trezor-firmware/firmware/u2f.c

/*
 * This file is part of the TREZOR project.
 *
 * Copyright (C) 2015 Mark Bryars <mbryars@google.com>
 *
 * This library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library.  If not, see <http://www.gnu.org/licenses/>.
 */


#include <string.h>
#include <ecdsa.h>

#include "debug.h"
#include "storage.h"
#include "bip32.h"
#include "layout2.h"
#include "usb.h"
#include "buttons.h"
#include "trezor.h"
#include "curves.h"
#include "nist256p1.h"
#include "rng.h"
#include "hmac.h"
#include "util.h"
#include "macros.h"

#include "u2f/u2f.h"
#include "u2f/u2f_hid.h"
#include "u2f/u2f_keys.h"
#include "u2f_knownapps.h"
#include "u2f.h"

#define MIN(a, b) (((a) < (b)) ? (a) : (b))

// About 1/2 Second according to values used in protect.c
#define U2F_TIMEOUT (800000/2)
#define U2F_OUT_PKT_BUFFER_LEN 128

// Initialise without a cid
static uint32_t cid = 0;

// Circular Output buffer
static uint32_t u2f_out_start = 0;
static uint32_t u2f_out_end = 0;
static uint8_t u2f_out_packets[U2F_OUT_PKT_BUFFER_LEN][HID_RPT_SIZE];

#define U2F_PUBKEY_LEN 65
#define KEY_PATH_LEN 32
#define KEY_HANDLE_LEN (KEY_PATH_LEN + SHA256_DIGEST_LENGTH)

// Derivation path is m/U2F'/r'/r'/r'/r'/r'/r'/r'/r'
#define KEY_PATH_ENTRIES (1 + KEY_PATH_LEN / sizeof(uint32_t))

// Defined as UsbSignHandler.BOGUS_APP_ID_HASH
// in https://github.com/google/u2f-ref-code/blob/master/u2f-chrome-extension/usbsignhandler.js#L118
#define BOGUS_APPID "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"

// Auth/Register request state machine
typedef enum {
	INIT = 0,
	AUTH = 10,
	AUTH_PASS = 11,
	REG = 20,
	REG_PASS = 21
} U2F_STATE;

static U2F_STATE last_req_state = INIT;

typedef struct {
	uint8_t reserved;
	uint8_t appId[U2F_APPID_SIZE];
	uint8_t chal[U2F_CHAL_SIZE];
	uint8_t keyHandle[KEY_HANDLE_LEN];
	uint8_t pubKey[U2F_PUBKEY_LEN];
} U2F_REGISTER_SIG_STR;

typedef struct {
	uint8_t appId[U2F_APPID_SIZE];
	uint8_t flags;
	uint8_t ctr[4];
	uint8_t chal[U2F_CHAL_SIZE];
} U2F_AUTHENTICATE_SIG_STR;


#if DEBUG_LOG
char *debugInt(const uint32_t i)
{
	static uint8_t n = 0;
	static char id[8][9];
	uint32hex(i, id[n]);
	debugLog(0, "", id[n]);
	char *ret = (char *)id[n];
	n = (n + 1) % 8;
	return ret;
}
#else
#define debugInt(I) do{}while(0)
#endif

static uint32_t dialog_timeout = 0;

uint32_t next_cid(void)
{
	// extremely unlikely but hey
	do {
		cid = random32();
	} while (cid == 0 || cid == CID_BROADCAST);
	return cid;
}

typedef struct {
	uint8_t buf[57+127*59];
	uint8_t *buf_ptr;
	uint32_t len;
	uint8_t seq;
	uint8_t cmd;
} U2F_ReadBuffer;

U2F_ReadBuffer *reader;

void u2fhid_read(char tiny, const U2FHID_FRAME *f)
{
	// Always handle init packets directly
	if (f->init.cmd == U2FHID_INIT) {
		u2fhid_init(f);
		if (tiny && reader && f->cid == cid) {
			// abort current channel
			reader->cmd = 0;
			reader->len = 0;
			reader->seq = 255;
		}
		return;
	}

	if (tiny) {
		// read continue packet
		if (reader == 0 || cid != f->cid) {
			send_u2fhid_error(f->cid, ERR_CHANNEL_BUSY);
			return;
		}

		if ((f->type & TYPE_INIT) && reader->seq == 255) {
			u2fhid_init_cmd(f);
			return;
		}

		if (reader->seq != f->cont.seq) {
			send_u2fhid_error(f->cid, ERR_INVALID_SEQ);
			reader->cmd = 0;
			reader->len = 0;
			reader->seq = 255;
			return;
		}

