/* $OpenBSD: ssh-verify-attestation.c,v 1.3 2025/05/12 05:42:02 tb Exp $ */
/*
 * Copyright (c) 2022-2024 Damien Miller
 *
 * 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.
 */

/*
 * This is a small program to verify FIDO attestation objects that
 * ssh-keygen(1) can record when enrolling a FIDO key. It requires that
 * the attestation object and challenge used when creating the key be
 * recorded.
 *
 * Example usage:
 *
 * $ # Generate a random challenge.
 * $ dd if=/dev/urandom of=key_ecdsa_sk.challenge bs=32 count=1
 * $ # Generate a key, record the attestation blob.
 * $ ssh-keygen -f key_ecdsa_sk -t ecdsa-sk \
 *       -Ochallenge=key_ecdsa_sk.challenge \
 *       -Owrite-attestation=key_ecdsa_sk.attest -N ''
 * $ # Validate the challenge (-A = print attestation CA cert)
 * $ ./obj/ssh-verify-attestation -A key_ecdsa_sk key_ecdsa_sk.challenge \
 *       key_ecdsa_sk.attest
 *
 * Limitations/TODO:
 *
 * 1) It doesn't automatically detect the attestation statement format. It
 *    assumes the "packed" format used by FIDO2 keys. If that doesn't work,
 *    then try using the -U option to select the "fido-u2f" format.
 * 2) It makes assumptions about RK, UV, etc status of the key/cred.
 * 3) Probably bugs.
 *
 * Thanks to Markus Friedl and Pedro Martelletto for help getting this
 * working.
 */

#include <stdint.h>
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <stdarg.h>

#include "xmalloc.h"
#include "log.h"
#include "sshbuf.h"
#include "sshkey.h"
#include "authfile.h"
#include "ssherr.h"
#include "misc.h"
#include "digest.h"
#include "crypto_api.h"

#include <fido.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/pem.h>

extern char *__progname;

#define ATTEST_MAGIC    "ssh-sk-attest-v01"

static int
prepare_fido_cred(fido_cred_t *cred, int credtype, const char *attfmt,
    const char *rp_id, struct sshbuf *b, const struct sshbuf *challenge,
    struct sshbuf **attestation_certp)
{
        struct sshbuf *attestation_cert = NULL, *sig = NULL, *authdata = NULL;
        char *magic = NULL;
        int r = SSH_ERR_INTERNAL_ERROR;

        *attestation_certp = NULL;

        /* Make sure it's the format we're expecting */
        if ((r = sshbuf_get_cstring(b, &magic, NULL)) != 0) {
                error_fr(r, "parse header");
                goto out;
        }
        if (strcmp(magic, ATTEST_MAGIC) != 0) {
                error_f("unsupported format");
                r = SSH_ERR_INVALID_FORMAT;
                goto out;
        }
        /* Parse the remaining fields */
        if ((r = sshbuf_froms(b, &attestation_cert)) != 0 ||
            (r = sshbuf_froms(b, &sig)) != 0 ||
            (r = sshbuf_froms(b, &authdata)) != 0 ||
            (r = sshbuf_get_u32(b, NULL)) != 0 || /* reserved flags */
            (r = sshbuf_get_string_direct(b, NULL, NULL)) != 0) { /* reserved */
                error_fr(r, "parse body");
                goto out;
        }
        debug3_f("attestation cert len=%zu, sig len=%zu, "
            "authdata len=%zu challenge len=%zu", sshbuf_len(attestation_cert),
            sshbuf_len(sig), sshbuf_len(authdata), sshbuf_len(challenge));

        fido_cred_set_type(cred, credtype);
        fido_cred_set_fmt(cred, attfmt);
        fido_cred_set_clientdata(cred, sshbuf_ptr(challenge),
            sshbuf_len(challenge));
        fido_cred_set_rp(cred, rp_id, NULL);
        fido_cred_set_authdata(cred, sshbuf_ptr(authdata),
            sshbuf_len(authdata));
        /* XXX set_extensions, set_rk, set_uv */
        fido_cred_set_x509(cred, sshbuf_ptr(attestation_cert),
            sshbuf_len(attestation_cert));
        fido_cred_set_sig(cred, sshbuf_ptr(sig), sshbuf_len(sig));

