ssh-sk.c

/* $OpenBSD: ssh-sk.c,v 1.23 2019/12/30 09:24:45 djm Exp $ */
/*
 * Copyright (c) 2019 Google LLC
 *
 * 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 DEBUG_SK 1 */

#include "includes.h"

#ifdef ENABLE_SK

#include <dlfcn.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>

#ifdef WITH_OPENSSL
#include <openssl/objects.h>
#include <openssl/ec.h>
#endif /* WITH_OPENSSL */

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

#include "ssh-sk.h"
#include "sk-api.h"
#include "crypto_api.h"

struct sshsk_provider {
	char *path;
	void *dlhandle;

	/* Return the version of the middleware API */
	uint32_t (*sk_api_version)(void);

	/* Enroll a U2F key (private key generation) */
	int (*sk_enroll)(int alg, const uint8_t *challenge,
	    size_t challenge_len, const char *application, uint8_t flags,
	    const char *pin, struct sk_enroll_response **enroll_response);

	/* Sign a challenge */
	int (*sk_sign)(int alg, const uint8_t *message, size_t message_len,
	    const char *application,
	    const uint8_t *key_handle, size_t key_handle_len,
	    uint8_t flags, const char *pin,
	    struct sk_sign_response **sign_response);

	/* Enumerate resident keys */
	int (*sk_load_resident_keys)(const char *pin,
	    struct sk_resident_key ***rks, size_t *nrks);
};

/* Built-in version */
int ssh_sk_enroll(int alg, const uint8_t *challenge,
    size_t challenge_len, const char *application, uint8_t flags,
    const char *pin, struct sk_enroll_response **enroll_response);
int ssh_sk_sign(int alg, const uint8_t *message, size_t message_len,
    const char *application,
    const uint8_t *key_handle, size_t key_handle_len,
    uint8_t flags, const char *pin, struct sk_sign_response **sign_response);
int ssh_sk_load_resident_keys(const char *pin,
    struct sk_resident_key ***rks, size_t *nrks);

static void
sshsk_free(struct sshsk_provider *p)
{
	if (p == NULL)
		return;
	free(p->path);
	if (p->dlhandle != NULL)
		dlclose(p->dlhandle);
	free(p);
}

static struct sshsk_provider *
sshsk_open(const char *path)
{
	struct sshsk_provider *ret = NULL;
	uint32_t version;

	if ((ret = calloc(1, sizeof(*ret))) == NULL) {
		error("%s: calloc failed", __func__);
		return NULL;
	}
	if ((ret->path = strdup(path)) == NULL) {
		error("%s: strdup failed", __func__);
		goto fail;
	}
	/* Skip the rest if we're using the linked in middleware */
	if (strcasecmp(ret->path, "internal") == 0) {
#ifdef ENABLE_SK_INTERNAL
		ret->sk_enroll = ssh_sk_enroll;
		ret->sk_sign = ssh_sk_sign;
		ret->sk_load_resident_keys = ssh_sk_load_resident_keys;
#else
		error("internal security key support not enabled");
#endif
		return ret;
	}
	if ((ret->dlhandle = dlopen(path, RTLD_NOW)) == NULL) {
		error("Security key provider \"%s\" dlopen failed: %s",
		    path, dlerror());
		goto fail;
	}
	if ((ret->sk_api_version = dlsym(ret->dlhandle,
	    "sk_api_version")) == NULL) {
		error("Security key provider \"%s\" dlsym(sk_api_version) "
		    "failed: %s", path, dlerror());
		goto fail;
	}
	version = ret->sk_api_version();
	debug("%s: provider %s implements version 0x%08lx", __func__,
	    ret->path, (u_long)version);
	if ((version & SSH_SK_VERSION_MAJOR_MASK) != SSH_SK_VERSION_MAJOR) {
		error("Security key provider \"%s\" implements unsupported "
		    "version 0x%08lx (supported: 0x%08lx)",
		    path, (u_long)version, (u_long)SSH_SK_VERSION_MAJOR);
		goto fail;
	}
	if ((ret->sk_enroll = dlsym(ret->dlhandle, "sk_enroll")) == NULL) {
		error("Security key  provider %s dlsym(sk_enroll) "
		    "failed: %s", path, dlerror());
		goto fail;
	}
	if ((ret->sk_sign = dlsym(ret->dlhandle, "sk_sign")) == NULL) {
		error("Security key provider \"%s\" dlsym(sk_sign) failed: %s",
		    path, dlerror());
		goto fail;
	}
	if ((ret->sk_load_resident_keys = dlsym(ret->dlhandle,
	    "sk_load_resident_keys")) == NULL) {
		error("Security key provider \"%s\" "
		    "dlsym(sk_load_resident_keys) failed: %s", path, dlerror());
		goto fail;
	}
	/* success */
	return ret;
fail:
	sshsk_free(ret);
	return NULL;
}

