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Update/replace the experimental post-quantim hybrid key exchange
method based on Streamlined NTRU Prime (coupled with X25519).

The previous sntrup4591761x25519-sha512@tinyssh.org method is
replaced with sntrup761x25519-sha512@openssh.com. Per the authors,
sntrup4591761 was replaced almost two years ago by sntrup761.

The sntrup761 implementaion, like sntrup4591761 before it, is public
domain code extracted from the SUPERCOP cryptography benchmark
suite (https://bench.cr.yp.to/supercop.html).

Thanks for Daniel J Bernstein for guidance on algorithm selection.
Patch from Tobias Heider; feedback & ok markus@ and myself

(note this both the updated method and the one that it replaced are
disabled by default)

/* $OpenBSD: kexgen.c,v 1.5 2020/12/29 00:59:15 djm Exp $ */
/*
 * Copyright (c) 2019 Markus Friedl.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/types.h>

#include <stdio.h>
#include <string.h>
#include <signal.h>

#include "sshkey.h"
#include "kex.h"
#include "log.h"
#include "packet.h"
#include "ssh2.h"
#include "sshbuf.h"
#include "digest.h"
#include "ssherr.h"

static int input_kex_gen_init(int, u_int32_t, struct ssh *);
static int input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh);

static int
kex_gen_hash(
    int hash_alg,
    const struct sshbuf *client_version,
    const struct sshbuf *server_version,
    const struct sshbuf *client_kexinit,
    const struct sshbuf *server_kexinit,
    const struct sshbuf *server_host_key_blob,
    const struct sshbuf *client_pub,
    const struct sshbuf *server_pub,
    const struct sshbuf *shared_secret,
    u_char *hash, size_t *hashlen)
{
	struct sshbuf *b;
	int r;

	if (*hashlen < ssh_digest_bytes(hash_alg))
		return SSH_ERR_INVALID_ARGUMENT;
	if ((b = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_stringb(b, client_version)) != 0 ||
	    (r = sshbuf_put_stringb(b, server_version)) != 0 ||
	    /* kexinit messages: fake header: len+SSH2_MSG_KEXINIT */
	    (r = sshbuf_put_u32(b, sshbuf_len(client_kexinit) + 1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_putb(b, client_kexinit)) != 0 ||
	    (r = sshbuf_put_u32(b, sshbuf_len(server_kexinit) + 1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_putb(b, server_kexinit)) != 0 ||
	    (r = sshbuf_put_stringb(b, server_host_key_blob)) != 0 ||
	    (r = sshbuf_put_stringb(b, client_pub)) != 0 ||
	    (r = sshbuf_put_stringb(b, server_pub)) != 0 ||
	    (r = sshbuf_putb(b, shared_secret)) != 0) {
		sshbuf_free(b);
		return r;
	}
#ifdef DEBUG_KEX
	sshbuf_dump(b, stderr);
#endif
	if (ssh_digest_buffer(hash_alg, b, hash, *hashlen) != 0) {
		sshbuf_free(b);
		return SSH_ERR_LIBCRYPTO_ERROR;
	}
	sshbuf_free(b);
	*hashlen = ssh_digest_bytes(hash_alg);
#ifdef DEBUG_KEX
	dump_digest("hash", hash, *hashlen);
#endif
	return 0;
}

int
kex_gen_client(struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	int r;

	switch (kex->kex_type) {
#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
		r = kex_dh_keypair(kex);
		break;
	case KEX_ECDH_SHA2:
		r = kex_ecdh_keypair(kex);
		break;
#endif /* WITH_OPENSSL */
	case KEX_C25519_SHA256:
		r = kex_c25519_keypair(kex);
		break;
	case KEX_KEM_SNTRUP761X25519_SHA512:
		r = kex_kem_sntrup761x25519_keypair(kex);
		break;
	default:
		r = SSH_ERR_INVALID_ARGUMENT;
		break;
	}
	if (r != 0)
		return r;
	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_ECDH_INIT)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, kex->client_pub)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		return r;
	debug("expecting SSH2_MSG_KEX_ECDH_REPLY");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_REPLY, &input_kex_gen_reply);
	return 0;
}

static int
input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	struct sshkey *server_host_key = NULL;
	struct sshbuf *shared_secret = NULL;
	struct sshbuf *server_blob = NULL;
	struct sshbuf *tmp = NULL, *server_host_key_blob = NULL;
	u_char *signature = NULL;
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	/* hostkey */
	if ((r = sshpkt_getb_froms(ssh, &server_host_key_blob)) != 0)
		goto out;
	/* sshkey_fromb() consumes its buffer, so make a copy */
	if ((tmp = sshbuf_fromb(server_host_key_blob)) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshkey_fromb(tmp, &server_host_key)) != 0)
		goto out;
	if ((r = kex_verify_host_key(ssh, server_host_key)) != 0)
		goto out;

