encode_rsa_pk8der.c

/*
 * Copyright (c) 2018 Thomas Pornin <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining 
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be 
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include "inner.h"

/* see bearssl_x509.h */
size_t
br_encode_rsa_pkcs8_der(void *dest, const br_rsa_private_key *sk,
	const br_rsa_public_key *pk, const void *d, size_t dlen)
{
	/*
	 * ASN.1 format:
	 *
	 *   OneAsymmetricKey ::= SEQUENCE {
	 *     version                   Version,
	 *     privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
	 *     privateKey                PrivateKey,
	 *     attributes            [0] Attributes OPTIONAL,
	 *     ...,
	 *     [[2: publicKey        [1] PublicKey OPTIONAL ]],
	 *     ...
	 *   }
	 *
	 * We don't include attributes or public key. The 'version' field
	 * is an INTEGER that we will set to 0 (meaning 'v1', compatible
	 * with previous versions of PKCS#8). The 'privateKeyAlgorithm'
	 * structure is an AlgorithmIdentifier whose OID should be
	 * rsaEncryption, with NULL parameters. The 'privateKey' is an
	 * OCTET STRING, whose value is the "raw DER" encoding of the
	 * key pair.
	 *
	 * Since the private key value comes last, this function really
	 * adds a header, which is mostly fixed (only some lengths have
	 * to be modified.
	 */

	/*
	 * Concatenation of:
	 *  - DER encoding of an INTEGER of value 0 (the 'version' field)
	 *  - DER encoding of a PrivateKeyAlgorithmIdentifier that uses
	 *    the rsaEncryption OID, and NULL parameters
	 *  - An OCTET STRING tag
	 */
	static const unsigned char PK8_HEAD[] = {
		0x02, 0x01, 0x00,
		0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86,
		0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00,
		0x04
	};

	size_t len_raw, len_seq;

	len_raw = br_encode_rsa_raw_der(NULL, sk, pk, d, dlen);
	len_seq = (sizeof PK8_HEAD) + len_of_len(len_raw) + len_raw;
	if (dest == NULL) {
		return 1 + len_of_len(len_seq) + len_seq;
	} else {
		unsigned char *buf;
		size_t lenlen;

		buf = dest;
		*buf ++ = 0x30;  /* SEQUENCE tag */
		lenlen = br_asn1_encode_length(buf, len_seq);
		buf += lenlen;

		/* version, privateKeyAlgorithm, privateKey tag */
		memcpy(buf, PK8_HEAD, sizeof PK8_HEAD);
		buf += sizeof PK8_HEAD;

		/* privateKey */
		buf += br_asn1_encode_length(buf, len_raw);
		br_encode_rsa_raw_der(buf, sk, pk, d, dlen);

		return 1 + lenlen + len_seq;
	}
}