, len );

    return( (int) len );
}

/*
 * ECParameters ::= CHOICE {
 *   namedCurve         OBJECT IDENTIFIER
 * }
 */
static int pk_write_ec_param( unsigned char **p, unsigned char *start,
                              mbedtls_ecp_keypair *ec )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    size_t len = 0;
    const char *oid;
    size_t oid_len;

    if( ( ret = mbedtls_oid_get_oid_by_ec_grp( ec->grp.id, &oid, &oid_len ) ) != 0 )
        return( ret );

    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_oid( p, start, oid, oid_len ) );

    return( (int) len );
}

/*
 * privateKey  OCTET STRING -- always of length ceil(log2(n)/8)
 */
static int pk_write_ec_private( unsigned char **p, unsigned char *start,
                                mbedtls_ecp_keypair *ec )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    size_t byte_length = ( ec->grp.pbits + 7 ) / 8;
    unsigned char tmp[MBEDTLS_ECP_MAX_BYTES];

    ret = mbedtls_ecp_write_key( ec, tmp, byte_length );
    if( ret != 0 )
        goto exit;
    ret = mbedtls_asn1_write_octet_string( p, start, tmp, byte_length );

exit:
    mbedtls_platform_zeroize( tmp, byte_length );
    return( ret );
}
#endif /* MBEDTLS_ECP_C */

/**
 * \brief           Write a subjectPublicKey to ASN.1 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 key       PK context which must contain a valid public or private key.
 *
 * \return          the length written or a negative error code
 */
int mbedtls_pk_write_pubkey( unsigned char **p, unsigned char *start,
                             const mbedtls_pk_context *key )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    size_t len = 0;

    PK_VALIDATE_RET( p != NULL );
    PK_VALIDATE_RET( *p != NULL );
    PK_VALIDATE_RET( start != NULL );
    PK_VALIDATE_RET( key != NULL );

#if defined(MBEDTLS_RSA_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_RSA )
        MBEDTLS_ASN1_CHK_ADD( len, pk_write_rsa_pubkey( p, start, mbedtls_pk_rsa( *key ) ) );
    else
#endif
#if defined(MBEDTLS_ECP_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_ECKEY )
        MBEDTLS_ASN1_CHK_ADD( len, pk_write_ec_pubkey( p, start, mbedtls_pk_ec( *key ) ) );
    else
#endif
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_OPAQUE )
    {
        size_t buffer_size;
        psa_key_id_t* key_id = (psa_key_id_t*) key->pk_ctx;

        if ( *p < start )
            return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );

        buffer_size = (size_t)( *p - start );
        if ( psa_export_public_key( *key_id, start, buffer_size, &len )
             != PSA_SUCCESS )
        {
            return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
        }
        else
        {
            *p -= len;
            memmove( *p, start, len );
        }
    }
    else
#endif /* MBEDTLS_USE_PSA_CRYPTO */
        return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );

    return( (int) len );
}

/**
 * \brief           Write a public key to a SubjectPublicKeyInfo DER structure
 *                  Note: data is written at the end of the buffer! Use the
 *                        return value to determine where you should start
 *                        using the buffer
 *
 * \param ctx       PK context which must contain a valid public or private key.
 * \param buf       buffer to write to
 * \param size      size of the buffer
 *
 * \return          length of data written if successful, or a specific
 *                  error code
 */
int mbedtls_pk_write_pubkey_der( mbedtls_pk_context *key, unsigned char *buf, size_t size )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    unsigned char *c;
    size_t len = 0, par_len = 0, oid_len;
    mbedtls_pk_type_t pk_type;
    const char *oid;

    PK_VALIDATE_RET( key != NULL );
    if( size == 0 )
        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
    PK_VALIDATE_RET( buf != NULL );

    c = buf + size;

    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_pk_write_pubkey( &c, buf, key ) );

    if( c - buf < 1 )
        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );

    /*
     *  SubjectPublicKeyInfo  ::=  SEQUENCE  {
     *       algorithm            AlgorithmIdentifier,
     *       subjectPublicKey     BIT STRING }
     */
    *--c = 0;
    len += 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_BIT_STRING ) );

    pk_type = mbedtls_pk_get_type( key );
#if defined(MBEDTLS_ECP_C)
    if( pk_type == MBEDTLS_PK_ECKEY )
    {
        MBEDTLS_ASN1_CHK_ADD( par_len, pk_write_ec_param( &c, buf, mbedtls_pk_ec( *key ) ) );
    }
#endif
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    if( pk_type == MBEDTLS_PK_OPAQUE )
    {
        psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
        psa_key_type_t key_type;
        psa_key_id_t key_id;
        psa_ecc_family_t curve;
        size_t bits;

        key_id = *((psa_key_id_t*) key->pk_ctx );
        if( PSA_SUCCESS != psa_get_key_attributes( key_id, &attributes ) )
            return( MBEDTLS_ERR_PK_HW_ACCEL_FAILED );
        key_type = psa_get_key_type( &attributes );
        bits = psa_get_key_bits( &attributes );
        psa_reset_key_attributes( &attributes );

        curve = PSA_KEY_TYPE_ECC_GET_FAMILY( key_type );
        if( curve == 0 )
            return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );

        ret = mbedtls_psa_get_ecc_oid_from_id( curve, bits, &oid, &oid_len );
        if( ret != 0 )
            return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );

        /* Write EC algorithm parameters; that's akin
         * to pk_write_ec_param() above. */
        MBEDTLS_ASN1_CHK_ADD( par_len, mbedtls_asn1_write_oid( &c, buf,
                                                               oid, oid_len ) );

        /* The rest of the function works as for legacy EC contexts. */
        pk_type = MBEDTLS_PK_ECKEY;
    }
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    if( ( ret = mbedtls_oid_get_oid_by_pk_alg( pk_type, &oid,
                                               &oid_len ) ) != 0 )
    {
        return( ret );
    }

    MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_algorithm_identifier( &c, buf, oid, oid_len,
                                                        par_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 );
}

/**
 * \brief           Write a private key to a PKCS#1 or SEC1 DER structure
 *                  Note: data is written at the end of the buffer! Use the
 *                        return value to determine where you should start
 *                        using the buffer
 *
 * \param ctx       PK context which must contain a valid private key.
 * \param buf       buffer to write to
 * \param size      size of the buffer
 *
 * \return          length of data written if successful, or a specific
 *                  error code
 */
int mbedtls_pk_write_key_der( mbedtls_pk_context *key, unsigned char *buf, size_t size )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    unsigned char *c;
    size_t len = 0;

    PK_VALIDATE_RET( key != NULL );
    if( size == 0 )
        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
    PK_VALIDATE_RET( buf != NULL );

    c = buf + size;

#if defined(MBEDTLS_RSA_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_RSA )
    {
        mbedtls_mpi T; /* Temporary holding the exported parameters */
        mbedtls_rsa_context *rsa = mbedtls_pk_rsa( *key );

        /*
         * Export the parameters one after another to avoid simultaneous copies.
         */

        mbedtls_mpi_init( &T );

        /* Export QP */
        if( ( ret = mbedtls_rsa_export_crt( rsa, NULL, NULL, &T ) ) != 0 ||
            ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export DQ */
        if( ( ret = mbedtls_rsa_export_crt( rsa, NULL, &T, NULL ) ) != 0 ||
            ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export DP */
        if( ( ret = mbedtls_rsa_export_crt( rsa, &T, NULL, NULL ) ) != 0 ||
            ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export Q */
        if ( ( ret = mbedtls_rsa_export( rsa, NULL, NULL,
                                         &T, NULL, NULL ) ) != 0 ||
             ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export P */
        if ( ( ret = mbedtls_rsa_export( rsa, NULL, &T,
                                         NULL, NULL, NULL ) ) != 0 ||
             ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export D */
        if ( ( ret = mbedtls_rsa_export( rsa, NULL, NULL,
                                         NULL, &T, NULL ) ) != 0 ||
             ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export E */
        if ( ( ret = mbedtls_rsa_export( rsa, NULL, NULL,
                                         NULL, NULL, &T ) ) != 0 ||
             ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

        /* Export N */
        if ( ( ret = mbedtls_rsa_export( rsa, &T, NULL,
                                         NULL, NULL, NULL ) ) != 0 ||
             ( ret = mbedtls_asn1_write_mpi( &c, buf, &T ) ) < 0 )
            goto end_of_export;
        len += ret;

    end_of_export:

        mbedtls_mpi_free( &T );
        if( ret < 0 )
            return( ret );

        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, buf, 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 ) );
    }
    else
#endif /* MBEDTLS_RSA_C */
#if defined(MBEDTLS_ECP_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_ECKEY )
    {
        mbedtls_ecp_keypair *ec = mbedtls_pk_ec( *key );
        size_t pub_len = 0, par_len = 0;

        /*
         * RFC 5915, or SEC1 Appendix C.4
         *
         * ECPrivateKey ::= SEQUENCE {
         *      version        INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
         *      privateKey     OCTET STRING,
         *      parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
         *      publicKey  [1] BIT STRING OPTIONAL
         *    }
         */

        /* publicKey */
        MBEDTLS_ASN1_CHK_ADD( pub_len, pk_write_ec_pubkey( &c, buf, ec ) );

        if( c - buf < 1 )
            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
        *--c = 0;
        pub_len += 1;

        MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_asn1_write_len( &c, buf, pub_len ) );
        MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_asn1_write_tag( &c, buf, MBEDTLS_ASN1_BIT_STRING ) );

        MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_asn1_write_len( &c, buf, pub_len ) );
        MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_asn1_write_tag( &c, buf,
                            MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_CONSTRUCTED | 1 ) );
        len += pub_len;

        /* parameters */
        MBEDTLS_ASN1_CHK_ADD( par_len, pk_write_ec_param( &c, buf, ec ) );

        MBEDTLS_ASN1_CHK_ADD( par_len, mbedtls_asn1_write_len( &c, buf, par_len ) );
        MBEDTLS_ASN1_CHK_ADD( par_len, mbedtls_asn1_write_tag( &c, buf,
                            MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_CONSTRUCTED | 0 ) );
        len += par_len;

        /* privateKey */
        MBEDTLS_ASN1_CHK_ADD( len, pk_write_ec_private( &c, buf, ec ) );

        /* version */
        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &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 ) );
    }
    else
#endif /* MBEDTLS_ECP_C */
        return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );

    return( (int) len );
}

#if defined(MBEDTLS_PEM_WRITE_C)

#define PEM_BEGIN_PUBLIC_KEY    "-----BEGIN PUBLIC KEY-----\n"
#define PEM_END_PUBLIC_KEY      "-----END PUBLIC KEY-----\n"

#define PEM_BEGIN_PRIVATE_KEY_RSA   "-----BEGIN RSA PRIVATE KEY-----\n"
#define PEM_END_PRIVATE_KEY_RSA     "-----END RSA PRIVATE KEY-----\n"
#define PEM_BEGIN_PRIVATE_KEY_EC    "-----BEGIN EC PRIVATE KEY-----\n"
#define PEM_END_PRIVATE_KEY_EC      "-----END EC PRIVATE KEY-----\n"

/*
 * Max sizes of key per types. Shown as tag + len (+ content).
 */

#if defined(MBEDTLS_RSA_C)
/*
 * RSA public keys:
 *  SubjectPublicKeyInfo  ::=  SEQUENCE  {          1 + 3
 *       algorithm            AlgorithmIdentifier,  1 + 1 (sequence)
 *                                                + 1 + 1 + 9 (rsa oid)
 *                                                + 1 + 1 (params null)
 *       subjectPublicKey     BIT STRING }          1 + 3 + (1 + below)
 *  RSAPublicKey ::= SEQUENCE {                     1 + 3
 *      modulus           INTEGER,  -- n            1 + 3 + MPI_MAX + 1
 *      publicExponent    INTEGER   -- e            1 + 3 + MPI_MAX + 1
 *  }
 */
#define RSA_PUB_DER_MAX_BYTES   ( 38 + 2 * MBEDTLS_MPI_MAX_SIZE )

/*
 * RSA private keys:
 *  RSAPrivateKey ::= SEQUENCE {                    1 + 3
 *      version           Version,                  1 + 1 + 1
 *      modulus           INTEGER,                  1 + 3 + MPI_MAX + 1
 *      publicExponent    INTEGER,                  1 + 3 + MPI_MAX + 1
 *      privateExponent   INTEGER,                  1 + 3 + MPI_MAX + 1
 *      prime1            INTEGER,                  1 + 3 + MPI_MAX / 2 + 1
 *      prime2            INTEGER,                  1 + 3 + MPI_MAX / 2 + 1
 *      exponent1         INTEGER,                  1 + 3 + MPI_MAX / 2 + 1
 *      exponent2         INTEGER,                  1 + 3 + MPI_MAX / 2 + 1
 *      coefficient       INTEGER,                  1 + 3 + MPI_MAX / 2 + 1
 *      otherPrimeInfos   OtherPrimeInfos OPTIONAL  0 (not supported)
 *  }
 */
#define MPI_MAX_SIZE_2          ( MBEDTLS_MPI_MAX_SIZE / 2 + \
                                  MBEDTLS_MPI_MAX_SIZE % 2 )
#define RSA_PRV_DER_MAX_BYTES   ( 47 + 3 * MBEDTLS_MPI_MAX_SIZE \
                                   + 5 * MPI_MAX_SIZE_2 )

#else /* MBEDTLS_RSA_C */

#define RSA_PUB_DER_MAX_BYTES   0
#define RSA_PRV_DER_MAX_BYTES   0

#endif /* MBEDTLS_RSA_C */

#if defined(MBEDTLS_ECP_C)
/*
 * EC public keys:
 *  SubjectPublicKeyInfo  ::=  SEQUENCE  {      1 + 2
 *    algorithm         AlgorithmIdentifier,    1 + 1 (sequence)
 *                                            + 1 + 1 + 7 (ec oid)
 *                                            + 1 + 1 + 9 (namedCurve oid)
 *    subjectPublicKey  BIT STRING              1 + 2 + 1               [1]
 *                                            + 1 (point format)        [1]
 *                                            + 2 * ECP_MAX (coords)    [1]
 *  }
 */
#define ECP_PUB_DER_MAX_BYTES   ( 30 + 2 * MBEDTLS_ECP_MAX_BYTES )

