path: root/common/u2f.c

/* Copyright 2017 The Chromium OS Authors. All rights reserved.
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

/* APDU dispatcher and U2F command handlers. */

#include "console.h"
#include "cryptoc/p256.h"
#include "cryptoc/sha256.h"
#include "dcrypto.h"
#include "extension.h"
#include "system.h"
#include "u2f_impl.h"
#include "u2f.h"
#include "util.h"

#define G2F_CERT_NAME "CrO2"

#define CPRINTF(format, args...) cprintf(CC_EXTENSION, format, ##args)

/* Crypto parameters */
#define AES_BLOCK_LEN 16
#define KH_LEN 64

static int individual_cert(const p256_int *d, const p256_int *pk_x,
			   const p256_int *pk_y, uint8_t *cert, const int n)
{
	p256_int *serial;

	if (system_get_chip_unique_id((uint8_t **)&serial) != P256_NBYTES)
		return 0;

	return DCRYPTO_x509_gen_u2f_cert_name(d, pk_x, pk_y, serial,
					      G2F_CERT_NAME, cert, n);
}

int g2f_attestation_cert(uint8_t *buf)
{
	p256_int d, pk_x, pk_y;

	if (g2f_individual_keypair(&d, &pk_x, &pk_y))
		return 0;

	/* Note that max length is not currently respected here. */
	return individual_cert(&d, &pk_x, &pk_y, buf,
			       G2F_ATTESTATION_CERT_MAX_LEN);
}

static void copy_kh_pubkey_out(p256_int *opk_x, p256_int *opk_y,
			       struct u2f_key_handle *kh, void *buf)
{
	struct u2f_generate_resp *resp = buf;

	/* Insert origin-specific public keys into the response */
	p256_to_bin(opk_x, resp->pubKey.x); /* endianness */
	p256_to_bin(opk_y, resp->pubKey.y); /* endianness */

	resp->pubKey.pointFormat = U2F_POINT_UNCOMPRESSED;

	/* Copy key handle to response. */
	memcpy(&resp->keyHandle, kh, sizeof(struct u2f_key_handle));
}

static void copy_versioned_kh_pubkey_out(p256_int *opk_x, p256_int *opk_y,
					 struct u2f_versioned_key_handle *kh,
					 void *buf)
{
	struct u2f_generate_versioned_resp *resp = buf;

	/* Insert origin-specific public keys into the response */
	p256_to_bin(opk_x, resp->pubKey.x); /* endianness */
	p256_to_bin(opk_y, resp->pubKey.y); /* endianness */

	resp->pubKey.pointFormat = U2F_POINT_UNCOMPRESSED;

	/* Copy key handle to response. */
	memcpy(&resp->keyHandle, kh, sizeof(struct u2f_versioned_key_handle));
}

/* U2F GENERATE command  */
static enum vendor_cmd_rc u2f_generate(enum vendor_cmd_cc code, void *buf,
				       size_t input_size, size_t *response_size)
{
	struct u2f_generate_req *req = buf;
	uint8_t kh_version =
		(req->flags & U2F_UV_ENABLED_KH) ? U2F_KH_VERSION_1 : 0;

	/* Origin keypair. Must be word aligned, otherwise TRNG will crash. */
	uint8_t od_seed[P256_NBYTES] __aligned(4);
	p256_int od, opk_x, opk_y;

	/* Buffer for generating key handle. */
	union {
		struct u2f_key_handle kh;
		struct u2f_versioned_key_handle vkh;
	} kh_buf;
	size_t keypair_input_size =
		(kh_version == 0) ?
			sizeof(kh_buf.kh) :
			sizeof(struct u2f_versioned_key_handle_header);

	/* Whether key handle generation succeeded */
	int generate_kh_rc;
	/* Whether keypair generation succeeded */
	int generate_keypair_rc;

	size_t response_buf_size = *response_size;

	/* Authorization salt for versioned KHs */
	uint8_t *authorization_salt;

	*response_size = 0;

	if (input_size != sizeof(struct u2f_generate_req))
		return VENDOR_RC_BOGUS_ARGS;

	if (kh_version == 0) {
		if (response_buf_size < sizeof(struct u2f_generate_resp))
			return VENDOR_RC_BOGUS_ARGS;
	} else {
		if (response_buf_size <
		    sizeof(struct u2f_generate_versioned_resp))
			return VENDOR_RC_BOGUS_ARGS;
	}

	/* Maybe enforce user presence, w/ optional consume */
	if (pop_check_presence(req->flags & G2F_CONSUME) != POP_TOUCH_YES &&
	    (req->flags & U2F_AUTH_FLAG_TUP) != 0)
		return VENDOR_RC_NOT_ALLOWED;