		// check out of bounds
		if ((reader->buf_ptr - reader->buf) >= (signed) reader->len
			|| (reader->buf_ptr + sizeof(f->cont.data) - reader->buf) > (signed) sizeof(reader->buf))
			return;
		reader->seq++;
		memcpy(reader->buf_ptr, f->cont.data, sizeof(f->cont.data));
		reader->buf_ptr += sizeof(f->cont.data);
		return;
	}

	u2fhid_read_start(f);
}

void u2fhid_init_cmd(const U2FHID_FRAME *f) {
	reader->seq = 0;
	reader->buf_ptr = reader->buf;
	reader->len = MSG_LEN(*f);
	reader->cmd = f->type;
	memcpy(reader->buf_ptr, f->init.data, sizeof(f->init.data));
	reader->buf_ptr += sizeof(f->init.data);
	cid = f->cid;
}

void u2fhid_read_start(const U2FHID_FRAME *f) {
	U2F_ReadBuffer readbuffer;
	if (!(f->type & TYPE_INIT)) {
		return;
	}

	// Broadcast is reserved for init
	if (f->cid == CID_BROADCAST || f->cid == 0) {
		send_u2fhid_error(f->cid, ERR_INVALID_CID);
		return;
	}

	if ((unsigned)MSG_LEN(*f) > sizeof(reader->buf)) {
		send_u2fhid_error(f->cid, ERR_INVALID_LEN);
		return;
	}

	reader = &readbuffer;
	u2fhid_init_cmd(f);

	usbTiny(1);
	for(;;) {
		// Do we need to wait for more data
		while ((reader->buf_ptr - reader->buf) < (signed)reader->len) {
			uint8_t lastseq = reader->seq;
			uint8_t lastcmd = reader->cmd;
			int counter = U2F_TIMEOUT;
			while (reader->seq == lastseq && reader->cmd == lastcmd) {
				if (counter-- == 0) {
					// timeout
					send_u2fhid_error(cid, ERR_MSG_TIMEOUT);
					cid = 0;
					reader = 0;
					usbTiny(0);
					layoutHome();
					return;
				}
				usbPoll();
			}
		}

		// We have all the data
		switch (reader->cmd) {
		case 0:
			// message was aborted by init
			break;
		case U2FHID_PING:
			u2fhid_ping(reader->buf, reader->len);
			break;
		case U2FHID_MSG:
			u2fhid_msg((APDU *)reader->buf, reader->len);
			break;
		case U2FHID_WINK:
			u2fhid_wink(reader->buf, reader->len);
			break;
		default:
			send_u2fhid_error(cid, ERR_INVALID_CMD);
			break;
		}

		// wait for next commmand/ button press
		reader->cmd = 0;
		reader->seq = 255;
		while (dialog_timeout > 0 && reader->cmd == 0) {
			dialog_timeout--;
			usbPoll(); // may trigger new request
			buttonUpdate();
			if (button.YesUp &&
				(last_req_state == AUTH || last_req_state == REG)) {
				last_req_state++;
				// standard requires to remember button press for 10 seconds.
				dialog_timeout = 10 * U2F_TIMEOUT;
			}
		}

		if (reader->cmd == 0) {
			last_req_state = INIT;
			cid = 0;
			reader = 0;
			usbTiny(0);
			layoutHome();
			return;
		}
	}
}

void u2fhid_ping(const uint8_t *buf, uint32_t len)
{
	debugLog(0, "", "u2fhid_ping");
	send_u2fhid_msg(U2FHID_PING, buf, len);
}

void u2fhid_wink(const uint8_t *buf, uint32_t len)
{
	debugLog(0, "", "u2fhid_wink");
	(void)buf;

	if (len > 0)
		return send_u2fhid_error(cid, ERR_INVALID_LEN);

	if (dialog_timeout > 0)
		dialog_timeout = U2F_TIMEOUT;