/*
 * EC private keys:
 * ECPrivateKey ::= SEQUENCE {                  1 + 2
 *      version        INTEGER ,                1 + 1 + 1
 *      privateKey     OCTET STRING,            1 + 1 + ECP_MAX
 *      parameters [0] ECParameters OPTIONAL,   1 + 1 + (1 + 1 + 9)
 *      publicKey  [1] BIT STRING OPTIONAL      1 + 2 + [1] above
 *    }
 */
#define ECP_PRV_DER_MAX_BYTES   ( 29 + 3 * MBEDTLS_ECP_MAX_BYTES )

#else /* MBEDTLS_ECP_C */

#define ECP_PUB_DER_MAX_BYTES   0
#define ECP_PRV_DER_MAX_BYTES   0

#endif /* MBEDTLS_ECP_C */

#define PUB_DER_MAX_BYTES   ( RSA_PUB_DER_MAX_BYTES > ECP_PUB_DER_MAX_BYTES ? \
                              RSA_PUB_DER_MAX_BYTES : ECP_PUB_DER_MAX_BYTES )
#define PRV_DER_MAX_BYTES   ( RSA_PRV_DER_MAX_BYTES > ECP_PRV_DER_MAX_BYTES ? \
                              RSA_PRV_DER_MAX_BYTES : ECP_PRV_DER_MAX_BYTES )

/**
 * \brief           Write a public key to a PEM string
 *
 * \param ctx       PK context which must contain a valid public or private key.
 * \param buf       Buffer to write to. The output includes a
 *                  terminating null byte.
 * \param size      Size of the buffer in bytes.
 *
 * \return          0 if successful, or a specific error code
 */
int mbedtls_pk_write_pubkey_pem( mbedtls_pk_context *key, unsigned char *buf, size_t size )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    unsigned char output_buf[PUB_DER_MAX_BYTES];
    size_t olen = 0;

    PK_VALIDATE_RET( key != NULL );
    PK_VALIDATE_RET( buf != NULL || size == 0 );

    if( ( ret = mbedtls_pk_write_pubkey_der( key, output_buf,
                                     sizeof(output_buf) ) ) < 0 )
    {
        return( ret );
    }

    if( ( ret = mbedtls_pem_write_buffer( PEM_BEGIN_PUBLIC_KEY, PEM_END_PUBLIC_KEY,
                                  output_buf + sizeof(output_buf) - ret,
                                  ret, buf, size, &olen ) ) != 0 )
    {
        return( ret );
    }

    return( 0 );
}

/**
 * \brief           Write a private key to a PKCS#1 or SEC1 PEM string
 *
 * \param ctx       PK context which must contain a valid private key.
 * \param buf       Buffer to write to. The output includes a
 *                  terminating null byte.
 * \param size      Size of the buffer in bytes.
 *
 * \return          0 if successful, or a specific error code
 */
int mbedtls_pk_write_key_pem( mbedtls_pk_context *key, unsigned char *buf, size_t size )
{
    int ret = MBEDTLS_ERR_THIS_CORRUPTION;
    unsigned char *output_buf;
    const char *begin, *end;
    size_t olen = 0;

    PK_VALIDATE_RET( key != NULL );
    PK_VALIDATE_RET( buf != NULL || size == 0 );

    output_buf = malloc(PRV_DER_MAX_BYTES);

    if( ( ret = mbedtls_pk_write_key_der( key, output_buf, PRV_DER_MAX_BYTES ) ) < 0 ) {
        free(output_buf);
        return( ret );
    }

#if defined(MBEDTLS_RSA_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_RSA )
    {
        begin = PEM_BEGIN_PRIVATE_KEY_RSA;
        end = PEM_END_PRIVATE_KEY_RSA;
    }
    else
#endif
#if defined(MBEDTLS_ECP_C)
    if( mbedtls_pk_get_type( key ) == MBEDTLS_PK_ECKEY )
    {
        begin = PEM_BEGIN_PRIVATE_KEY_EC;
        end = PEM_END_PRIVATE_KEY_EC;
    }
    else
#endif
        return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );

    if( ( ret = mbedtls_pem_write_buffer( begin, end,
                                  output_buf + PRV_DER_MAX_BYTES - ret,
                                  ret, buf, size, &olen ) ) != 0 )
    {
        free(output_buf);
        return( ret );
    }

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

#endif /* MBEDTLS_PK_WRITE_C */