static void
sshsk_free_enroll_response(struct sk_enroll_response *r)
{
	if (r == NULL)
		return;
	freezero(r->key_handle, r->key_handle_len);
	freezero(r->public_key, r->public_key_len);
	freezero(r->signature, r->signature_len);
	freezero(r->attestation_cert, r->attestation_cert_len);
	freezero(r, sizeof(*r));
}

static void
sshsk_free_sign_response(struct sk_sign_response *r)
{
	if (r == NULL)
		return;
	freezero(r->sig_r, r->sig_r_len);
	freezero(r->sig_s, r->sig_s_len);
	freezero(r, sizeof(*r));
}

#ifdef WITH_OPENSSL
/* Assemble key from response */
static int
sshsk_ecdsa_assemble(struct sk_enroll_response *resp, struct sshkey **keyp)
{
	struct sshkey *key = NULL;
	struct sshbuf *b = NULL;
	EC_POINT *q = NULL;
	int r;

	*keyp = NULL;
	if ((key = sshkey_new(KEY_ECDSA_SK)) == NULL) {
		error("%s: sshkey_new failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	key->ecdsa_nid = NID_X9_62_prime256v1;
	if ((key->ecdsa = EC_KEY_new_by_curve_name(key->ecdsa_nid)) == NULL ||
	    (q = EC_POINT_new(EC_KEY_get0_group(key->ecdsa))) == NULL ||
	    (b = sshbuf_new()) == NULL) {
		error("%s: allocation failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshbuf_put_string(b,
	    resp->public_key, resp->public_key_len)) != 0) {
		error("%s: buffer error: %s", __func__, ssh_err(r));
		goto out;
	}
	if ((r = sshbuf_get_ec(b, q, EC_KEY_get0_group(key->ecdsa))) != 0) {
		error("%s: parse key: %s", __func__, ssh_err(r));
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if (sshkey_ec_validate_public(EC_KEY_get0_group(key->ecdsa), q) != 0) {
		error("Security key returned invalid ECDSA key");
		r = SSH_ERR_KEY_INVALID_EC_VALUE;
		goto out;
	}
	if (EC_KEY_set_public_key(key->ecdsa, q) != 1) {
		/* XXX assume it is a allocation error */
		error("%s: allocation failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	/* success */
	*keyp = key;
	key = NULL; /* transferred */
	r = 0;
 out:
	EC_POINT_free(q);
	sshkey_free(key);
	sshbuf_free(b);
	return r;
}
#endif /* WITH_OPENSSL */

static int
sshsk_ed25519_assemble(struct sk_enroll_response *resp, struct sshkey **keyp)
{
	struct sshkey *key = NULL;
	int r;

