The Text Message Payload, prior to encryption, varies from 74 to 896 bits in length. Conforming to the AES Encryption scheme with a 128-bit cipher key, the encrypted part of the payload then effectively becomes 128, 256, 384, 512, 640, 768 or 896 bits in length to create blocks of 128 bits.
|
Parameter |
Number of bits |
Description |
Encryption Status |
|
DAC |
10 |
Identifier for this message: 366 |
Unencrypted |
|
Function ID |
6 |
Function Identifier: 55 |
Unencrypted |
|
Source ID (MMSI) |
30 |
Source MMSI number. |
AES encrypted |
|
Acknowledgement Flag |
1 |
Reserved for future use. |
AES encrypted |
|
Segment Number |
11 |
Reserved for future use. |
AES encrypted |
|
Text length |
8 |
Length of Text Message |
AES encrypted |
|
Text Message |
0-822 Variable |
User Text Message (6 bits times 0-137 characters) |
AES encrypted |
|
Checksum |
8 |
Exclusive OR (XOR) 8-bit |
AES encrypted |
|
Stuffed bits |
Variable |
Variable; stuffed bits are inserted so that the number of bits in the encrypted field is a multiple of 128 bits; set to zero. |
AES encrypted |
|
TOTAL |
144 272 400 528 656 784 912 |
16 unencrypted bits plus 128, 256, 384, 512, 640, 768 or 896 encrypted bits |
|
Considering the constraints imposed by the AES encryption algorithm and recommended bit- stuffing guidelines contained in Section 3.2.2.1 of Annex 2 to ITU-R M.1371-5, this results in the following slot usage as a function of number of 6-bit ASCII characters:
|
Number of Slots |
Number of 6-bit ASCII characters |
|
1 |
not possible to have a 1-slot message |
|
2 |
0 - 30 |
|
3 |
31 - 73 |
|
4 |
74 - 115 |
|
5 |
116 - 137 |