        /* success */
        *attestation_certp = attestation_cert;
        attestation_cert = NULL;
        r = 0;
 out:
        free(magic);
        sshbuf_free(attestation_cert);
        sshbuf_free(sig);
        sshbuf_free(authdata);
        return r;
}

static uint8_t *
get_pubkey_from_cred_ecdsa(const fido_cred_t *cred, size_t *pubkey_len)
{
        const uint8_t *ptr;
        uint8_t *pubkey = NULL, *ret = NULL;
        BIGNUM *x = NULL, *y = NULL;
        EC_POINT *q = NULL;
        EC_GROUP *g = NULL;

        if ((x = BN_new()) == NULL ||
            (y = BN_new()) == NULL ||
            (g = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1)) == NULL ||
            (q = EC_POINT_new(g)) == NULL) {
                error_f("libcrypto setup failed");
                goto out;
        }
        if ((ptr = fido_cred_pubkey_ptr(cred)) == NULL) {
                error_f("fido_cred_pubkey_ptr failed");
                goto out;
        }
        if (fido_cred_pubkey_len(cred) != 64) {
                error_f("bad fido_cred_pubkey_len %zu",
                    fido_cred_pubkey_len(cred));
                goto out;
        }

        if (BN_bin2bn(ptr, 32, x) == NULL ||
            BN_bin2bn(ptr + 32, 32, y) == NULL) {
                error_f("BN_bin2bn failed");
                goto out;
        }
        if (EC_POINT_set_affine_coordinates(g, q, x, y, NULL) != 1) {
                error_f("EC_POINT_set_affine_coordinates failed");
                goto out;
        }
        *pubkey_len = EC_POINT_point2oct(g, q,
            POINT_CONVERSION_UNCOMPRESSED, NULL, 0, NULL);
        if (*pubkey_len == 0 || *pubkey_len > 2048) {
                error_f("bad pubkey length %zu", *pubkey_len);
                goto out;
        }
        if ((pubkey = malloc(*pubkey_len)) == NULL) {
                error_f("malloc pubkey failed");
                goto out;
        }
        if (EC_POINT_point2oct(g, q, POINT_CONVERSION_UNCOMPRESSED,
            pubkey, *pubkey_len, NULL) == 0) {
                error_f("EC_POINT_point2oct failed");
                goto out;
        }
        /* success */
        ret = pubkey;
        pubkey = NULL;
 out:
        free(pubkey);
        EC_POINT_free(q);
        EC_GROUP_free(g);
        BN_clear_free(x);
        BN_clear_free(y);
        return ret;
}

/* copied from sshsk_ecdsa_assemble() */
static int
cred_matches_key_ecdsa(const fido_cred_t *cred, const struct sshkey *k)
{
        struct sshkey *key = NULL;
        struct sshbuf *b = NULL;
        EC_KEY *ec = NULL;
        uint8_t *pubkey = NULL;
        size_t pubkey_len;
        int r;

        if ((key = sshkey_new(KEY_ECDSA_SK)) == NULL) {
                error_f("sshkey_new failed");
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
        key->ecdsa_nid = NID_X9_62_prime256v1;
        if ((key->pkey = EVP_PKEY_new()) == NULL ||
            (ec = EC_KEY_new_by_curve_name(key->ecdsa_nid)) == NULL ||
            (b = sshbuf_new()) == NULL) {
                error_f("allocation failed");
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
        if ((pubkey = get_pubkey_from_cred_ecdsa(cred, &pubkey_len)) == NULL) {
                error_f("get_pubkey_from_cred_ecdsa failed");
                r = SSH_ERR_INVALID_FORMAT;
                goto out;
        }
        if ((r = sshbuf_put_string(b, pubkey, pubkey_len)) != 0) {
                error_fr(r, "sshbuf_put_string");
                goto out;
        }
        if ((r = sshbuf_get_eckey(b, ec)) != 0) {
                error_fr(r, "parse");
                r = SSH_ERR_INVALID_FORMAT;
                goto out;
        }
        if (sshkey_ec_validate_public(EC_KEY_get0_group(ec),
            EC_KEY_get0_public_key(ec)) != 0) {
                error("Authenticator returned invalid ECDSA key");
                r = SSH_ERR_KEY_INVALID_EC_VALUE;
                goto out;
        }
        if (EVP_PKEY_set1_EC_KEY(key->pkey, ec) != 1) {
                /* XXX assume it is a allocation error */
                error_f("allocation failed");
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
        key->sk_application = xstrdup(k->sk_application); /* XXX */
        if (!sshkey_equal_public(key, k)) {
                error("sshkey_equal_public failed");
                r = SSH_ERR_INVALID_ARGUMENT;
                goto out;
        }
        r = 0; /* success */
 out:
        EC_KEY_free(ec);
        free(pubkey);
        sshkey_free(key);
        sshbuf_free(b);
        return r;
}


/* copied from sshsk_ed25519_assemble() */
static int
cred_matches_key_ed25519(const fido_cred_t *cred, const struct sshkey *k)
{
        struct sshkey *key = NULL;
        const uint8_t *ptr;
        int r = -1;

        if ((ptr = fido_cred_pubkey_ptr(cred)) == NULL) {
                error_f("fido_cred_pubkey_ptr failed");
                goto out;
        }
        if (fido_cred_pubkey_len(cred) != ED25519_PK_SZ) {
                error_f("bad fido_cred_pubkey_len %zu",
                    fido_cred_pubkey_len(cred));
                goto out;
        }