	*keyp = NULL;
	if (resp->public_key_len != ED25519_PK_SZ) {
		error("%s: invalid size: %zu", __func__, resp->public_key_len);
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if ((key = sshkey_new(KEY_ED25519_SK)) == NULL) {
		error("%s: sshkey_new failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((key->ed25519_pk = malloc(ED25519_PK_SZ)) == NULL) {
		error("%s: malloc failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	memcpy(key->ed25519_pk, resp->public_key, ED25519_PK_SZ);
	/* success */
	*keyp = key;
	key = NULL; /* transferred */
	r = 0;
 out:
	sshkey_free(key);
	return r;
}

static int
sshsk_key_from_response(int alg, const char *application, uint8_t flags,
    struct sk_enroll_response *resp, struct sshkey **keyp)
{
	struct sshkey *key = NULL;
	int r = SSH_ERR_INTERNAL_ERROR;

	*keyp = NULL;

	/* Check response validity */
	if (resp->public_key == NULL || resp->key_handle == NULL) {
		error("%s: sk_enroll response invalid", __func__);
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	switch (alg) {
#ifdef WITH_OPENSSL
	case SSH_SK_ECDSA:
		if ((r = sshsk_ecdsa_assemble(resp, &key)) != 0)
			goto out;
		break;
#endif /* WITH_OPENSSL */
	case SSH_SK_ED25519:
		if ((r = sshsk_ed25519_assemble(resp, &key)) != 0)
			goto out;
		break;
	default:
		error("%s: unsupported algorithm %d", __func__, alg);
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	key->sk_flags = flags;
	if ((key->sk_key_handle = sshbuf_new()) == NULL ||
	    (key->sk_reserved = sshbuf_new()) == NULL) {
		error("%s: allocation failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((key->sk_application = strdup(application)) == NULL) {
		error("%s: strdup application failed", __func__);
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshbuf_put(key->sk_key_handle, resp->key_handle,
	    resp->key_handle_len)) != 0) {
		error("%s: buffer error: %s", __func__, ssh_err(r));
		goto out;
	}
	/* success */
	r = 0;
	*keyp = key;
	key = NULL;
 out:
	sshkey_free(key);
	return r;
}

static int
skerr_to_ssherr(int skerr)
{
	switch (skerr) {
	case SSH_SK_ERR_UNSUPPORTED:
		return SSH_ERR_FEATURE_UNSUPPORTED;
	case SSH_SK_ERR_PIN_REQUIRED:
		return SSH_ERR_KEY_WRONG_PASSPHRASE;
	case SSH_SK_ERR_GENERAL:
	default:
		return SSH_ERR_INVALID_FORMAT;
	}
}

int
sshsk_enroll(int type, const char *provider_path, const char *application,
    uint8_t flags, const char *pin, struct sshbuf *challenge_buf,
    struct sshkey **keyp, struct sshbuf *attest)
{
	struct sshsk_provider *skp = NULL;
	struct sshkey *key = NULL;
	u_char randchall[32];
	const u_char *challenge;
	size_t challenge_len;
	struct sk_enroll_response *resp = NULL;
	int r = SSH_ERR_INTERNAL_ERROR;
	int alg;

	debug("%s: provider \"%s\", application \"%s\", flags 0x%02x, "
	    "challenge len %zu%s", __func__, provider_path, application,
	    flags, challenge_buf == NULL ? 0 : sshbuf_len(challenge_buf),
	    (pin != NULL && *pin != '\0') ? " with-pin" : "");

	*keyp = NULL;
	if (attest)
		sshbuf_reset(attest);
	switch (type) {
#ifdef WITH_OPENSSL
	case KEY_ECDSA_SK:
		alg = SSH_SK_ECDSA;
		break;
#endif /* WITH_OPENSSL */
	case KEY_ED25519_SK:
		alg = SSH_SK_ED25519;
		break;
	default:
		error("%s: unsupported key type", __func__);
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if (provider_path == NULL) {
		error("%s: missing provider", __func__);
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if (application == NULL || *application == '\0') {
		error("%s: missing application", __func__);
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if (challenge_buf == NULL) {
		debug("%s: using random challenge", __func__);
		arc4random_buf(randchall, sizeof(randchall));
		challenge = randchall;
		challenge_len = sizeof(randchall);
	} else if (sshbuf_len(challenge_buf) == 0) {
		error("Missing enrollment challenge");
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	} else {
		challenge = sshbuf_ptr(challenge_buf);
		challenge_len = sshbuf_len(challenge_buf);
		debug3("%s: using explicit challenge len=%zd",
		    __func__, challenge_len);
	}
	if ((skp = sshsk_open(provider_path)) == NULL) {
		r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
		goto out;
	}
	/* XXX validate flags? */
	/* enroll key */
	if ((r = skp->sk_enroll(alg, challenge, challenge_len, application,
	    flags, pin, &resp)) != 0) {
		error("Security key provider \"%s\" returned failure %d",
		    provider_path, r);
		r = skerr_to_ssherr(r);
		goto out;
	}

