described above. Messages Messages sent and received over the air All AIS messages transmit 3 basic elements of information: The MMSI number of the ship or equipment that holds the transmitter (base station, buoy, etc.) The identification of the message being transmitted (See below table) A repeat indicator that was designed to be used for repeating messages over obstacles by relay devices. The following table gives a summary of all the currently used AIS messages. Messages sent to other equipment in the ship AIS equipment exchange information with other equipment using NMEA 0183 sentences. The NMEA 0183 standard uses two primary sentences for AIS data !AIVDM (received data from other vessels) !AIVDO (own vessel's information) Typical NMEA 0183 standard AIS message: !AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D In order: !AIVDM: The NMEA message type, other NMEA device messages are restricted 1 Number of sentences (some messages need more than one, maximum generally is 9) 1 Sentence number (1 unless it is a multi-sentence message) The blank is the sequential message ID (for multi-sentence messages) A The AIS channel (A or B), for dual channel transponders it must match the channel used 14eG;... The encoded AIS data, using AIS-ASCII6 0* End of data, number of unused bits at end of encoded data (0-5) 7D NMEA checksum (NMEA 0183 Standard CRC16) Security AIS was designed as an intentionally open standard and due to the unauthenticated and unencrypted nature of AIS, recently Balduzzi, Pasta, Wilhoit et al. showed that AIS is vulnerable to different threats like spoofing, hijacking and availability disruption. These threats affect both the implementation in online providers and the protocol specification, which make the problems relevant to all transponder installations (estimated at 300,000+). Publicly available ship monitoring websites rely on largely unauthenticated data feeds from volunteer-operated AIS receiver network, whose messages can be relatively easily faked by means of injecting AIS packets into the raw data stream, or on-air using slightly more complex equipment such as SDR. Ship-to-ship communications are however sent by Class B transponders which are certified to only supply GPS position from integrated receiver, so circumventing these messages would require SDR or GPS spoofing. Spoofing On 18 June 2021 AIS receivers in Chornomorsk, Ukraine, reported HMS Defender and HNLMS Evertsen allegedly sailing towards Sevastopol Russian military base in annexed Crimea while the ships were safely moored in Odessa, according to numerous live port webcam feeds and witnesses, which implied that falsified AIS