asterisk/res/res_pjsip_stir_shaken.c

/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2020, Sangoma Technologies Corporation
 *
 * Ben Ford <bford@sangoma.com>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*** MODULEINFO
	<depend>pjproject</depend>
	<depend>res_pjsip</depend>
	<depend>res_pjsip_session</depend>
	<depend>res_stir_shaken</depend>
	<support_level>core</support_level>
 ***/

#include "asterisk.h"

#include "asterisk/res_pjsip.h"
#include "asterisk/res_pjsip_session.h"
#include "asterisk/module.h"

#include "asterisk/res_stir_shaken.h"

/*! The Date header will not be valid after this many milliseconds (60 seconds recommended) */
#define STIR_SHAKEN_DATE_HEADER_TIMEOUT 60000

/*!
 * \brief Get the attestation from the payload
 *
 * \param json_str The JSON string representation of the payload
 *
 * \retval Empty string on failure
 * \retval The attestation on success
 */
static char *get_attestation_from_payload(const char *json_str)
{
	RAII_VAR(struct ast_json *, json, NULL, ast_json_free);
	char *attestation;

	json = ast_json_load_string(json_str, NULL);
	attestation = (char *)ast_json_string_get(ast_json_object_get(json, "attest"));

	if (!ast_strlen_zero(attestation)) {
		return attestation;
	}

	return "";
}

/*!
 * \brief Compare the caller ID from the INVITE with the one in the payload
 *
 * \param caller_id
 * \param json_str The JSON string representation of the payload
 *
 * \retval -1 on failure
 * \retval 0 on success
 */
static int compare_caller_id(char *caller_id, const char *json_str)
{
	RAII_VAR(struct ast_json *, json, NULL, ast_json_free);
	char *caller_id_other;

	json = ast_json_load_string(json_str, NULL);
	caller_id_other = (char *)ast_json_string_get(ast_json_object_get(
		ast_json_object_get(json, "orig"), "tn"));

	if (strcmp(caller_id, caller_id_other)) {
		return -1;
	}

	return 0;
}

/*!
 * \brief Compare the current timestamp with the one in the payload. If the difference
 * is greater than the signature timeout, it's not valid anymore
 *
 * \param json_str The JSON string representation of the payload
 *
 * \retval -1 on failure
 * \retval 0 on success
 */
static int compare_timestamp(const char *json_str)
{
	RAII_VAR(struct ast_json *, json, NULL, ast_json_free);
	long int timestamp;
	struct timeval now = ast_tvnow();

#ifdef TEST_FRAMEWORK
	ast_debug(3, "Ignoring STIR/SHAKEN timestamp\n");
	return 0;
#endif

	json = ast_json_load_string(json_str, NULL);
	timestamp = ast_json_integer_get(ast_json_object_get(json, "iat"));

	if (now.tv_sec - timestamp > ast_stir_shaken_get_signature_timeout()) {
		return -1;
	}

	return 0;
}

static int check_date_header(pjsip_rx_data *rdata)
{
	static const pj_str_t date_hdr_str = { "Date", 4 };
	char *date_hdr_val;
	struct ast_tm date_hdr_tm;
	struct timeval date_hdr_timeval;
	struct timeval current_timeval;
	char *remainder;
	char timezone[80] = { 0 };
	int64_t time_diff;

	date_hdr_val = ast_sip_rdata_get_header_value(rdata, date_hdr_str);
	if (ast_strlen_zero(date_hdr_val)) {
		ast_log(LOG_ERROR, "Failed to get Date header from incoming INVITE for STIR/SHAKEN\n");
		return -1;
	}

	if (!(remainder = ast_strptime(date_hdr_val, "%a, %d %b %Y %T", &date_hdr_tm))) {
		ast_log(LOG_ERROR, "Failed to parse Date header\n");
		return -1;
	}

	sscanf(remainder, "%79s", timezone);