	if ((r = sshsk_key_from_response(alg, application, flags,
	    resp, &key)) != 0)
		goto out;

	/* Optionally fill in the attestation information */
	if (attest != NULL) {
		if ((r = sshbuf_put_cstring(attest, "sk-attest-v00")) != 0 ||
		    (r = sshbuf_put_u32(attest, 1)) != 0 || /* XXX U2F ver */
		    (r = sshbuf_put_string(attest,
		    resp->attestation_cert, resp->attestation_cert_len)) != 0 ||
		    (r = sshbuf_put_string(attest,
		    resp->signature, resp->signature_len)) != 0 ||
		    (r = sshbuf_put_u32(attest, flags)) != 0 || /* XXX right? */
		    (r = sshbuf_put_string(attest, NULL, 0)) != 0) {
			error("%s: buffer error: %s", __func__, ssh_err(r));
			goto out;
		}
	}
	/* success */
	*keyp = key;
	key = NULL; /* transferred */
	r = 0;
 out:
	sshsk_free(skp);
	sshkey_free(key);
	sshsk_free_enroll_response(resp);
	explicit_bzero(randchall, sizeof(randchall));
	return r;
}

#ifdef WITH_OPENSSL
static int
sshsk_ecdsa_sig(struct sk_sign_response *resp, struct sshbuf *sig)
{
	struct sshbuf *inner_sig = NULL;
	int r = SSH_ERR_INTERNAL_ERROR;

	/* Check response validity */
	if (resp->sig_r == NULL || resp->sig_s == NULL) {
		error("%s: sk_sign response invalid", __func__);
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if ((inner_sig = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	/* Prepare and append inner signature object */
	if ((r = sshbuf_put_bignum2_bytes(inner_sig,
	    resp->sig_r, resp->sig_r_len)) != 0 ||
	    (r = sshbuf_put_bignum2_bytes(inner_sig,
	    resp->sig_s, resp->sig_s_len)) != 0) {
		debug("%s: buffer error: %s", __func__, ssh_err(r));
		goto out;
	}
	if ((r = sshbuf_put_stringb(sig, inner_sig)) != 0 ||
	    (r = sshbuf_put_u8(sig, resp->flags)) != 0 ||
	    (r = sshbuf_put_u32(sig, resp->counter)) != 0) {
		debug("%s: buffer error: %s", __func__, ssh_err(r));
		goto out;
	}
#ifdef DEBUG_SK
	fprintf(stderr, "%s: sig_r:\n", __func__);
	sshbuf_dump_data(resp->sig_r, resp->sig_r_len, stderr);
	fprintf(stderr, "%s: sig_s:\n", __func__);
	sshbuf_dump_data(resp->sig_s, resp->sig_s_len, stderr);
	fprintf(stderr, "%s: inner:\n", __func__);
	sshbuf_dump(inner_sig, stderr);
#endif
	r = 0;
 out:
	sshbuf_free(inner_sig);
	return r;
}
#endif /* WITH_OPENSSL */

static int
sshsk_ed25519_sig(struct sk_sign_response *resp, struct sshbuf *sig)
{
	int r = SSH_ERR_INTERNAL_ERROR;