	/* Generate origin-specific keypair */
	do {
		if (!DCRYPTO_ladder_random(&od_seed))
			return VENDOR_RC_INTERNAL_ERROR;

		if (kh_version == 0)
			generate_kh_rc = u2f_origin_user_keyhandle(
				req->appId, req->userSecret, od_seed,
				&kh_buf.kh);
		else
			generate_kh_rc = u2f_origin_user_versioned_keyhandle(
				req->appId, req->userSecret, od_seed,
				kh_version, &kh_buf.vkh.header);

		if (generate_kh_rc != EC_SUCCESS)
			return VENDOR_RC_INTERNAL_ERROR;

		generate_keypair_rc = u2f_origin_user_keypair(
			(uint8_t *)&kh_buf, keypair_input_size, &od, &opk_x,
			&opk_y);
	} while (generate_keypair_rc == EC_ERROR_TRY_AGAIN);

	if (generate_keypair_rc != EC_SUCCESS)
		return VENDOR_RC_INTERNAL_ERROR;

	/*
	 * From this point: the request 'req' content is invalid as it is
	 * overridden by the response we are building in the same buffer.
	 */
	if (kh_version == 0) {
		copy_kh_pubkey_out(&opk_x, &opk_y, &kh_buf.kh, buf);
		*response_size = sizeof(struct u2f_generate_resp);
	} else {
		authorization_salt = od_seed;
		/* Generate in word-aligned array so that TRNG doesn't crash */
		if (!DCRYPTO_ladder_random(authorization_salt))
			return VENDOR_RC_INTERNAL_ERROR;

		if (u2f_authorization_hmac(
			    authorization_salt, &kh_buf.vkh.header,
			    req->authTimeSecretHash,
			    kh_buf.vkh.authorization_hmac) != EC_SUCCESS)
			return VENDOR_RC_INTERNAL_ERROR;

		memcpy(&kh_buf.vkh.authorization_salt, authorization_salt,
		       U2F_AUTHORIZATION_SALT_SIZE);
		copy_versioned_kh_pubkey_out(&opk_x, &opk_y, &kh_buf.vkh, buf);
		*response_size = sizeof(struct u2f_generate_versioned_resp);
	}

	return VENDOR_RC_SUCCESS;
}
DECLARE_VENDOR_COMMAND(VENDOR_CC_U2F_GENERATE, u2f_generate);

static int verify_kh_pubkey(const uint8_t *key_handle, size_t key_handle_size,
			    const struct u2f_ec_point *public_key, int *matches)
{
	int rc;
	struct u2f_ec_point kh_pubkey;
	p256_int od, opk_x, opk_y;

	rc = u2f_origin_user_keypair(key_handle, key_handle_size, &od, &opk_x,
				     &opk_y);
	if (rc != EC_SUCCESS)
		return rc;

	/* Reconstruct the public key. */
	p256_to_bin(&opk_x, kh_pubkey.x);
	p256_to_bin(&opk_y, kh_pubkey.y);
	kh_pubkey.pointFormat = U2F_POINT_UNCOMPRESSED;

	*matches = safe_memcmp(&kh_pubkey, public_key,
			       sizeof(struct u2f_ec_point)) == 0;

	return EC_SUCCESS;
}

static int verify_kh_owned(const uint8_t *user_secret, const uint8_t *app_id,
			   const struct u2f_key_handle *key_handle, int *owned)
{
	int rc;
	/* Re-created key handle. */
	struct u2f_key_handle recreated_kh;

	/*
	 * Re-create the key handle and compare against that which
	 * was provided. This allows us to verify that the key handle
	 * is owned by this combination of device, current user and app_id.
	 */

	rc = u2f_origin_user_keyhandle(app_id, user_secret,
				       key_handle->origin_seed, &recreated_kh);

	if (rc == EC_SUCCESS)
		*owned = safe_memcmp(&recreated_kh, key_handle,
				     sizeof(recreated_kh)) == 0;

	return rc;
}

static int verify_versioned_kh_owned(
	const uint8_t *user_secret, const uint8_t *app_id,
	const struct u2f_versioned_key_handle_header *key_handle_header,
	int *owned)
{
	int rc;
	/* Re-created key handle. */
	struct u2f_versioned_key_handle_header recreated_kh_header;