	if (ast_strlen_zero(timezone)) {
		ast_log(LOG_ERROR, "A timezone is required for STIR/SHAKEN Date header, but we didn't get one\n");
		return -1;
	}

	date_hdr_timeval = ast_mktime(&date_hdr_tm, timezone);
	current_timeval = ast_tvnow();

	time_diff = ast_tvdiff_ms(current_timeval, date_hdr_timeval);
	if (time_diff < 0) {
		/* An INVITE from the future! */
		ast_log(LOG_ERROR, "STIR/SHAKEN Date header has a future date\n");
		return -1;
	} else if (time_diff > STIR_SHAKEN_DATE_HEADER_TIMEOUT) {
		ast_log(LOG_ERROR, "STIR/SHAKEN Date header was outside of the allowable range (60 seconds)\n");
		return -1;
	}

	return 0;
}

/* Send a response back and end the session */
static void stir_shaken_inv_end_session(struct ast_sip_session *session, pjsip_rx_data *rdata, int response_code, const pj_str_t response_str)
{
	pjsip_tx_data *tdata;

	if (pjsip_inv_end_session(session->inv_session, response_code, &response_str, &tdata) == PJ_SUCCESS) {
		pjsip_endpt_send_response2(ast_sip_get_pjsip_endpoint(), rdata, tdata, NULL, NULL);
	}
	ast_hangup(session->channel);
}

/*!
 * \internal
 * \brief Session supplement callback on an incoming INVITE request
 *
 * When we receive an INVITE, check it for STIR/SHAKEN information and
 * decide what to do from there
 *
 * \param session The session that has received an INVITE
 * \param rdata The incoming INVITE
 */
static int stir_shaken_incoming_request(struct ast_sip_session *session, pjsip_rx_data *rdata)
{
	static const pj_str_t identity_str = { "Identity", 8 };
	const pj_str_t bad_identity_info_str = {
		AST_STIR_SHAKEN_RESPONSE_STR_BAD_IDENTITY_INFO,
		strlen(AST_STIR_SHAKEN_RESPONSE_STR_BAD_IDENTITY_INFO)
	};
	const pj_str_t unsupported_credential_str = {
		AST_STIR_SHAKEN_RESPONSE_STR_UNSUPPORTED_CREDENTIAL,
		strlen(AST_STIR_SHAKEN_RESPONSE_STR_UNSUPPORTED_CREDENTIAL)
	};
	const pj_str_t stale_date_str = {
		AST_STIR_SHAKEN_RESPONSE_STR_STALE_DATE,
		strlen(AST_STIR_SHAKEN_RESPONSE_STR_STALE_DATE)
	};
	const pj_str_t use_supported_passport_format_str = {
		AST_STIR_SHAKEN_RESPONSE_STR_USE_SUPPORTED_PASSPORT_FORMAT,
		strlen(AST_STIR_SHAKEN_RESPONSE_STR_USE_SUPPORTED_PASSPORT_FORMAT)
	};
	const pj_str_t invalid_identity_hdr_str = {
		AST_STIR_SHAKEN_RESPONSE_STR_INVALID_IDENTITY_HEADER,
		strlen(AST_STIR_SHAKEN_RESPONSE_STR_INVALID_IDENTITY_HEADER)
	};
	const pj_str_t server_internal_error_str = { "Server Internal Error", 21 };
	char *identity_hdr_val;
	char *encoded_val;
	struct ast_channel *chan = session->channel;
	char *caller_id = session->id.number.str;
	RAII_VAR(char *, header, NULL, ast_free);
	RAII_VAR(char *, payload, NULL, ast_free);
	char *signature;
	char *algorithm;
	char *public_cert_url;
	char *attestation;
	char *ppt;
	int mismatch = 0;
	struct ast_stir_shaken_payload *ss_payload;
	int failure_code = 0;
	RAII_VAR(struct stir_shaken_profile *, profile, NULL, ao2_cleanup);

