/*
 * openvpn.c
 *
 * Copyright (C) 2011-18 - ntop.org
 *
 * OpenVPN TCP / UDP Detection - 128/160 hmac
 *
 * Detection based upon these openvpn protocol properties:
 *   - opcode
 *   - packet ID
 *   - session ID
 *
 * Two (good) packets are needed to perform detection.
 *  - First packet from client: save session ID
 *  - Second packet from server: report saved session ID
 *
 * TODO
 *  - Support PSK only mode (instead of TLS)
 *  - Support PSK + TLS mode (PSK used for early authentication)
 *  - TLS certificate extraction
 *
 */

#include "ndpi_protocol_ids.h"

#define NDPI_CURRENT_PROTO NDPI_PROTOCOL_OPENVPN

#include "ndpi_api.h"


#define P_CONTROL_HARD_RESET_CLIENT_V1  (0x01 << 3)
#define P_CONTROL_HARD_RESET_CLIENT_V2  (0x07 << 3)
#define P_CONTROL_HARD_RESET_SERVER_V1  (0x02 << 3)
#define P_CONTROL_HARD_RESET_SERVER_V2  (0x08 << 3)
#define P_OPCODE_MASK 0xF8
#define P_SHA1_HMAC_SIZE 20
#define P_HMAC_128 16                            // (RSA-)MD5, (RSA-)MD4, ..others
#define P_HMAC_160 20                            // (RSA-|DSA-)SHA(1), ..others, SHA1 is openvpn default
#define P_HARD_RESET_PACKET_ID_OFFSET(hmac_size) (9 + hmac_size)
#define P_PACKET_ID_ARRAY_LEN_OFFSET(hmac_size)  (P_HARD_RESET_PACKET_ID_OFFSET(hmac_size) + 8)
#define P_HARD_RESET_CLIENT_MAX_COUNT  5

static
#ifndef WIN32
inline
#endif
u_int32_t get_packet_id(const u_int8_t * payload, u_int8_t hms) {
    return ntohl(*(u_int32_t*)(payload + P_HARD_RESET_PACKET_ID_OFFSET(hms)));
}

static
#ifndef WIN32
inline
#endif
int8_t check_pkid_and_detect_hmac_size(const u_int8_t * payload) {
    // try to guess
    if (get_packet_id(payload, P_HMAC_160) == 1)
        return P_HMAC_160;
    if (get_packet_id(payload, P_HMAC_128) == 1)
        return P_HMAC_128;
    return -1;
}

void ndpi_search_openvpn(struct ndpi_detection_module_struct* ndpi_struct,
        struct ndpi_flow_struct* flow) {
    struct ndpi_packet_struct* packet = &flow->packet;
    const u_int8_t * ovpn_payload = packet->payload;
    const u_int8_t * session_remote;
    u_int8_t opcode;
    u_int8_t alen;
    int8_t hmac_size;
    int8_t failed = 0;

    if (packet->payload_packet_len >= 40) {
        // skip openvpn TCP transport packet size
        if (packet->tcp != NULL)
            ovpn_payload += 2;

        opcode = ovpn_payload[0] & P_OPCODE_MASK;

        if (flow->ovpn_counter < P_HARD_RESET_CLIENT_MAX_COUNT && (opcode == P_CONTROL_HARD_RESET_CLIENT_V1 ||
                    opcode == P_CONTROL_HARD_RESET_CLIENT_V2)) {

            if (check_pkid_and_detect_hmac_size(ovpn_payload) > 0) {
                memcpy(flow->ovpn_session_id, ovpn_payload+1, 8);

                NDPI_LOG_DBG2(ndpi_struct,
                        "session key: %02x%02x%02x%02x%02x%02x%02x%02x\n",
                        flow->ovpn_session_id[0], flow->ovpn_session_id[1], flow->ovpn_session_id[2], flow->ovpn_session_id[3],
                        flow->ovpn_session_id[4], flow->ovpn_session_id[5], flow->ovpn_session_id[6], flow->ovpn_session_id[7]);
            }
        } else if (flow->ovpn_counter >= 1 && flow->ovpn_counter <= P_HARD_RESET_CLIENT_MAX_COUNT &&
                (opcode == P_CONTROL_HARD_RESET_SERVER_V1 || opcode == P_CONTROL_HARD_RESET_SERVER_V2)) {

            hmac_size = check_pkid_and_detect_hmac_size(ovpn_payload);

            if (hmac_size > 0) {
                alen = ovpn_payload[P_PACKET_ID_ARRAY_LEN_OFFSET(hmac_size)];
                session_remote = ovpn_payload + P_PACKET_ID_ARRAY_LEN_OFFSET(hmac_size) + 1 + alen * 4;

                if (memcmp(flow->ovpn_session_id, session_remote, 8) == 0) {
                    NDPI_LOG_INFO(ndpi_struct,"found openvpn\n");
                    ndpi_set_detected_protocol(ndpi_struct, flow, NDPI_PROTOCOL_OPENVPN);
                }
                else {
                    NDPI_LOG_DBG2(ndpi_struct,
                            "key mismatch: %02x%02x%02x%02x%02x%02x%02x%02x\n",
                            session_remote[0], session_remote[1], session_remote[2], session_remote[3],
                            session_remote[4], session_remote[5], session_remote[6], session_remote[7]);
                    failed = 1;
                }
            } else
                failed = 1;
        } else
            failed = 1;

        flow->ovpn_counter++;

        if (failed) {
            NDPI_EXCLUDE_PROTO(ndpi_struct, flow);
        }
    }
}

void init_openvpn_dissector(struct ndpi_detection_module_struct *ndpi_struct, u_int32_t *id, NDPI_PROTOCOL_BITMASK *detection_bitmask)
{
    ndpi_set_bitmask_protocol_detection("OpenVPN", ndpi_struct, detection_bitmask, *id,
            NDPI_PROTOCOL_OPENVPN,
            ndpi_search_openvpn,
            NDPI_SELECTION_BITMASK_PROTOCOL_TCP_OR_UDP_WITH_PAYLOAD,
            SAVE_DETECTION_BITMASK_AS_UNKNOWN,
            ADD_TO_DETECTION_BITMASK);

    *id += 1;
}