	U2FHID_FRAME f;
	MEMSET_BZERO(&f, sizeof(f));
	f.cid = cid;
	f.init.cmd = U2FHID_WINK;
	f.init.bcntl = 0;
	queue_u2f_pkt(&f);
}

void u2fhid_init(const U2FHID_FRAME *in)
{
	const U2FHID_INIT_REQ *init_req = (const U2FHID_INIT_REQ *)&in->init.data;
	U2FHID_FRAME f;
	U2FHID_INIT_RESP resp;

	debugLog(0, "", "u2fhid_init");

	if (in->cid == 0) {
		send_u2fhid_error(in->cid, ERR_INVALID_CID);
		return;
	}

	MEMSET_BZERO(&f, sizeof(f));
	f.cid = in->cid;
	f.init.cmd = U2FHID_INIT;
	f.init.bcnth = 0;
	f.init.bcntl = U2FHID_INIT_RESP_SIZE;

	memcpy(resp.nonce, init_req->nonce, sizeof(init_req->nonce));
	resp.cid = in->cid == CID_BROADCAST ? next_cid() : in->cid;
	resp.versionInterface = U2FHID_IF_VERSION;
	resp.versionMajor = VERSION_MAJOR;
	resp.versionMinor = VERSION_MINOR;
	resp.versionBuild = VERSION_PATCH;
	resp.capFlags = CAPFLAG_WINK;
	memcpy(&f.init.data, &resp, sizeof(resp));

	queue_u2f_pkt(&f);
}

void queue_u2f_pkt(const U2FHID_FRAME *u2f_pkt)
{
	// debugLog(0, "", "u2f_write_pkt");
	uint32_t next = (u2f_out_end + 1) % U2F_OUT_PKT_BUFFER_LEN;
	if (u2f_out_start == next) {
		debugLog(0, "", "u2f_write_pkt full");
		return; // Buffer full :(
	}
	memcpy(u2f_out_packets[u2f_out_end], u2f_pkt, HID_RPT_SIZE);
	u2f_out_end = next;
}

uint8_t *u2f_out_data(void)
{
	if (u2f_out_start == u2f_out_end)
		return NULL; // No data
	// debugLog(0, "", "u2f_out_data");
	uint32_t t = u2f_out_start;
	u2f_out_start = (u2f_out_start + 1) % U2F_OUT_PKT_BUFFER_LEN;
	return u2f_out_packets[t];
}

void u2fhid_msg(const APDU *a, uint32_t len)
{
	if ((APDU_LEN(*a) + sizeof(APDU)) > len) {
		debugLog(0, "", "BAD APDU LENGTH");
		debugInt(APDU_LEN(*a));
		debugInt(len);
		return;
	}

	if (a->cla != 0) {
		send_u2f_error(U2F_SW_CLA_NOT_SUPPORTED);
		return;
	}

	switch (a->ins) {
		case U2F_REGISTER:
			u2f_register(a);
			break;
		case U2F_AUTHENTICATE:
			u2f_authenticate(a);
			break;
		case U2F_VERSION:
			u2f_version(a);
			break;
		default:
			debugLog(0, "", "u2f unknown cmd");
			send_u2f_error(U2F_SW_INS_NOT_SUPPORTED);
	}
}

void send_u2fhid_msg(const uint8_t cmd, const uint8_t *data, const uint32_t len)
{
	U2FHID_FRAME f;
	uint8_t *p = (uint8_t *)data;
	uint32_t l = len;
	uint32_t psz;
	uint8_t seq = 0;

	// debugLog(0, "", "send_u2fhid_msg");

	MEMSET_BZERO(&f, sizeof(f));
	f.cid = cid;
	f.init.cmd = cmd;
	f.init.bcnth = len >> 8;
	f.init.bcntl = len & 0xff;

	// Init packet
	psz = MIN(sizeof(f.init.data), l);
	memcpy(f.init.data, p, psz);
	queue_u2f_pkt(&f);
	l -= psz;
	p += psz;

	// Cont packet(s)
	for (; l > 0; l -= psz, p += psz) {
		// debugLog(0, "", "send_u2fhid_msg con");
		MEMSET_BZERO(&f.cont.data, sizeof(f.cont.data));
		f.cont.seq = seq++;
		psz = MIN(sizeof(f.cont.data), l);
		memcpy(f.cont.data, p, psz);
		queue_u2f_pkt(&f);
	}

	if (data + len != p) {
		debugLog(0, "", "send_u2fhid_msg is bad");
		debugInt(data + len - p);
	}
}

void send_u2fhid_error(uint32_t fcid, uint8_t err)
{
	U2FHID_FRAME f;

	MEMSET_BZERO(&f, sizeof(f));
	f.cid = fcid;
	f.init.cmd = U2FHID_ERROR;
	f.init.bcntl = 1;
	f.init.data[0] = err;
	queue_u2f_pkt(&f);
}

void u2f_version(const APDU *a)
{
	if (APDU_LEN(*a) != 0) {
		debugLog(0, "", "u2f version - badlen");
		send_u2f_error(U2F_SW_WRONG_LENGTH);
		return;
	}

