Change RSI Command (encrypted)
This ASM implements the Project 25 OTAR Change Radio Set Identifier (RSI) message.
Usage Notes
- This message is sent from the Key Management Facility to the Mobile Radio to support the changeover of an individual or key management group Radio Set Identifier.
- This message is broadcast as a non-addressed message, to reduce the message size, and to avoid issues from a relocated RSI to a different MMSI.
- The “Subsequent RSI Change” field is optional and unlimited per Project 25. In order to fix message size at 5 slots, the number of instructions is limited to 8 per message.
Version 0:
This is an RTCM compliant implementation of the Project 25 OTAR Change RSI (Radio Set Identifier) Command.
This ASM only has an AIS Message 26 variant.
Change RSI Command (Message 26, broadcast)
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Parameter |
# of Bits |
Description |
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Standard Message Header |
Message ID |
6 |
Identifier for Message 26; always 26. |
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Repeat Indicator |
2 |
Indicates how many times a message has been repeated. 0 – 3; 0 = default; 3 = do not repeat any more. Set to zero (default). |
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Source MMSI |
30 |
MMSI number of source station. Varies according to the transmitter ID. |
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Destination Indicator |
1 |
0 = broadcast (no Destination ID field) 1 = Addressed (30 bit Destination ID field). Set to zero (broadcast). |
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Binary Data Flag |
1 |
0 = unstructured binary data (no Application Identifier bits used) 1 = binary data coded as defined by the 16-bit AI. Set to 1 (structured). |
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Destination ID |
0 |
MMSI number of destination station. Not used. |
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Spare |
0 |
Not used. |
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Binary Data |
DAC |
10 |
Designated Area Code. Set to 366. |
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Function Identifier |
6 |
Function identifier. Set to 19. |
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KMM Preamble |
Reserved |
3 |
Reserved for future use. Set to 0. |
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Version |
5 |
Version of KMM Preamble Message Body. Set to 0. |
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MFID |
8 |
Manufacturer’s ID, indicates conformance to standard. Set to 0 |
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Algorithm ID |
8 |
Identifies the algorithm used to encrypt payload. Set to (hex) 85 = AES-128 |
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Key ID |
16 |
Identifies the TEK used to encrypt the KMM. |
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Message Indicator |
72 |
Provides the message indicator to synchronize the encryption of the OTAR KMM |
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Encrypted Payload |
OTAR Message ID |
8 |
Identifies the Project 25 “Message ID” type. One of 255 possible values assigned as the message type. Set to (decimal) 3 = Change RSI Command |
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Message Length |
16 |
A 16 bit binary number that defines the length (in octets) of the subsequent OTAR fields, including MAC. Set to (decimal) 31 - 87. |
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Message Format |
RSP |
2 |
Response Kind defines acknowledgment to be returned to the sender of the KMM. 00 =Rsp Kind 1, 01 = Rsp Kind 2, 10 = Rsp Kind 3 and 11 is undefined. |
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MN |
2 |
Message Number defines the size of the Message Number field in the KMM. 00 = no message number, 10 = a 2 octet message number. Set to “10” |
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MAC Processing |
2 |
Defines the type of MAC processing performed over the entire KMM. Set to “11” (Type 3 MAC) |
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Spare |
1 |
Set to zero. |
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Done |
1 |
Indicates if KMM is last in a series. 1 = More to follow (Not Done), 0 = done. |
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Destination RSI |
24 |
Radio Set Identifier number |
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Source RSI |
24 |
Radio Set Identifier Number |
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KMF Message Number |
16 |
A rolling sequence number for the KMF to prevent message playback. Binary number representing values from 0 – 65535. |
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Change RSI Instruction Count |
8 |
The number of Change-RSI instructions in this message Set to (decimal) 1 - 8. |
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Old RSI |
24 |
Identifies the individual or group identifier that is to be changed or deleted. |
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New RSI |
24 |
Identifies the individual or group identifier which is to be added. |
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RSI Message Number |
16 |
Initial value for the inbound and outbound message numbers for RSI transaction. This is a rolling sequence number to prevent message playback. Binary number representing values from 0 – 65535. Set to all zeroes if RSI is being deleted. |
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Subsequent Change- RSI Instructions |
0 – 448 |
Subsequent items are 64 bits apiece, Old RSI, New RSI, RSI message Number repeated for each item. |
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Binary Data |
Encrypted Payload |
Message Authentication Code |
MAC |
64 |
OTAR Message authentication code, concatenated to key length/2. Calculated per paragraph 13.5.2 of TIA 102.AACA-A, September 2014. |
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MAC Length |
8 |
Length of message authentication code in octets. Set to (decimal) 8 |
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MAC Algorithm ID |
8 |
Used with MAC Key ID to uniquely determine key to used to produce MAC. Set to (hex) 85 = AES-128. |
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MAC Key ID |
16 |
Identifies the Key to be used to compute the MAC |
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T |
1 |
Not used. Set to zero. |
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D |
1 |
Derived Key. Set to zero = Dedicated MAC Key |
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R |
1 |
Reserved for future use. Set to zero. |
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Version |
5 |
Defines the version of the MAC message body. Set to zero. |
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Encryption Bit Padding |
variable |
Sufficient spare bits to ensure that the binary data is a multiple of 128 bits for block encryption (384, 512, 640, 768). Set to zero. |
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Checksum |
16 |
16-bit CRC calculated as per EAIS Specification |
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Footer |
Spare |
4 |
Four extra bits to ensure the message ends on a byte boundary. Set to zero. |
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Communications State Selector |
1 |
0 = SOTDMA communication state follows 1 = ITDMA communication state follows |
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Communications State |
19 |
SOTDMA communication state (see ITU 1371-5 § 3.3.7.2.1, Annex 2), if communication state selector flag is set to 0, or ITDMA communication state (§ 3.3.7.3.2, Annex 2), if communication state selector flag is set to 1. |
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Total bits |
592 – 976 |
3 - 5 Slot Binary Message |
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