	/* Check if this is a reinvite. If it is, we don't need to do anything */
	if (rdata->msg_info.to->tag.slen) {
		return 0;
	}

	profile = ast_stir_shaken_get_profile(session->endpoint->stir_shaken_profile);
	/* Profile should be checked first as it takes priority over anything else.
	 * If there is a profile and it doesn't have verification enabled, do nothing.
	 * If there is no profile and the stir_shaken option is either not set or does
	 * not support verification, do nothing.
	 */
	if ((profile && !ast_stir_shaken_profile_supports_verification(profile))
		|| (!profile && (session->endpoint->stir_shaken & AST_SIP_STIR_SHAKEN_VERIFY) == 0)) {
		return 0;
	}

	identity_hdr_val = ast_sip_rdata_get_header_value(rdata, identity_str);
	if (ast_strlen_zero(identity_hdr_val)) {
		ast_stir_shaken_add_verification(chan, caller_id, "", AST_STIR_SHAKEN_VERIFY_NOT_PRESENT);
		return 0;
	}

	encoded_val = strtok_r(identity_hdr_val, ".", &identity_hdr_val);
	header = ast_base64url_decode_string(encoded_val);
	if (ast_strlen_zero(header)) {
		ast_debug(3, "STIR/SHAKEN INVITE for %s is missing header\n",
			ast_sorcery_object_get_id(session->endpoint));
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_BAD_IDENTITY_INFO, bad_identity_info_str);
		return 1;
	}

	encoded_val = strtok_r(identity_hdr_val, ".", &identity_hdr_val);
	payload = ast_base64url_decode_string(encoded_val);
	if (ast_strlen_zero(payload)) {
		ast_debug(3, "STIR/SHAKEN INVITE for %s is missing payload\n",
			ast_sorcery_object_get_id(session->endpoint));
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_BAD_IDENTITY_INFO, bad_identity_info_str);
		return 1;
	}

	/* It's fine to leave the signature encoded */
	signature = strtok_r(identity_hdr_val, ";", &identity_hdr_val);
	if (ast_strlen_zero(signature)) {
		ast_debug(3, "STIR/SHAKEN INVITE for %s is missing signature\n",
			ast_sorcery_object_get_id(session->endpoint));
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_BAD_IDENTITY_INFO, bad_identity_info_str);
		return 1;
	}

	/* Trim "info=<" to get public cert URL */
	strtok_r(identity_hdr_val, "<", &identity_hdr_val);
	public_cert_url = strtok_r(identity_hdr_val, ">", &identity_hdr_val);

	/* Make sure the public URL is actually a URL */
	if (ast_strlen_zero(public_cert_url) || !ast_begins_with(public_cert_url, "http")) {
		/* RFC8224 states that if we can't acquire the credentials needed
		 * by the verification service, we should send a 436 */
		ast_debug(3, "STIR/SHAKEN INVITE for %s did not  have valid URL (%s)\n",
			ast_sorcery_object_get_id(session->endpoint), public_cert_url);
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_BAD_IDENTITY_INFO, bad_identity_info_str);
		return 1;
	}

	algorithm = strtok_r(identity_hdr_val, ";", &identity_hdr_val);
	if (ast_strlen_zero(algorithm)) {
		/* RFC8224 states that if the algorithm is not specified, use ES256 */
		algorithm = STIR_SHAKEN_ENCRYPTION_ALGORITHM;
	} else {
		strtok_r(algorithm, "=", &algorithm);
		if (strcmp(algorithm, STIR_SHAKEN_ENCRYPTION_ALGORITHM)) {
			/* RFC8224 states that if we don't support the algorithm, send a 437 */
			ast_debug(3, "STIR/SHAKEN INVITE for %s uses an unsupported algorithm (%s)\n",
				ast_sorcery_object_get_id(session->endpoint), algorithm);
			stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_UNSUPPORTED_CREDENTIAL, unsupported_credential_str);
			return 1;
		}
	}