	// INCLUDES SW_NO_ERROR
	static const uint8_t version_response[] = {'U', '2', 'F',  '_',
						   'V', '2', 0x90, 0x00};
	debugLog(0, "", "u2f version");
	send_u2f_msg(version_response, sizeof(version_response));
}

void getReadableAppId(const uint8_t appid[U2F_APPID_SIZE], const char **appname, const BITMAP **appicon) {
	unsigned int i;
	static char buf[8+2+8+1];

	for (i = 0; i < sizeof(u2f_well_known)/sizeof(U2FWellKnown); i++) {
		if (memcmp(appid, u2f_well_known[i].appid, U2F_APPID_SIZE) == 0) {
			*appname = u2f_well_known[i].appname;
			*appicon = u2f_well_known[i].appicon;
			return;
		}
	}

	data2hex(appid, 4, &buf[0]);
	buf[8] = buf[9] = '.';
	data2hex(appid + (U2F_APPID_SIZE - 4), 4, &buf[10]);
	*appname = buf;
	*appicon = NULL;
}

const HDNode *getDerivedNode(uint32_t *address_n, size_t address_n_count)
{
	static HDNode node;
	if (!storage_getRootNode(&node, NIST256P1_NAME, false)) {
		layoutHome();
		debugLog(0, "", "ERR: Device not init");
		return 0;
	}
	if (!address_n || address_n_count == 0) {
		return &node;
	}
	if (hdnode_private_ckd_cached(&node, address_n, address_n_count) == 0) {
		layoutHome();
		debugLog(0, "", "ERR: Derive private failed");
		return 0;
	}
	return &node;
}

const HDNode *generateKeyHandle(const uint8_t app_id[], uint8_t key_handle[])
{
	uint8_t keybase[U2F_APPID_SIZE + KEY_PATH_LEN];

	// Derivation path is m/U2F'/r'/r'/r'/r'/r'/r'/r'/r'
	uint32_t i, key_path[KEY_PATH_ENTRIES];
	key_path[0] = U2F_KEY_PATH;
	for (i = 1; i < KEY_PATH_ENTRIES; i++) {
		// high bit for hardened keys
		key_path[i]= 0x80000000 | random32();
	}

	// First half of keyhandle is key_path
	memcpy(key_handle, &key_path[1], KEY_PATH_LEN);

	// prepare keypair from /random data
	const HDNode *node = getDerivedNode(key_path, KEY_PATH_ENTRIES);
	if (!node)
		return NULL;

	// For second half of keyhandle
	// Signature of app_id and random data
	memcpy(&keybase[0], app_id, U2F_APPID_SIZE);
	memcpy(&keybase[U2F_APPID_SIZE], key_handle, KEY_PATH_LEN);
	hmac_sha256(node->private_key, sizeof(node->private_key),
					keybase, sizeof(keybase), &key_handle[KEY_PATH_LEN]);

	// Done!
	return node;
}


const HDNode *validateKeyHandle(const uint8_t app_id[], const uint8_t key_handle[])
{
	uint32_t key_path[KEY_PATH_ENTRIES];
	key_path[0] = U2F_KEY_PATH;
	memcpy(&key_path[1], key_handle, KEY_PATH_LEN);

	const HDNode *node = getDerivedNode(key_path, KEY_PATH_ENTRIES);
	if (!node)
		return NULL;

	uint8_t keybase[U2F_APPID_SIZE + KEY_PATH_LEN];
	memcpy(&keybase[0], app_id, U2F_APPID_SIZE);
	memcpy(&keybase[U2F_APPID_SIZE], key_handle, KEY_PATH_LEN);


	uint8_t hmac[SHA256_DIGEST_LENGTH];
	hmac_sha256(node->private_key, sizeof(node->private_key),
				keybase, sizeof(keybase), hmac);

	if (memcmp(&key_handle[KEY_PATH_LEN], hmac, SHA256_DIGEST_LENGTH) != 0)
		return NULL;

	// Done!
	return node;
}


void u2f_register(const APDU *a)
{
	static U2F_REGISTER_REQ last_req;
	const U2F_REGISTER_REQ *req = (U2F_REGISTER_REQ *)a->data;

	if (!storage_isInitialized()) {
		send_u2f_error(U2F_SW_CONDITIONS_NOT_SATISFIED);
		return;
	}