	/* Check response validity */
	if (resp->sig_r == NULL) {
		error("%s: sk_sign response invalid", __func__);
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if ((r = sshbuf_put_string(sig,
	    resp->sig_r, resp->sig_r_len)) != 0 ||
	    (r = sshbuf_put_u8(sig, resp->flags)) != 0 ||
	    (r = sshbuf_put_u32(sig, resp->counter)) != 0) {
		debug("%s: buffer error: %s", __func__, ssh_err(r));
		goto out;
	}
#ifdef DEBUG_SK
	fprintf(stderr, "%s: sig_r:\n", __func__);
	sshbuf_dump_data(resp->sig_r, resp->sig_r_len, stderr);
#endif
	r = 0;
 out:
	return 0;
}

int
sshsk_sign(const char *provider_path, struct sshkey *key,
    u_char **sigp, size_t *lenp, const u_char *data, size_t datalen,
    u_int compat, const char *pin)
{
	struct sshsk_provider *skp = NULL;
	int r = SSH_ERR_INTERNAL_ERROR;
	int type, alg;
	struct sk_sign_response *resp = NULL;
	struct sshbuf *inner_sig = NULL, *sig = NULL;
	uint8_t message[32];

	debug("%s: provider \"%s\", key %s, flags 0x%02x%s", __func__,
	    provider_path, sshkey_type(key), key->sk_flags,
	    (pin != NULL && *pin != '\0') ? " with-pin" : "");

	if (sigp != NULL)
		*sigp = NULL;
	if (lenp != NULL)
		*lenp = 0;
	type = sshkey_type_plain(key->type);
	switch (type) {
#ifdef WITH_OPENSSL
	case KEY_ECDSA_SK:
		alg = SSH_SK_ECDSA;
		break;
#endif /* WITH_OPENSSL */
	case KEY_ED25519_SK:
		alg = SSH_SK_ED25519;
		break;
	default:
		return SSH_ERR_INVALID_ARGUMENT;
	}
	if (provider_path == NULL ||
	    key->sk_key_handle == NULL ||
	    key->sk_application == NULL || *key->sk_application == '\0') {
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if ((skp = sshsk_open(provider_path)) == NULL) {
		r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
		goto out;
	}

	/* hash data to be signed before it goes to the security key */
	if ((r = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
	    message, sizeof(message))) != 0) {
		error("%s: hash application failed: %s", __func__, ssh_err(r));
		r = SSH_ERR_INTERNAL_ERROR;
		goto out;
	}
	if ((r = skp->sk_sign(alg, message, sizeof(message),
	    key->sk_application,
	    sshbuf_ptr(key->sk_key_handle), sshbuf_len(key->sk_key_handle),
	    key->sk_flags, pin, &resp)) != 0) {
		debug("%s: sk_sign failed with code %d", __func__, r);
		r = skerr_to_ssherr(r);
		goto out;
	}
	/* Assemble signature */
	if ((sig = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshbuf_put_cstring(sig, sshkey_ssh_name_plain(key))) != 0) {
		debug("%s: buffer error (outer): %s", __func__, ssh_err(r));
		goto out;
	}
	switch (type) {
#ifdef WITH_OPENSSL
	case KEY_ECDSA_SK:
		if ((r = sshsk_ecdsa_sig(resp, sig)) != 0)
			goto out;
		break;
#endif /* WITH_OPENSSL */
	case KEY_ED25519_SK:
		if ((r = sshsk_ed25519_sig(resp, sig)) != 0)
			goto out;
		break;
	}
#ifdef DEBUG_SK
	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
	    __func__, resp->flags, resp->counter);
	fprintf(stderr, "%s: hashed message:\n", __func__);
	sshbuf_dump_data(message, sizeof(message), stderr);
	fprintf(stderr, "%s: sigbuf:\n", __func__);
	sshbuf_dump(sig, stderr);
#endif
	if (sigp != NULL) {
		if ((*sigp = malloc(sshbuf_len(sig))) == NULL) {
			r = SSH_ERR_ALLOC_FAIL;
			goto out;
		}
		memcpy(*sigp, sshbuf_ptr(sig), sshbuf_len(sig));
	}
	if (lenp != NULL)
		*lenp = sshbuf_len(sig);
	/* success */
	r = 0;
 out:
	explicit_bzero(message, sizeof(message));
	sshsk_free(skp);
	sshsk_free_sign_response(resp);
	sshbuf_free(sig);
	sshbuf_free(inner_sig);
	return r;
}