	/* The only thing left should be ppt=shaken (which could have more values later),
	 * unless using the compact PASSport form */
	strtok_r(identity_hdr_val, "=", &identity_hdr_val);
	ppt = ast_strip(identity_hdr_val);
	if (!ast_strlen_zero(ppt) && strcmp(ppt, STIR_SHAKEN_PPT)) {
		ast_log(LOG_ERROR, "STIR/SHAKEN INVITE for %s has unsupported ppt (%s)\n",
			ast_sorcery_object_get_id(session->endpoint), ppt);
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_USE_SUPPORTED_PASSPORT_FORMAT, use_supported_passport_format_str);
		return 1;
	}

	if (check_date_header(rdata)) {
		ast_debug(3, "STIR/SHAKEN INVITE for %s has old Date header\n",
			ast_sorcery_object_get_id(session->endpoint));
		stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_STALE_DATE, stale_date_str);
		return 1;
	}

	attestation = get_attestation_from_payload(payload);

	ss_payload = ast_stir_shaken_verify_with_profile(header, payload, signature, algorithm, public_cert_url, &failure_code, profile);

	if (!ss_payload) {

		if (failure_code == AST_STIR_SHAKEN_VERIFY_FAILED_TO_GET_CERT) {
			/* RFC8224 states that if we can't get the credentials we need, send a 437 */
			ast_debug(3, "STIR/SHAKEN INVITE for %s failed to acquire cert during verification process\n",
				ast_sorcery_object_get_id(session->endpoint));
			stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_UNSUPPORTED_CREDENTIAL, unsupported_credential_str);
		} else if (failure_code == AST_STIR_SHAKEN_VERIFY_FAILED_MEMORY_ALLOC) {
			ast_log(LOG_ERROR, "Failed to allocate memory during STIR/SHAKEN verification"
				" for %s\n", ast_sorcery_object_get_id(session->endpoint));
			stir_shaken_inv_end_session(session, rdata, 500, server_internal_error_str);
		} else if (failure_code == AST_STIR_SHAKEN_VERIFY_FAILED_SIGNATURE_VALIDATION) {
			/* RFC8224 states that if we can't validate the signature, send a 438 */
			ast_debug(3, "STIR/SHAKEN INVITE for %s failed signature validation during verification process\n",
				ast_sorcery_object_get_id(session->endpoint));
			ast_stir_shaken_add_verification(chan, caller_id, attestation, AST_STIR_SHAKEN_VERIFY_SIGNATURE_FAILED);
			stir_shaken_inv_end_session(session, rdata, AST_STIR_SHAKEN_RESPONSE_CODE_INVALID_IDENTITY_HEADER, invalid_identity_hdr_str);
		}

		return 1;
	}
	ast_stir_shaken_payload_free(ss_payload);

	mismatch |= compare_caller_id(caller_id, payload);
	mismatch |= compare_timestamp(payload);

	if (mismatch) {
		ast_stir_shaken_add_verification(chan, caller_id, attestation, AST_STIR_SHAKEN_VERIFY_MISMATCH);
		return 0;
	}

	ast_stir_shaken_add_verification(chan, caller_id, attestation, AST_STIR_SHAKEN_VERIFY_PASSED);

	return 0;
}

static int add_identity_header(const struct ast_sip_session *session, pjsip_tx_data *tdata)
{
	static const pj_str_t identity_str = { "Identity", 8 };
	pjsip_generic_string_hdr *identity_hdr;
	pj_str_t identity_val;
	pjsip_fromto_hdr *old_identity;
	pjsip_fromto_hdr *to;
	pjsip_sip_uri *uri;
	char *signature;
	char *public_cert_url;
	struct ast_json *header;
	struct ast_json *payload;
	char *dumped_string;
	RAII_VAR(char *, dest_tn, NULL, ast_free);
	RAII_VAR(struct ast_json *, json, NULL, ast_json_free);
	RAII_VAR(struct ast_stir_shaken_payload *, ss_payload, NULL, ast_stir_shaken_payload_free);
	RAII_VAR(char *, encoded_header, NULL, ast_free);
	RAII_VAR(char *, encoded_payload, NULL, ast_free);
	RAII_VAR(char *, combined_str, NULL, ast_free);
	size_t combined_size;