	/*
	 * Re-create the key handle and compare against that which
	 * was provided. This allows us to verify that the key handle
	 * is owned by this combination of device, current user and app_id.
	 */

	rc = u2f_origin_user_versioned_keyhandle(app_id, user_secret,
						 key_handle_header->origin_seed,
						 key_handle_header->version,
						 &recreated_kh_header);

	if (rc == EC_SUCCESS)
		*owned = safe_memcmp(&recreated_kh_header, key_handle_header,
				     sizeof(recreated_kh_header)) == 0;

	return rc;
}

/* Below, we depend on the response not being larger than than the request. */
BUILD_ASSERT(sizeof(struct u2f_sign_resp) <= sizeof(struct u2f_sign_req));

/* U2F SIGN command */
static enum vendor_cmd_rc u2f_sign(enum vendor_cmd_cc code, void *buf,
				   size_t input_size, size_t *response_size)
{
	const struct u2f_sign_req *req = buf;
	const struct u2f_sign_versioned_req *req_versioned = buf;
	const uint8_t *key_handle, *hash;
	uint8_t flags;
	struct u2f_sign_resp *resp;

	struct drbg_ctx ctx;

	/* Whether the key handle is owned by this device. */
	int kh_owned = 0;

	/* Origin private key. */
	p256_int origin_d;

	/* Hash, and corresponding signature. */
	p256_int h, r, s;

	/* Version of KH; 0 if KH is not versioned. */
	uint8_t version;

	/* Size of the part of KH used to derive keypair, in bytes. */
	size_t keypair_input_size;

	int verify_owned_rc;

	/* Response is smaller than request, so no need to check this. */
	*response_size = 0;

	if (input_size == sizeof(struct u2f_sign_req)) {
		version = 0;
		key_handle = (uint8_t *)&req->keyHandle;
		hash = req->hash;
		flags = req->flags;
		keypair_input_size = sizeof(struct u2f_key_handle);
		verify_owned_rc = verify_kh_owned(req->userSecret, req->appId,
						  &req->keyHandle, &kh_owned);
	} else if (input_size == sizeof(struct u2f_sign_versioned_req)) {
		version = req_versioned->keyHandle.header.version;
		key_handle = (uint8_t *)&req_versioned->keyHandle;
		hash = req_versioned->hash;
		flags = req_versioned->flags;
		keypair_input_size =
			sizeof(struct u2f_versioned_key_handle_header);
		verify_owned_rc = verify_versioned_kh_owned(
			req_versioned->userSecret, req_versioned->appId,
			&req_versioned->keyHandle.header, &kh_owned);
	} else {
		return VENDOR_RC_BOGUS_ARGS;
	}

	if (verify_owned_rc != EC_SUCCESS)
		return VENDOR_RC_INTERNAL_ERROR;

	if (!kh_owned)
		return VENDOR_RC_PASSWORD_REQUIRED;

	/* We might not actually need to sign anything. */
	if ((flags & U2F_AUTH_CHECK_ONLY) == U2F_AUTH_CHECK_ONLY)
		return VENDOR_RC_SUCCESS;

	/*
	 * Enforce user presence for version 0 KHs, with optional consume.
	 */
	if (pop_check_presence(flags & G2F_CONSUME) != POP_TOUCH_YES) {
		if (version != U2F_KH_VERSION_1)
			return VENDOR_RC_NOT_ALLOWED;
		if ((flags & U2F_AUTH_FLAG_TUP) != 0)
			return VENDOR_RC_NOT_ALLOWED;
		/*
		 * TODO(yichengli): When auth-time secrets is ready, enforce
		 * authorization hmac when no power button press.
		 */
	}

	/* Re-create origin-specific key. */
	if (u2f_origin_user_keypair(key_handle, keypair_input_size, &origin_d,
				    NULL, NULL) != EC_SUCCESS)
		return VENDOR_RC_INTERNAL_ERROR;

	/* Prepare hash to sign. */
	p256_from_bin(hash, &h);

	/* Sign. */
	hmac_drbg_init_rfc6979(&ctx, &origin_d, &h);
	if (!dcrypto_p256_ecdsa_sign(&ctx, &origin_d, &h, &r, &s)) {
		p256_clear(&origin_d);
		return VENDOR_RC_INTERNAL_ERROR;
	}
	p256_clear(&origin_d);

	/*
	 * From this point: the request 'req' content is invalid as it is
	 * overridden by the response we are building in the same buffer.
	 * The response is smaller than the request, so we have the space.
	 */
	resp = buf;

