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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: kexsntrup761x25519.c,v 1.1 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 "sshbuf.h"
#include "digest.h"
#include "ssherr.h"

int
kex_kem_sntrup761x25519_keypair(struct kex *kex)
{
	struct sshbuf *buf = NULL;
	u_char *cp = NULL;
	size_t need;
	int r;

	if ((buf = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	need = crypto_kem_sntrup761_PUBLICKEYBYTES + CURVE25519_SIZE;
	if ((r = sshbuf_reserve(buf, need, &cp)) != 0)
		goto out;
	crypto_kem_sntrup761_keypair(cp, kex->sntrup761_client_key);
#ifdef DEBUG_KEXECDH
	dump_digest("client public key sntrup761:", cp,
	    crypto_kem_sntrup761_PUBLICKEYBYTES);
#endif
	cp += crypto_kem_sntrup761_PUBLICKEYBYTES;
	kexc25519_keygen(kex->c25519_client_key, cp);
#ifdef DEBUG_KEXECDH
	dump_digest("client public key c25519:", cp, CURVE25519_SIZE);
#endif
	kex->client_pub = buf;
	buf = NULL;
 out:
	sshbuf_free(buf);
	return r;
}

int
kex_kem_sntrup761x25519_enc(struct kex *kex,
   const struct sshbuf *client_blob, struct sshbuf **server_blobp,
   struct sshbuf **shared_secretp)
{
	struct sshbuf *server_blob = NULL;
	struct sshbuf *buf = NULL;
	const u_char *client_pub;
	u_char *kem_key, *ciphertext, *server_pub;
	u_char server_key[CURVE25519_SIZE];
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t need;
	int r;

	*server_blobp = NULL;
	*shared_secretp = NULL;

	/* client_blob contains both KEM and ECDH client pubkeys */
	need = crypto_kem_sntrup761_PUBLICKEYBYTES + CURVE25519_SIZE;
	if (sshbuf_len(client_blob) != need) {
		r = SSH_ERR_SIGNATURE_INVALID;
		goto out;
	}
	client_pub = sshbuf_ptr(client_blob);
#ifdef DEBUG_KEXECDH
	dump_digest("client public key sntrup761:", client_pub,
	    crypto_kem_sntrup761_PUBLICKEYBYTES);
	dump_digest("client public key 25519:",
	    client_pub + crypto_kem_sntrup761_PUBLICKEYBYTES,
	    CURVE25519_SIZE);
#endif
	/* allocate buffer for concatenation of KEM key and ECDH shared key */
	/* the buffer will be hashed and the result is the shared secret */
	if ((buf = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshbuf_reserve(buf, crypto_kem_sntrup761_BYTES,
	    &kem_key)) != 0)
		goto out;
	/* allocate space for encrypted KEM key and ECDH pub key */
	if ((server_blob = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	need = crypto_kem_sntrup761_CIPHERTEXTBYTES + CURVE25519_SIZE;
	if ((r = sshbuf_reserve(server_blob, need, &ciphertext)) != 0)
		goto out;
	/* generate and encrypt KEM key with client key */
	crypto_kem_sntrup761_enc(ciphertext, kem_key, client_pub);
	/* generate ECDH key pair, store server pubkey after ciphertext */
	server_pub = ciphertext + crypto_kem_sntrup761_CIPHERTEXTBYTES;
	kexc25519_keygen(server_key, server_pub);
	/* append ECDH shared key */
	client_pub += crypto_kem_sntrup761_PUBLICKEYBYTES;
	if ((r = kexc25519_shared_key_ext(server_key, client_pub, buf, 1)) < 0)
		goto out;
	if ((r = ssh_digest_buffer(kex->hash_alg, buf, hash, sizeof(hash))) != 0)
		goto out;
#ifdef DEBUG_KEXECDH
	dump_digest("server public key 25519:", server_pub, CURVE25519_SIZE);
	dump_digest("server cipher text:", ciphertext,
	    crypto_kem_sntrup761_CIPHERTEXTBYTES);
	dump_digest("server kem key:", kem_key, sizeof(kem_key));
	dump_digest("concatenation of KEM key and ECDH shared key:",
	    sshbuf_ptr(buf), sshbuf_len(buf));
#endif
	/* string-encoded hash is resulting shared secret */
	sshbuf_reset(buf);
	if ((r = sshbuf_put_string(buf, hash,
	    ssh_digest_bytes(kex->hash_alg))) != 0)
		goto out;
#ifdef DEBUG_KEXECDH
	dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));
#endif
	*server_blobp = server_blob;
	*shared_secretp = buf;
	server_blob = NULL;
	buf = NULL;
 out:
	explicit_bzero(hash, sizeof(hash));
	explicit_bzero(server_key, sizeof(server_key));
	sshbuf_free(server_blob);
	sshbuf_free(buf);
	return r;
}

int
kex_kem_sntrup761x25519_dec(struct kex *kex,
    const struct sshbuf *server_blob, struct sshbuf **shared_secretp)
{
	struct sshbuf *buf = NULL;
	u_char *kem_key = NULL;
	const u_char *ciphertext, *server_pub;
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t need;
	int r, decoded;

	*shared_secretp = NULL;

	need = crypto_kem_sntrup761_CIPHERTEXTBYTES + CURVE25519_SIZE;
	if (sshbuf_len(server_blob) != need) {
		r = SSH_ERR_SIGNATURE_INVALID;
		goto out;
	}
	ciphertext = sshbuf_ptr(server_blob);
	server_pub = ciphertext + crypto_kem_sntrup761_CIPHERTEXTBYTES;
#ifdef DEBUG_KEXECDH
	dump_digest("server cipher text:", ciphertext,
	    crypto_kem_sntrup761_CIPHERTEXTBYTES);
	dump_digest("server public key c25519:", server_pub, CURVE25519_SIZE);
#endif
	/* hash concatenation of KEM key and ECDH shared key */
	if ((buf = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshbuf_reserve(buf, crypto_kem_sntrup761_BYTES,
	    &kem_key)) != 0)
		goto out;
	decoded = crypto_kem_sntrup761_dec(kem_key, ciphertext,
	    kex->sntrup761_client_key);
	if ((r = kexc25519_shared_key_ext(kex->c25519_client_key, server_pub,
	    buf, 1)) < 0)
		goto out;
	if ((r = ssh_digest_buffer(kex->hash_alg, buf, hash, sizeof(hash))) != 0)
		goto out;
#ifdef DEBUG_KEXECDH
	dump_digest("client kem key:", kem_key, crypto_kem_sntrup761_BYTES);
	dump_digest("concatenation of KEM key and ECDH shared key:",
	    sshbuf_ptr(buf), sshbuf_len(buf));
#endif
	sshbuf_reset(buf);
	if ((r = sshbuf_put_string(buf, hash,
	    ssh_digest_bytes(kex->hash_alg))) != 0)
		goto out;
#ifdef DEBUG_KEXECDH
	dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));
#endif
	if (decoded != 0) {
		r = SSH_ERR_SIGNATURE_INVALID;
		goto out;
	}
	*shared_secretp = buf;
	buf = NULL;
 out:
	explicit_bzero(hash, sizeof(hash));
	sshbuf_free(buf);
	return r;
}