static void
sshsk_free_sk_resident_keys(struct sk_resident_key **rks, size_t nrks)
{
	size_t i;

	if (nrks == 0 || rks == NULL)
		return;
	for (i = 0; i < nrks; i++) {
		free(rks[i]->application);
		freezero(rks[i]->key.key_handle, rks[i]->key.key_handle_len);
		freezero(rks[i]->key.public_key, rks[i]->key.public_key_len);
		freezero(rks[i]->key.signature, rks[i]->key.signature_len);
		freezero(rks[i]->key.attestation_cert,
		    rks[i]->key.attestation_cert_len);
		freezero(rks[i], sizeof(**rks));
	}
	free(rks);
}

int
sshsk_load_resident(const char *provider_path, const char *pin,
    struct sshkey ***keysp, size_t *nkeysp)
{
	struct sshsk_provider *skp = NULL;
	int r = SSH_ERR_INTERNAL_ERROR;
	struct sk_resident_key **rks = NULL;
	size_t i, nrks = 0, nkeys = 0;
	struct sshkey *key = NULL, **keys = NULL, **tmp;
	uint8_t flags;

	debug("%s: provider \"%s\"%s", __func__, provider_path,
	    (pin != NULL && *pin != '\0') ? ", have-pin": "");

	if (keysp == NULL || nkeysp == NULL)
		return SSH_ERR_INVALID_ARGUMENT;
	*keysp = NULL;
	*nkeysp = 0;

	if ((skp = sshsk_open(provider_path)) == NULL) {
		r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
		goto out;
	}
	if ((r = skp->sk_load_resident_keys(pin, &rks, &nrks)) != 0) {
		error("Security key provider \"%s\" returned failure %d",
		    provider_path, r);
		r = skerr_to_ssherr(r);
		goto out;
	}
	for (i = 0; i < nrks; i++) {
		debug3("%s: rk %zu: slot = %zu, alg = %d, application = \"%s\"",
		    __func__, i, rks[i]->slot, rks[i]->alg,
		    rks[i]->application);
		/* XXX need better filter here */
		if (strncmp(rks[i]->application, "ssh:", 4) != 0)
			continue;
		switch (rks[i]->alg) {
		case SSH_SK_ECDSA:
		case SSH_SK_ED25519:
			break;
		default:
			continue;
		}
		/* XXX where to get flags? */
		flags = SSH_SK_USER_PRESENCE_REQD|SSH_SK_RESIDENT_KEY;
		if ((r = sshsk_key_from_response(rks[i]->alg,
		    rks[i]->application, flags, &rks[i]->key, &key)) != 0)
			goto out;
		if ((tmp = recallocarray(keys, nkeys, nkeys + 1,
		    sizeof(*tmp))) == NULL) {
			error("%s: recallocarray failed", __func__);
			r = SSH_ERR_ALLOC_FAIL;
			goto out;
		}
		keys = tmp;
		keys[nkeys++] = key;
		key = NULL;
		/* XXX synthesise comment */
	}
	/* success */
	*keysp = keys;
	*nkeysp = nkeys;
	keys = NULL;
	nkeys = 0;
	r = 0;
 out:
	sshsk_free(skp);
	sshsk_free_sk_resident_keys(rks, nrks);
	sshkey_free(key);
	if (nkeys != 0) {
		for (i = 0; i < nkeys; i++)
			sshkey_free(keys[i]);
		free(keys);
	}
	return r;
}

#endif /* ENABLE_SK */