	// Validate basic request parameters
	debugLog(0, "", "u2f register");
	if (APDU_LEN(*a) != sizeof(U2F_REGISTER_REQ)) {
		debugLog(0, "", "u2f register - badlen");
		send_u2f_error(U2F_SW_WRONG_LENGTH);
		return;
	}

	// If this request is different from last request, reset state machine
	if (memcmp(&last_req, req, sizeof(last_req)) != 0) {
		memcpy(&last_req, req, sizeof(last_req));
		last_req_state = INIT;
	}

	// First Time request, return not present and display request dialog
	if (last_req_state == INIT) {
		// wake up crypto system to be ready for signing
		getDerivedNode(NULL, 0);
		// error: testof-user-presence is required
		buttonUpdate(); // Clear button state
		if (0 == memcmp(req->appId, BOGUS_APPID, U2F_APPID_SIZE)) {
			layoutDialog(&bmp_icon_warning, NULL, "OK", NULL, "Another U2F device", "was used to register", "in this application.", NULL, NULL, NULL);
		} else {
			const char *appname;
			const BITMAP *appicon;
			getReadableAppId(req->appId, &appname, &appicon);
			layoutU2FDialog("Register", appname, appicon);
		}
		last_req_state = REG;
	}

	// Still awaiting Keypress
	if (last_req_state == REG) {
		// error: testof-user-presence is required
		send_u2f_error(U2F_SW_CONDITIONS_NOT_SATISFIED);
		dialog_timeout = U2F_TIMEOUT;
		return;
	}

	// Buttons said yes
	if (last_req_state == REG_PASS) {
		uint8_t data[sizeof(U2F_REGISTER_RESP) + 2];
		U2F_REGISTER_RESP *resp = (U2F_REGISTER_RESP *)&data;
		MEMSET_BZERO(data, sizeof(data));


		resp->registerId = U2F_REGISTER_ID;
		resp->keyHandleLen = KEY_HANDLE_LEN;
		// Generate keypair for this appId
		const HDNode *node =
			generateKeyHandle(req->appId, (uint8_t*)&resp->keyHandleCertSig);

		if (!node) {
			debugLog(0, "", "getDerivedNode Fail");
			send_u2f_error(U2F_SW_WRONG_DATA); // error:bad key handle
			return;
		}

		ecdsa_get_public_key65(node->curve->params, node->private_key,
				       (uint8_t *)&resp->pubKey);

		memcpy(resp->keyHandleCertSig + resp->keyHandleLen,
		       U2F_ATT_CERT, sizeof(U2F_ATT_CERT));

		uint8_t sig[64];
		U2F_REGISTER_SIG_STR sig_base;
		sig_base.reserved = 0;
		memcpy(sig_base.appId, req->appId, U2F_APPID_SIZE);
		memcpy(sig_base.chal, req->chal, U2F_CHAL_SIZE);
		memcpy(sig_base.keyHandle, &resp->keyHandleCertSig, KEY_HANDLE_LEN);
		memcpy(sig_base.pubKey, &resp->pubKey, U2F_PUBKEY_LEN);
		if (ecdsa_sign(&nist256p1, U2F_ATT_PRIV_KEY, (uint8_t *)&sig_base, sizeof(sig_base), sig, NULL, NULL) != 0) {
			send_u2f_error(U2F_SW_WRONG_DATA);
			return;
		}

		// Where to write the signature in the response
		uint8_t *resp_sig = resp->keyHandleCertSig +
				    resp->keyHandleLen + sizeof(U2F_ATT_CERT);
		// Convert to der for the response
		const uint8_t sig_len = ecdsa_sig_to_der(sig, resp_sig);

		// Append success bytes
		memcpy(resp->keyHandleCertSig + resp->keyHandleLen +
			   sizeof(U2F_ATT_CERT) + sig_len,
		       "\x90\x00", 2);

		int l = 1 /* registerId */ + U2F_PUBKEY_LEN +
			1 /* keyhandleLen */ + resp->keyHandleLen +
			sizeof(U2F_ATT_CERT) + sig_len + 2;

		last_req_state = INIT;
		dialog_timeout = 0;
		send_u2f_msg(data, l);
		return;
	}