	old_identity = pjsip_msg_find_hdr_by_name(tdata->msg, &identity_str, NULL);
	if (old_identity) {
		return 0;
	}

	to = pjsip_msg_find_hdr(tdata->msg, PJSIP_H_TO, NULL);
	if (!to) {
		ast_log(LOG_ERROR, "Failed to find To header while adding STIR/SHAKEN Identity header\n");
		return -1;
	}

	uri = pjsip_uri_get_uri(to->uri);
	if (!uri) {
		ast_log(LOG_ERROR, "Failed to retrieve URI from To header while adding STIR/SHAKEN Identity header\n");
		return -1;
	}

	dest_tn = ast_malloc(uri->user.slen + 1);
	if (!dest_tn) {
		ast_log(LOG_ERROR, "Failed to allocate memory for STIR/SHAKEN dest->tn\n");
		return -1;
	}

	/* Remove everything except 0-9, *, and # in telephone number according to RFC 8224
	 * (required by RFC 8225 as part of canonicalization) */
	{
		int i;
		const char *s = uri->user.ptr;
		char *new_tn = dest_tn;
		/* We're only removing characters, if anything, so the buffer is guaranteed to be large enough */
		for (i = 0; i < uri->user.slen; i++) {
			if (isdigit(*s) || *s == '#' || *s == '*') { /* Only characters allowed */
				*new_tn++ = *s;
			}
			s++;
		}
		*new_tn = '\0';
		ast_debug(4, "Canonicalized telephone number %.*s -> %s\n", (int) uri->user.slen, uri->user.ptr, dest_tn);
	}

	/* x5u (public key URL), attestation, and origid will be added by ast_stir_shaken_sign */
	json = ast_json_pack("{s: {s: s, s: s, s: s}, s: {s: {s: [s]}, s: {s: s}}}",
		"header", "alg", "ES256", "ppt", "shaken", "typ", "passport",
		"payload", "dest", "tn", dest_tn, "orig", "tn",
		session->id.number.str);
	if (!json) {
		ast_log(LOG_ERROR, "Failed to allocate memory for STIR/SHAKEN JSON\n");
		return -1;
	}

	ss_payload = ast_stir_shaken_sign(json);
	if (!ss_payload) {
		ast_log(LOG_ERROR, "Failed to sign STIR/SHAKEN payload\n");
		return -1;
	}

	header = ast_json_object_get(json, "header");
	dumped_string = ast_json_dump_string(header);
	encoded_header = ast_base64url_encode_string(dumped_string);
	ast_json_free(dumped_string);
	if (!encoded_header) {
		ast_log(LOG_ERROR, "Failed to encode STIR/SHAKEN header\n");
		return -1;
	}

	payload = ast_json_object_get(json, "payload");
	/* Fields must appear in lexiographic order: https://www.rfc-editor.org/rfc/rfc8588.html#section-6
	 * https://www.rfc-editor.org/rfc/rfc8225.html#section-9 */
	dumped_string = ast_json_dump_string_sorted(payload);
	encoded_payload = ast_base64url_encode_string(dumped_string);
	ast_json_free(dumped_string);
	if (!encoded_payload) {
		ast_log(LOG_ERROR, "Failed to encode STIR/SHAKEN payload\n");
		return -1;
	}

	signature = (char *)ast_stir_shaken_payload_get_signature(ss_payload);
	public_cert_url = ast_stir_shaken_payload_get_public_cert_url(ss_payload);

