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/* kem.c - KEM helper functions
* Copyright (C) 2024 g10 Code GmbH.
*
* This file is part of GnuPG.
*
* GnuPG is free software; you can redistribute and/or modify this
* part of GnuPG under the terms of either
*
* - the GNU Lesser General Public License as published by the Free
* Software Foundation; either version 3 of the License, or (at
* your option) any later version.
*
* or
*
* - the GNU General Public License as published by the Free
* Software Foundation; either version 2 of the License, or (at
* your option) any later version.
*
* or both in parallel, as here.
*
* GnuPG is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received copies of the GNU General Public License
* and the GNU Lesser General Public License along with this program;
* if not, see <https://www.gnu.org/licenses/>.
* SPDX-License-Identifier: (LGPL-3.0-or-later OR GPL-2.0-or-later)
*/
#include <config.h>
#include <stdlib.h>
#include <string.h>
#include <gpg-error.h>
#include <gcrypt.h>
#include "mischelp.h"
/* domSeperation as per *PGP specs. */
#define KMAC_KEY "OpenPGPCompositeKeyDerivationFunction"
/* customizationString as per *PGP specs. */
#define KMAC_CUSTOM "KDF"
/* The blocksize used for Keccak by compute_kmac256. */
#define KECCAK512_BLOCKSIZE 136
static gpg_error_t
compute_kmac256 (void *digest, size_t digestlen,
const void *key, size_t keylen,
const void *custom, size_t customlen,
gcry_buffer_t *data_iov, int data_iovlen)
{
gpg_error_t err;
gcry_buffer_t iov[20];
const unsigned char headPAD[2] = { 1, KECCAK512_BLOCKSIZE };
unsigned char headK[3];
const unsigned char pad[KECCAK512_BLOCKSIZE] = { 0 };
unsigned char right_encode_L[3];
unsigned int len;
int iovcnt;
if (data_iovlen >= DIM(iov) - 6)
return gpg_error (GPG_ERR_TOO_LARGE);
/* Check the validity conditions of NIST SP 800-185 */
if (keylen >= 255 || customlen >= 255 || digestlen >= 255)
return gpg_error (GPG_ERR_TOO_LARGE);
iovcnt = 0;
iov[iovcnt].data = "KMAC";
iov[iovcnt].off = 0;
iov[iovcnt].len = 4;
iovcnt++;
iov[iovcnt].data = (void *)custom;
iov[iovcnt].off = 0;
iov[iovcnt].len = customlen;
iovcnt++;
iov[iovcnt].data = (void *)headPAD;
iov[iovcnt].off = 0;
iov[iovcnt].len = sizeof (headPAD);
iovcnt++;
if (keylen < 32)
{
headK[0] = 1;
headK[1] = (keylen*8)&0xff;
iov[iovcnt].data = headK;
iov[iovcnt].off = 0;
iov[iovcnt].len = 2;
}
else
{
headK[0] = 2;
headK[1] = (keylen*8)>>8;
headK[2] = (keylen*8)&0xff;
iov[iovcnt].data = headK;
iov[iovcnt].off = 0;
iov[iovcnt].len = 3;
}
iovcnt++;
iov[iovcnt].data = (void *)key;
iov[iovcnt].off = 0;
iov[iovcnt].len = keylen;
iovcnt++;
len = iov[2].len + iov[3].len + iov[4].len;
len %= KECCAK512_BLOCKSIZE;
iov[iovcnt].data = (unsigned char *)pad;
iov[iovcnt].off = 0;
iov[iovcnt].len = sizeof (pad) - len;
iovcnt++;
memcpy (&iov[iovcnt], data_iov, data_iovlen * sizeof (gcry_buffer_t));
iovcnt += data_iovlen;
if (digestlen < 32)
{
right_encode_L[0] = (digestlen * 8) & 0xff;
right_encode_L[1] = 1;
}
else
{
right_encode_L[0] = (digestlen * 8) >> 8;
right_encode_L[1] = (digestlen * 8) & 0xff;
right_encode_L[2] = 2;
}
iov[iovcnt].data = right_encode_L;
iov[iovcnt].off = 0;
iov[iovcnt].len = 3;
iovcnt++;
err = gcry_md_hash_buffers_ext (GCRY_MD_CSHAKE256, 0,
digest, digestlen, iov, iovcnt);
return err;
}
/* Compute KEK (shared secret) for ECC with HASHALGO, ECDH result,
ciphertext in ECC_CT, public key in ECC_PK. */
gpg_error_t
gnupg_ecc_kem_kdf (void *kek, size_t kek_len,
int hashalgo, const void *ecdh, size_t ecdh_len,
const void *ecc_ct, size_t ecc_ct_len,
const void *ecc_pk, size_t ecc_pk_len)
{
gcry_buffer_t iov[3];
unsigned int dlen;
dlen = gcry_md_get_algo_dlen (hashalgo);
if (kek_len != dlen)
return gpg_error (GPG_ERR_INV_LENGTH);
memset (iov, 0, sizeof (iov));
iov[0].data = (unsigned char *)ecdh;
iov[0].len = ecdh_len;
iov[1].data = (unsigned char *)ecc_ct;
iov[1].len = ecc_ct_len;
iov[2].data = (unsigned char *)ecc_pk;
iov[2].len = ecc_pk_len;
gcry_md_hash_buffers (hashalgo, 0, kek, iov, 3);
return 0;
}
/* Compute KEK by combining two KEMs. The caller provides a buffer
* KEK allocated with size KEK_LEN which will receive the computed
* KEK. (ECC_SS, ECC_SS_LEN) is the shared secret of the first key.
* (ECC_CT, ECC_CT_LEN) is the ciphertext of the first key.
* (MLKEM_SS, ECC_SS_LEN) is the shared secret of the second key.
* (MLKEM_CT, MLKEM_CT_LEN) is the ciphertext of the second key.
* (FIXEDINFO, FIXEDINFO_LEN) is an octet string used to bind the KEK
* to a the key; for PGP we use the concatenation of the session key's
* algorithm id and the v5 fingerprint of the key.
*/
gpg_error_t
gnupg_kem_combiner (void *kek, size_t kek_len,
const void *ecc_ss, size_t ecc_ss_len,
const void *ecc_ct, size_t ecc_ct_len,
const void *mlkem_ss, size_t mlkem_ss_len,
const void *mlkem_ct, size_t mlkem_ct_len,
const void *fixedinfo, size_t fixedinfo_len)
{
gpg_error_t err;
gcry_buffer_t iov[6];
memset (iov, 0, sizeof (iov));
iov[0].data = "\x00\x00\x00\x01"; /* Counter */
iov[0].len = 4;
iov[1].data = (unsigned char *)ecc_ss;
iov[1].len = ecc_ss_len;
iov[2].data = (unsigned char *)ecc_ct;
iov[2].len = ecc_ct_len;
iov[3].data = (unsigned char *)mlkem_ss;
iov[3].len = mlkem_ss_len;
iov[4].data = (unsigned char *)mlkem_ct;
iov[4].len = mlkem_ct_len;
iov[5].data = (unsigned char *)fixedinfo;
iov[5].len = fixedinfo_len;
err = compute_kmac256 (kek, kek_len,
KMAC_KEY, strlen (KMAC_KEY),
KMAC_CUSTOM, strlen (KMAC_CUSTOM), iov, 6);
return err;
}
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