	// Didnt expect to get here
	dialog_timeout = 0;
}

void u2f_authenticate(const APDU *a)
{
	const U2F_AUTHENTICATE_REQ *req = (U2F_AUTHENTICATE_REQ *)a->data;
	static U2F_AUTHENTICATE_REQ last_req;

	if (!storage_isInitialized()) {
		send_u2f_error(U2F_SW_CONDITIONS_NOT_SATISFIED);
		return;
	}

	if (APDU_LEN(*a) < 64) { /// FIXME: decent value
		debugLog(0, "", "u2f authenticate - badlen");
		send_u2f_error(U2F_SW_WRONG_LENGTH);
		return;
	}

	if (req->keyHandleLen != KEY_HANDLE_LEN) {
		debugLog(0, "", "u2f auth - bad keyhandle len");
		send_u2f_error(U2F_SW_WRONG_DATA); // error:bad key handle
		return;
	}

	const HDNode *node =
	    validateKeyHandle(req->appId, req->keyHandle);

	if (!node) {
		debugLog(0, "", "u2f auth - bad keyhandle len");
		send_u2f_error(U2F_SW_WRONG_DATA); // error:bad key handle
		return;
	}

	if (a->p1 == U2F_AUTH_CHECK_ONLY) {
		debugLog(0, "", "u2f authenticate check");
		// This is a success for a good keyhandle
		// A failed check would have happened earlier
		// error: testof-user-presence is required
		send_u2f_error(U2F_SW_CONDITIONS_NOT_SATISFIED);
		return;
	}

	if (a->p1 != U2F_AUTH_ENFORCE) {
		debugLog(0, "", "u2f authenticate unknown");
		// error:bad key handle
		send_u2f_error(U2F_SW_WRONG_DATA);
		return;
	}

	debugLog(0, "", "u2f authenticate enforce");

	if (memcmp(&last_req, req, sizeof(last_req)) != 0) {
		memcpy(&last_req, req, sizeof(last_req));
		last_req_state = INIT;
	}

	if (last_req_state == INIT) {
		// error: testof-user-presence is required
		buttonUpdate(); // Clear button state
		const char *appname;
		const BITMAP *appicon;
		getReadableAppId(req->appId, &appname, &appicon);
		layoutU2FDialog("Authenticate", appname, appicon);
		last_req_state = AUTH;
	}

	// Awaiting Keypress
	if (last_req_state == AUTH) {
		// error: testof-user-presence is required
		send_u2f_error(U2F_SW_CONDITIONS_NOT_SATISFIED);
		dialog_timeout = U2F_TIMEOUT;
		return;
	}

	// Buttons said yes
	if (last_req_state == AUTH_PASS) {
		uint8_t buf[sizeof(U2F_AUTHENTICATE_RESP) + 2];
		U2F_AUTHENTICATE_RESP *resp =
			(U2F_AUTHENTICATE_RESP *)&buf;

		const uint32_t ctr = storage_nextU2FCounter();
		resp->flags = U2F_AUTH_FLAG_TUP;
		resp->ctr[0] = ctr >> 24 & 0xff;
		resp->ctr[1] = ctr >> 16 & 0xff;
		resp->ctr[2] = ctr >> 8 & 0xff;
		resp->ctr[3] = ctr & 0xff;

		// Build and sign response
		U2F_AUTHENTICATE_SIG_STR sig_base;
		uint8_t sig[64];
		memcpy(sig_base.appId, req->appId, U2F_APPID_SIZE);
		sig_base.flags = resp->flags;
		memcpy(sig_base.ctr, resp->ctr, 4);
		memcpy(sig_base.chal, req->chal, U2F_CHAL_SIZE);
		if (ecdsa_sign(&nist256p1, node->private_key, (uint8_t *)&sig_base, sizeof(sig_base), sig, NULL, NULL) != 0) {
			send_u2f_error(U2F_SW_WRONG_DATA);
			return;
		}

		// Copy DER encoded signature into response
		const uint8_t sig_len = ecdsa_sig_to_der(sig, resp->sig);

		// Append OK
		memcpy(buf + sizeof(U2F_AUTHENTICATE_RESP) -
			   U2F_MAX_EC_SIG_SIZE + sig_len,
			   "\x90\x00", 2);
		last_req_state = INIT;
		dialog_timeout = 0;
		send_u2f_msg(buf, sizeof(U2F_AUTHENTICATE_RESP) -
					  U2F_MAX_EC_SIG_SIZE + sig_len +
					  2);
	}
}

void send_u2f_error(const uint16_t err)
{
	uint8_t data[2];
	data[0] = err >> 8 & 0xFF;
	data[1] = err & 0xFF;
	send_u2f_msg(data, 2);
}

void send_u2f_msg(const uint8_t *data, const uint32_t len)
{
	send_u2fhid_msg(U2FHID_MSG, data, len);
}