	*response_size = sizeof(*resp);

	p256_to_bin(&r, resp->sig_r);
	p256_to_bin(&s, resp->sig_s);

	return VENDOR_RC_SUCCESS;
}
DECLARE_VENDOR_COMMAND(VENDOR_CC_U2F_SIGN, u2f_sign);

struct g2f_register_msg {
	uint8_t reserved;
	uint8_t app_id[U2F_APPID_SIZE];
	uint8_t challenge[U2F_CHAL_SIZE];
	uint8_t key_handle[U2F_APPID_SIZE + sizeof(p256_int)];
	struct u2f_ec_point public_key;
};

static inline int u2f_attest_verify_reg_resp(const uint8_t *user_secret,
					     uint8_t data_size,
					     const uint8_t *data)
{
	struct g2f_register_msg *msg = (void *)data;
	int verified;
	/* We only do u2f_attest on non-versioned KHs. */
	const int key_handle_size = sizeof(struct u2f_key_handle);

	if (data_size != sizeof(struct g2f_register_msg))
		return VENDOR_RC_NOT_ALLOWED;

	if (msg->reserved != 0)
		return VENDOR_RC_NOT_ALLOWED;

	if (verify_kh_owned(user_secret, msg->app_id,
			    (struct u2f_key_handle *)&msg->key_handle,
			    &verified) != EC_SUCCESS)
		return VENDOR_RC_INTERNAL_ERROR;

	if (!verified)
		return VENDOR_RC_NOT_ALLOWED;

	if (verify_kh_pubkey(msg->key_handle, key_handle_size, &msg->public_key,
			     &verified) != EC_SUCCESS)
		return VENDOR_RC_INTERNAL_ERROR;

	if (!verified)
		return VENDOR_RC_NOT_ALLOWED;

	return VENDOR_RC_SUCCESS;
}

static int u2f_attest_verify(const uint8_t *user_secret, uint8_t format,
			     uint8_t data_size, const uint8_t *data)
{
	switch (format) {
	case U2F_ATTEST_FORMAT_REG_RESP:
		return u2f_attest_verify_reg_resp(user_secret, data_size, data);
	default:
		return VENDOR_RC_NOT_ALLOWED;
	}
}

static inline size_t u2f_attest_format_size(uint8_t format)
{
	switch (format) {
	case U2F_ATTEST_FORMAT_REG_RESP:
		return sizeof(struct g2f_register_msg);
	default:
		return 0;
	}
}

/* U2F ATTEST command */
static enum vendor_cmd_rc u2f_attest(enum vendor_cmd_cc code, void *buf,
				     size_t input_size, size_t *response_size)
{
	const struct u2f_attest_req *req = buf;
	struct u2f_attest_resp *resp;

	int verify_ret;

	HASH_CTX h_ctx;
	struct drbg_ctx dr_ctx;

	/* Data hash, and corresponding signature. */
	p256_int h, r, s;

	/* Attestation key */
	p256_int d, pk_x, pk_y;

	size_t response_buf_size = *response_size;

	*response_size = 0;

	if (input_size < offsetof(struct u2f_attest_req, data) ||
	    input_size <
		    (offsetof(struct u2f_attest_req, data) + req->dataLen) ||
	    input_size > sizeof(struct u2f_attest_req) ||
	    response_buf_size < sizeof(*resp))
		return VENDOR_RC_BOGUS_ARGS;

	verify_ret = u2f_attest_verify(req->userSecret, req->format,
				       req->dataLen, req->data);

	if (verify_ret != VENDOR_RC_SUCCESS)
		return verify_ret;

	/* Message signature */
	DCRYPTO_SHA256_init(&h_ctx, 0);
	HASH_update(&h_ctx, req->data, u2f_attest_format_size(req->format));
	p256_from_bin(HASH_final(&h_ctx), &h);

	/* Derive G2F Attestation Key */
	if (g2f_individual_keypair(&d, &pk_x, &pk_y)) {
		CPRINTF("G2F Attestation key generation failed");
		return VENDOR_RC_INTERNAL_ERROR;
	}

	/* Sign over the response w/ the attestation key */
	hmac_drbg_init_rfc6979(&dr_ctx, &d, &h);
	if (!dcrypto_p256_ecdsa_sign(&dr_ctx, &d, &h, &r, &s)) {
		CPRINTF("Signing error");
		return VENDOR_RC_INTERNAL_ERROR;
	}
	p256_clear(&d);

	/*
	 * From this point: the request 'req' content is invalid as it is
	 * overridden by the response we are building in the same buffer.
	 * The response is smaller than the request, so we have the space.
	 */
	resp = buf;

	*response_size = sizeof(*resp);

	p256_to_bin(&r, resp->sig_r);
	p256_to_bin(&s, resp->sig_s);

	return VENDOR_RC_SUCCESS;
}
DECLARE_VENDOR_COMMAND(VENDOR_CC_U2F_ATTEST, u2f_attest);