	/* Q_S, server public key */
	/* signed H */
	if ((r = sshpkt_getb_froms(ssh, &server_blob)) != 0 ||
	    (r = sshpkt_get_string(ssh, &signature, &slen)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;

	/* compute shared secret */
	switch (kex->kex_type) {
#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
		r = kex_dh_dec(kex, server_blob, &shared_secret);
		break;
	case KEX_ECDH_SHA2:
		r = kex_ecdh_dec(kex, server_blob, &shared_secret);
		break;
#endif /* WITH_OPENSSL */
	case KEX_C25519_SHA256:
		r = kex_c25519_dec(kex, server_blob, &shared_secret);
		break;
	case KEX_KEM_SNTRUP761X25519_SHA512:
		r = kex_kem_sntrup761x25519_dec(kex, server_blob,
		    &shared_secret);
		break;
	default:
		r = SSH_ERR_INVALID_ARGUMENT;
		break;
	}
	if (r !=0 )
		goto out;

	/* calc and verify H */
	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	    kex->hash_alg,
	    kex->client_version,
	    kex->server_version,
	    kex->my,
	    kex->peer,
	    server_host_key_blob,
	    kex->client_pub,
	    server_blob,
	    shared_secret,
	    hash, &hashlen)) != 0)
		goto out;

	if ((r = sshkey_verify(server_host_key, signature, slen, hash, hashlen,
	    kex->hostkey_alg, ssh->compat, NULL)) != 0)
		goto out;

	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
		r = kex_send_newkeys(ssh);
out:
	explicit_bzero(hash, sizeof(hash));
	explicit_bzero(kex->c25519_client_key, sizeof(kex->c25519_client_key));
	explicit_bzero(kex->sntrup761_client_key,
	    sizeof(kex->sntrup761_client_key));
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshbuf_free(tmp);
	sshkey_free(server_host_key);
	sshbuf_free(server_blob);
	sshbuf_free(shared_secret);
	sshbuf_free(kex->client_pub);
	kex->client_pub = NULL;
	return r;
}

int
kex_gen_server(struct ssh *ssh)
{
	debug("expecting SSH2_MSG_KEX_ECDH_INIT");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_INIT, &input_kex_gen_init);
	return 0;
}

static int
input_kex_gen_init(int type, u_int32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	struct sshkey *server_host_private, *server_host_public;
	struct sshbuf *shared_secret = NULL;
	struct sshbuf *server_pubkey = NULL;
	struct sshbuf *client_pubkey = NULL;
	struct sshbuf *server_host_key_blob = NULL;
	u_char *signature = NULL, hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	if ((r = kex_load_hostkey(ssh, &server_host_private,
	    &server_host_public)) != 0)
		goto out;

	if ((r = sshpkt_getb_froms(ssh, &client_pubkey)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;

	/* compute shared secret */
	switch (kex->kex_type) {
#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
		r = kex_dh_enc(kex, client_pubkey, &server_pubkey,
		    &shared_secret);
		break;
	case KEX_ECDH_SHA2:
		r = kex_ecdh_enc(kex, client_pubkey, &server_pubkey,
		    &shared_secret);
		break;
#endif /* WITH_OPENSSL */
	case KEX_C25519_SHA256:
		r = kex_c25519_enc(kex, client_pubkey, &server_pubkey,
		    &shared_secret);
		break;
	case KEX_KEM_SNTRUP761X25519_SHA512:
		r = kex_kem_sntrup761x25519_enc(kex, client_pubkey,
		    &server_pubkey, &shared_secret);
		break;
	default:
		r = SSH_ERR_INVALID_ARGUMENT;
		break;
	}
	if (r !=0 )
		goto out;

	/* calc H */
	if ((server_host_key_blob = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshkey_putb(server_host_public, server_host_key_blob)) != 0)
		goto out;
	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	    kex->hash_alg,
	    kex->client_version,
	    kex->server_version,
	    kex->peer,
	    kex->my,
	    server_host_key_blob,
	    client_pubkey,
	    server_pubkey,
	    shared_secret,
	    hash, &hashlen)) != 0)
		goto out;

	/* sign H */
	if ((r = kex->sign(ssh, server_host_private, server_host_public,
	     &signature, &slen, hash, hashlen, kex->hostkey_alg)) != 0)
		goto out;

	/* send server hostkey, ECDH pubkey 'Q_S' and signed H */
	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_ECDH_REPLY)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, server_host_key_blob)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, server_pubkey)) != 0 ||
	    (r = sshpkt_put_string(ssh, signature, slen)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		goto out;

	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
		r = kex_send_newkeys(ssh);
out:
	explicit_bzero(hash, sizeof(hash));
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshbuf_free(shared_secret);
	sshbuf_free(client_pubkey);
	sshbuf_free(server_pubkey);
	return r;
}