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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-ed25519-sk.c,v 1.3 2019/11/25 00:51:37 djm Exp $ */
/*
 * Copyright (c) 2019 Markus Friedl.  All rights reserved.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#define SSHKEY_INTERNAL
#include <sys/types.h>
#include <limits.h>

#include "crypto_api.h"

#include <string.h>
#include <stdarg.h>

#include "log.h"
#include "sshbuf.h"
#include "sshkey.h"
#include "ssherr.h"
#include "ssh.h"
#include "digest.h"

int
ssh_ed25519_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)
{
	struct sshbuf *b = NULL;
	struct sshbuf *encoded = NULL;
	char *ktype = NULL;
	const u_char *sigblob;
	const u_char *sm;
	u_char *m = NULL;
	u_char apphash[32];
	u_char msghash[32];
	u_char sig_flags;
	u_int sig_counter;
	size_t len;
	unsigned long long smlen = 0, mlen = 0;
	int r = SSH_ERR_INTERNAL_ERROR;
	int ret;
	struct sshkey_sig_details *details = NULL;

	if (detailsp != NULL)
		*detailsp = NULL;

	if (key == NULL ||
	    sshkey_type_plain(key->type) != KEY_ED25519_SK ||
	    key->ed25519_pk == NULL ||
	    signature == NULL || signaturelen == 0)
		return SSH_ERR_INVALID_ARGUMENT;

	if ((b = sshbuf_from(signature, signaturelen)) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if (sshbuf_get_cstring(b, &ktype, NULL) != 0 ||
	    sshbuf_get_string_direct(b, &sigblob, &len) != 0 ||
	    sshbuf_get_u8(b, &sig_flags) != 0 ||
	    sshbuf_get_u32(b, &sig_counter) != 0) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if (strcmp(sshkey_ssh_name_plain(key), ktype) != 0) {
		r = SSH_ERR_KEY_TYPE_MISMATCH;
		goto out;
	}
	if (sshbuf_len(b) != 0) {
		r = SSH_ERR_UNEXPECTED_TRAILING_DATA;
		goto out;
	}
	if (len > crypto_sign_ed25519_BYTES) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if (ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
	    strlen(key->sk_application), apphash, sizeof(apphash)) != 0 ||
	    ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
	    msghash, sizeof(msghash)) != 0) {
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if ((details = calloc(1, sizeof(*details))) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	details->sk_counter = sig_counter;
	details->sk_flags = sig_flags;
	if ((encoded = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if (sshbuf_put(encoded, sigblob, len) != 0 ||
	    sshbuf_put(encoded, apphash, sizeof(apphash)) != 0 ||
	    sshbuf_put_u8(encoded, sig_flags) != 0 ||
	    sshbuf_put_u32(encoded, sig_counter) != 0 ||
	    sshbuf_put(encoded, msghash, sizeof(msghash)) != 0) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	sm = sshbuf_ptr(encoded);
	smlen = sshbuf_len(encoded);
	mlen = smlen;
	if ((m = malloc(smlen)) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((ret = crypto_sign_ed25519_open(m, &mlen, sm, smlen,
	    key->ed25519_pk)) != 0) {
		debug2("%s: crypto_sign_ed25519_open failed: %d",
		    __func__, ret);
	}
	if (ret != 0 || mlen != smlen - len) {
		r = SSH_ERR_SIGNATURE_INVALID;
		goto out;
	}
	/* XXX compare 'm' and 'sm + len' ? */
	/* success */
	r = 0;
	if (detailsp != NULL) {
		*detailsp = details;
		details = NULL;
	}
 out:
	if (m != NULL) {
		explicit_bzero(m, smlen); /* NB mlen may be invalid if r != 0 */
		free(m);
	}
	sshkey_sig_details_free(details);
	sshbuf_free(b);
	sshbuf_free(encoded);
	free(ktype);
	return r;
}