Changeover Command (encrypted)
This ASM implements the Project 25 OTAR Changeover Command message.
Usage Notes
- This message is sent from the Key Management Facility to the Mobile Radio (MR) to support the change of the active keyset(s) at an MR or group of MR.
- This message instructs a MR to start using an already held keyset. Thus, this message must be preceded by a successful Modify Key transaction.
- 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 Changeover” field is optional and unlimited per Project 25. In order to limit message size to 5 slots, the count of instructions is limited to 35 per message.
Version 0:
This is an RTCM compliant implementation of the Project 25 OTAR Changeover Command. This ASM only has an AIS Message 26 variant.
Changeover 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 21. |
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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) 5 = Changeover 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) 25 - 93. |
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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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Changeover Count |
8 |
Number of changeover instructions in this message. Set to (decimal) 1 - 35. |
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Superceded Keyset ID |
8 |
Identifies the keyset which is superceded and must be deactivated. 1-255 |
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Activated Keyset ID |
8 |
Identifies the keyset which must be activated. 1-255 |
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Subsequent Changeover Instructions |
0 - 544 |
Subsequent items are 16 bits apiece, Superceded Keyset ID, and Activated Keyset ID 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 (256, 384, 512). 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 |
464 – 976 |
3 – 5 Slot Binary Message |
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