        if ((key = sshkey_new(KEY_ED25519_SK)) == NULL) {
                error_f("sshkey_new failed");
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
        if ((key->ed25519_pk = malloc(ED25519_PK_SZ)) == NULL) {
                error_f("malloc failed");
                r = SSH_ERR_ALLOC_FAIL;
                goto out;
        }
        memcpy(key->ed25519_pk, ptr, ED25519_PK_SZ);
        key->sk_application = xstrdup(k->sk_application); /* XXX */
        if (!sshkey_equal_public(key, k)) {
                error("sshkey_equal_public failed");
                r = SSH_ERR_INVALID_ARGUMENT;
                goto out;
        }
        r = 0; /* success */
 out:
        sshkey_free(key);
        return r;
}

static int
cred_matches_key(const fido_cred_t *cred, const struct sshkey *k)
{
        switch (sshkey_type_plain(k->type)) {
        case KEY_ECDSA_SK:
                switch (k->ecdsa_nid) {
                case NID_X9_62_prime256v1:
                        return cred_matches_key_ecdsa(cred, k);
                        break;
                default:
                        fatal("Unsupported ECDSA key size");
                }
                break;
        case KEY_ED25519_SK:
                return cred_matches_key_ed25519(cred, k);
        default:
                error_f("key type %s not supported", sshkey_type(k));
                return -1;
        }
}

int
main(int argc, char **argv)
{
        LogLevel log_level = SYSLOG_LEVEL_INFO;
        int r, ch, credtype = -1;
        struct sshkey *k = NULL;
        struct sshbuf *attestation = NULL, *challenge = NULL;
        struct sshbuf *attestation_cert = NULL;
        char *fp;
        const char *attfmt = "packed", *style = NULL;
        fido_cred_t *cred = NULL;
        int write_attestation_cert = 0;
        extern int optind;
        /* extern char *optarg; */

        ERR_load_crypto_strings();

        sanitise_stdfd();
        log_init(__progname, log_level, SYSLOG_FACILITY_AUTH, 1);

        while ((ch = getopt(argc, argv, "UAv")) != -1) {
                switch (ch) {
                case 'U':
                        attfmt = "fido-u2f";
                        break;
                case 'A':
                        write_attestation_cert = 1;
                        break;
                case 'v':
                        if (log_level == SYSLOG_LEVEL_ERROR)
                                log_level = SYSLOG_LEVEL_DEBUG1;
                        else if (log_level < SYSLOG_LEVEL_DEBUG3)
                                log_level++;
                        break;
                default:
                        goto usage;
                }
        }
        log_init(__progname, log_level, SYSLOG_FACILITY_AUTH, 1);
        argv += optind;
        argc -= optind;

        if (argc < 3) {
 usage:
                fprintf(stderr, "usage: %s [-vAU] "
                   "pubkey challenge attestation-blob\n", __progname);
                exit(1);
        }
        if ((r = sshkey_load_public(argv[0], &k, NULL)) != 0)
                fatal_r(r, "load key %s", argv[0]);
        if ((fp = sshkey_fingerprint(k, SSH_FP_HASH_DEFAULT,
            SSH_FP_DEFAULT)) == NULL)
                fatal("sshkey_fingerprint failed");
        debug2("key %s: %s %s", argv[2], sshkey_type(k), fp);
        free(fp);
        if ((r = sshbuf_load_file(argv[1], &challenge)) != 0)
                fatal_r(r, "load challenge %s", argv[1]);
        if ((r = sshbuf_load_file(argv[2], &attestation)) != 0)
                fatal_r(r, "load attestation %s", argv[2]);
        if ((cred = fido_cred_new()) == NULL)
                fatal("fido_cred_new failed");

        switch (sshkey_type_plain(k->type)) {
        case KEY_ECDSA_SK:
                switch (k->ecdsa_nid) {
                case NID_X9_62_prime256v1:
                        credtype = COSE_ES256;
                        break;
                default:
                        fatal("Unsupported ECDSA key size");
                }
                break;
        case KEY_ED25519_SK:
                credtype = COSE_EDDSA;
                break;
        default:
                fatal("unsupported key type %s", sshkey_type(k));
        }

        if ((r = prepare_fido_cred(cred, credtype, attfmt, k->sk_application,
            attestation, challenge, &attestation_cert)) != 0)
                fatal_r(r, "prepare_fido_cred %s", argv[2]);
        if (fido_cred_x5c_ptr(cred) != NULL) {
                debug("basic attestation");
                if ((r = fido_cred_verify(cred)) != FIDO_OK)
                        fatal("basic attestation failed");
                style = "basic";
        } else {
                debug("self attestation");
                if ((r = fido_cred_verify_self(cred)) != FIDO_OK)
                        fatal("self attestation failed");
                style = "self";
        }
        if (cred_matches_key(cred, k) != 0)
                fatal("cred authdata does not match key");

        fido_cred_free(&cred);

        if (write_attestation_cert) {
                PEM_write(stdout, "CERTIFICATE", NULL,
                    sshbuf_ptr(attestation_cert), sshbuf_len(attestation_cert));
        }
        sshbuf_free(attestation_cert);

        logit("%s: verified %s attestation", argv[2], style);

        return (0);
}