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AIS message authentication

Introduction/Additional information: 

This VDE Protocol Format may be used by equipment transmitting a digital signature for a message previously sent via the AIS channels, i.e., the transmission being signed. The AIS messages may not be older than 59 seconds when the authentication message is transmitted. The transmission may be triggered by interrogation from a coast station or be scheduled by the transmitting application.

The AIS channel content is not repeated but used to generate a signature for authentication by receiving equipment. 

To allow receiving equipment to link the authentication message with the transmission being signed, the message ID, MMSI, channel ID, slot number and time stamp is provided in the authentication message.

Equipment receiving this VDE Protocol Format should output the message as VDE message at the PI.

Applications may use the VDE message to augment received AIS messages on shore and ship.

Permitted as from: 
01/12/2022
Registrant: 
IALA
Physical link: 
VDE-TER
Message number (0=VDES-VDE): 
0
DAC or VPFI: 
2
FI / Message ID: 
6
Technical point of contact: 

IALA

G1117-Ed3.0-VHF-Data-Exchange-System-VDES-Overview

State: 
proposal
Number of slots (max): 
1
FEC: 
No
Last modified: 
29/03/2023 - 15:11
Details: 

Parameter

Number of bits

Description

VPFI

16

2

Message ID

16

6

AIS Message ID

6

AIS Message ID 1, 2, 3, … 27

MMSI

30

Ref. 1371-5

Channel ID

2

Channel where the transmission being signed occurred:

0: AIS 1

1: AIS 2

Slot number

12

Slot number value 0 .. 2249, for the transmission being signed, on the specified channel, as defined in ITU-R M.2092-1.

To be set to 2250 if unknown.

Time stamp

32

Unsigned integer number of seconds after the 1st of January

1970 00:00:00 UTC. After the 19th of January 2038 03:14:07

UTC, the number is the number of seconds after this new date (wraps around);

Set to the time when the transmission being signed was transmitted.

To avoid replay attacks, this value shall be used by the receiver application to verify the validity of the data

Signature

512

Signature of a concatenation of the above data (all bits from the start of the VPFI field to the end of the time stamp field) and all message bits of the AIS transmission being signed, as defined in Rec. ITU-R M.1371-5. The bits corresponding to the AIS message shall form the least significant bits of the concatenated bitstring.

String_to_sign = this_msg + ais_message;

The signature generation, algorithm to be used is as described for the terrestrial bulletin board, see Rec. ITU-R M.2092-1, Annex 4, §4.15

Total number of bits

626