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Add new structure for signature options

This is populated during signature verification with additional fields
that are present in and covered by the signature. At the moment, it is
only used to record security key-specific options, especially the flags
field.

with and ok markus@

/* $OpenBSD: ssh-ecdsa-sk.c,v 1.4 2019/11/25 00:51:37 djm Exp $ */
/*
 * Copyright (c) 2000 Markus Friedl.  All rights reserved.
 * Copyright (c) 2010 Damien Miller.  All rights reserved.
 * Copyright (c) 2019 Google Inc.  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.
 */

/* #define DEBUG_SK 1 */

#include <sys/types.h>

#include <openssl/bn.h>
#include <openssl/ec.h>
#include <openssl/ecdsa.h>
#include <openssl/evp.h>

#include <string.h>
#include <stdio.h> /* needed for DEBUG_SK only */

#include "sshbuf.h"
#include "ssherr.h"
#include "digest.h"
#define SSHKEY_INTERNAL
#include "sshkey.h"

/* ARGSUSED */
int
ssh_ecdsa_sk_verify(const struct sshkey *key,
    const u_char *signature, size_t signaturelen,
    const u_char *data, size_t datalen, u_int compat,
    struct sshkey_sig_details **detailsp)
{
	ECDSA_SIG *sig = NULL;
	BIGNUM *sig_r = NULL, *sig_s = NULL;
	u_char sig_flags;
	u_char msghash[32], apphash[32], sighash[32];
	u_int sig_counter;
	int ret = SSH_ERR_INTERNAL_ERROR;
	struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
	char *ktype = NULL;
	struct sshkey_sig_details *details = NULL;
#ifdef DEBUG_SK
	char *tmp = NULL;
#endif

	if (detailsp != NULL)
		*detailsp = NULL;
	if (key == NULL || key->ecdsa == NULL ||
	    sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
	    signature == NULL || signaturelen == 0)
		return SSH_ERR_INVALID_ARGUMENT;

	if (key->ecdsa_nid != NID_X9_62_prime256v1)
		return SSH_ERR_INTERNAL_ERROR;

	/* fetch signature */
	if ((b = sshbuf_from(signature, signaturelen)) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if (sshbuf_get_cstring(b, &ktype, NULL) != 0 ||
	    sshbuf_froms(b, &sigbuf) != 0 ||
	    sshbuf_get_u8(b, &sig_flags) != 0 ||
	    sshbuf_get_u32(b, &sig_counter) != 0) {
		ret = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if (strcmp(sshkey_ssh_name_plain(key), ktype) != 0) {
		ret = SSH_ERR_KEY_TYPE_MISMATCH;
		goto out;
	}
	if (sshbuf_len(b) != 0) {
		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
		goto out;
	}

	/* parse signature */
	if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
	    sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
		ret = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if ((sig = ECDSA_SIG_new()) == NULL) {
		ret = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if (!ECDSA_SIG_set0(sig, sig_r, sig_s)) {
		ret = SSH_ERR_LIBCRYPTO_ERROR;
		goto out;
	}
#ifdef DEBUG_SK
	fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
	free(tmp);
	fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
	free(tmp);
	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
	    __func__, sig_flags, sig_counter);
#endif
	sig_r = sig_s = NULL; /* transferred */

	if (sshbuf_len(sigbuf) != 0) {
		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
		goto out;
	}

	/* Reconstruct data that was supposedly signed */
	if ((original_signed = sshbuf_new()) == NULL) {
		ret = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
	    msghash, sizeof(msghash))) != 0)
		goto out;
	/* Application value is hashed before signature */
	if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
	    strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
		goto out;
#ifdef DEBUG_SK
	fprintf(stderr, "%s: hashed message:\n", __func__);
	sshbuf_dump_data(msghash, sizeof(msghash), stderr);
#endif
	if ((ret = sshbuf_put(original_signed,
	    apphash, sizeof(apphash))) != 0 ||
	    (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
	    (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
	    (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
		goto out;
	/* Signature is over H(original_signed) */
	if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
	    sighash, sizeof(sighash))) != 0)
		goto out;
	if ((details = calloc(1, sizeof(*details))) == NULL) {
		ret = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	details->sk_counter = sig_counter;
	details->sk_flags = sig_flags;
#ifdef DEBUG_SK
	fprintf(stderr, "%s: signed buf:\n", __func__);
	sshbuf_dump(original_signed, stderr);
	fprintf(stderr, "%s: signed hash:\n", __func__);
	sshbuf_dump_data(sighash, sizeof(sighash), stderr);
#endif

	/* Verify it */
	switch (ECDSA_do_verify(sighash, sizeof(sighash), sig, key->ecdsa)) {
	case 1:
		ret = 0;
		break;
	case 0:
		ret = SSH_ERR_SIGNATURE_INVALID;
		goto out;
	default:
		ret = SSH_ERR_LIBCRYPTO_ERROR;
		goto out;
	}
	/* success */
	if (detailsp != NULL) {
		*detailsp = details;
		details = NULL;
	}
 out:
	explicit_bzero(&sig_flags, sizeof(sig_flags));
	explicit_bzero(&sig_counter, sizeof(sig_counter));
	explicit_bzero(msghash, sizeof(msghash));
	explicit_bzero(sighash, sizeof(msghash));
	explicit_bzero(apphash, sizeof(apphash));
	sshkey_sig_details_free(details);
	sshbuf_free(original_signed);
	sshbuf_free(sigbuf);
	sshbuf_free(b);
	ECDSA_SIG_free(sig);
	BN_clear_free(sig_r);
	BN_clear_free(sig_s);
	free(ktype);
	return ret;
}