	/* The format for the identity header:
	 * header.payload.signature;info=<public_cert_url>alg=STIR_SHAKEN_ENCRYPTION_ALGORITHM;ppt=STIR_SHAKEN_PPT
	 */
	combined_size = strlen(encoded_header) + 1 + strlen(encoded_payload) + 1
		+ strlen(signature) + strlen(";info=<>alg=;ppt=") + strlen(public_cert_url)
		+ strlen(STIR_SHAKEN_ENCRYPTION_ALGORITHM) + strlen(STIR_SHAKEN_PPT) + 1;
	combined_str = ast_calloc(1, combined_size);
	if (!combined_str) {
		ast_log(LOG_ERROR, "Failed to allocate memory for STIR/SHAKEN identity string\n");
		return -1;
	}
	snprintf(combined_str, combined_size, "%s.%s.%s;info=<%s>alg=%s;ppt=%s", encoded_header,
		encoded_payload, signature, public_cert_url, STIR_SHAKEN_ENCRYPTION_ALGORITHM, STIR_SHAKEN_PPT);

	identity_val = pj_str(combined_str);
	identity_hdr = pjsip_generic_string_hdr_create(tdata->pool, &identity_str, &identity_val);
	if (!identity_hdr) {
		ast_log(LOG_ERROR, "Failed to create STIR/SHAKEN Identity header\n");
		return -1;
	}

	pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr *)identity_hdr);

	return 0;
}

static void add_date_header(const struct ast_sip_session *session, pjsip_tx_data *tdata)
{
	static const pj_str_t date_str = { "Date", 4 };
	pjsip_fromto_hdr *old_date;

	old_date = pjsip_msg_find_hdr_by_name(tdata->msg, &date_str, NULL);
	if (old_date) {
		ast_debug(3, "Found old STIR/SHAKEN date header, no need to add one\n");
		return;
	}

	ast_sip_add_date_header(tdata);
}

static void stir_shaken_outgoing_request(struct ast_sip_session *session, pjsip_tx_data *tdata)
{
	RAII_VAR(struct stir_shaken_profile *, profile, NULL, ao2_cleanup);

	profile = ast_stir_shaken_get_profile(session->endpoint->stir_shaken_profile);
	/* Profile should be checked first as it takes priority over anything else.
	 * If there is a profile and it doesn't have attestation enabled, do nothing.
	 * If there is no profile and the stir_shaken option is either not set or does
	 * not support attestation, do nothing.
	 */
	if ((profile && !ast_stir_shaken_profile_supports_attestation(profile))
		|| (!profile && (session->endpoint->stir_shaken & AST_SIP_STIR_SHAKEN_ATTEST) == 0)) {
		return;
	}

	if (ast_strlen_zero(session->id.number.str) && session->id.number.valid) {
		return;
	}

	/* If adding the Identity header fails for some reason, there's no point
	 * adding the Date header.
	 */
	if ((add_identity_header(session, tdata)) != 0) {
		return;
	}
	add_date_header(session, tdata);
}

static struct ast_sip_session_supplement stir_shaken_supplement = {
	.method = "INVITE",
	.priority = AST_SIP_SUPPLEMENT_PRIORITY_CHANNEL + 1, /* Run AFTER channel creation */
	.incoming_request = stir_shaken_incoming_request,
	.outgoing_request = stir_shaken_outgoing_request,
};

static int unload_module(void)
{
	ast_sip_session_unregister_supplement(&stir_shaken_supplement);
	return 0;
}

static int load_module(void)
{
	ast_sip_session_register_supplement(&stir_shaken_supplement);
	return AST_MODULE_LOAD_SUCCESS;
}

#undef AST_BUILDOPT_SUM
#define AST_BUILDOPT_SUM ""

AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_GLOBAL_SYMBOLS | AST_MODFLAG_LOAD_ORDER, "PJSIP STIR/SHAKEN Module for Asterisk",
	.support_level = AST_MODULE_SUPPORT_CORE,
	.load = load_module,
	.unload = unload_module,
	.load_pri = AST_MODPRI_DEFAULT,
	.requires = "res_pjsip,res_pjsip_session,res_stir_shaken",
);