library/pkwrite.c

/*
 *  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.
 */

#include "common.h"

#if defined(MBEDTLS_PK_WRITE_C)

#include "mbedtls/pk.h"
#include "mbedtls/asn1write.h"
#include "mbedtls/oid.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/error.h"

#include <string.h>

#if defined(MBEDTLS_RSA_C)
#include "mbedtls/rsa.h"
#endif
#if defined(MBEDTLS_ECP_C)
#include "mbedtls/bignum.h"
#include "mbedtls/ecp.h"
#include "mbedtls/platform_util.h"
#endif
#if defined(MBEDTLS_ECDSA_C)
#include "mbedtls/ecdsa.h"
#endif
#if defined(MBEDTLS_PEM_WRITE_C)
#include "mbedtls/pem.h"
#endif

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

/* 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_ERROR_CORRUPTION_DETECTED;
    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_ERROR_CORRUPTION_DETECTED;
    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_ERROR_CORRUPTION_DETECTED;
    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_ERROR_CORRUPTION_DETECTED;
    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 */

int mbedtls_pk_write_pubkey(unsigned char **p, unsigned char *start,
                            const mbedtls_pk_context *key)
{
    size_t len = 0;

    (void) p;
    (void) start;
    (void) key;

    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;
}

int mbedtls_pk_write_pubkey_der(mbedtls_pk_context *key, unsigned char *buf, size_t size)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    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;
}

int mbedtls_pk_write_key_der(mbedtls_pk_context *key, unsigned char *buf, size_t size)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char *c;
    size_t len = 0;

    (void) ret;
    (void) c;
    (void) key;

    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)

int mbedtls_pk_write_pubkey_pem(mbedtls_pk_context *key, unsigned char *buf, size_t size)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    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;
}

int mbedtls_pk_write_key_pem(mbedtls_pk_context *key, unsigned char *buf, size_t size)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char output_buf[PRV_DER_MAX_BYTES];
    const char *begin, *end;
    size_t olen = 0;

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

    if ((ret = mbedtls_pk_write_key_der(key, output_buf, sizeof(output_buf))) < 0) {
        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 + sizeof(output_buf) - ret,
                                        ret, buf, size, &olen)) != 0) {
        return ret;
    }

    return 0;
}
#endif /* MBEDTLS_PEM_WRITE_C */

#endif /* MBEDTLS_PK_WRITE_C */