Changeover Command (encrypted)

Introduction/Additional information:

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.

Permitted as from:

12/05/2016

Registrant:

USCG

Physical link:

AIS

Message number (0=VDES-VDE):

DAC or VPFI:

366

FI / Message ID:

Version:

State:

in force

Number of slots (max):

Last modified:

29/01/2021 - 23:19

Used by:

USCG

Details:

Changeover Command (Message 26, broadcast)

	Parameter	# of Bits	Description
	Parameter	# of Bits	Description
Standard Message Header	Message ID	6	Identifier for Message 26; always 26.
	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).
	Source MMSI	30	MMSI number of source station. Varies according to the transmitter ID.
	Destination Indicator	1	0 = broadcast (no Destination ID field) 1 = Addressed (30 bit Destination ID field). Set to zero (broadcast).
	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).
	Destination ID	0	MMSI number of destination station. Not used.
	Spare	0	Not used.

Binary Data	DAC			10	Designated Area Code. Set to 366.
	Function Identifier			6	Function identifier. Set to 21.
	KMM Preamble	Reserved		3	Reserved for future use. Set to 0.
		Version		5	Version of KMM Preamble Message Body. Set to 0.
		MFID		8	Manufacturer’s ID, indicates conformance to standard. Set to 0.
		Algorithm ID		8	Identifies the algorithm used to encrypt payload. Set to (hex) 85 = AES-128.
		Key ID		16	Identifies the TEK used to encrypt the KMM.
		Message Indicator		72	Provides the message indicator to synchronize the encryption of the OTAR KMM
	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
		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.
		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.
			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”
			MAC Processing	2	Defines the type of MAC processing performed over the entire KMM. Set to “11” (Type 3 MAC)
			Spare	1	Set to zero.
			Done	1	Indicates if KMM is last in a series. 1 = More to follow (Not Done), 0 = done.
		Destination RSI		24	Radio Set Identifier number
		Source RSI		24	Radio Set Identifier Number
		KMF Message Number		16	A rolling sequence number for the KMF to prevent message playback. Binary number representing values from 0 – 65535.
		Changeover Count		8	Number of changeover instructions in this message. Set to (decimal) 1 - 35.
		Superceded Keyset ID		8	Identifies the keyset which is superceded and must be deactivated. 1-255
		Activated Keyset ID		8	Identifies the keyset which must be activated. 1-255
		Subsequent Changeover Instructions		0 - 544	Subsequent items are 16 bits apiece, Superceded Keyset ID, and Activated Keyset ID repeated for each item.

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.
			MAC Length	8	Length of message authentication code in octets. Set to (decimal) 8
			MAC Algorithm ID	8	Used with MAC Key ID to uniquely determine key to used to produce MAC. Set to (hex) 85 = AES-128.
			MAC Key ID	16	Identifies the Key to be used to compute the MAC
			T	1	Not used. Set to zero.
			D	1	Derived Key. Set to zero = Dedicated MAC Key
			R	1	Reserved for future use. Set to zero.
			Version	5	Defines the version of the MAC message body. Set to zero.
		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.
	Checksum			16	16-bit CRC calculated as per EAIS Specification
Footer	Spare			4	Four extra bits to ensure the message ends on a byte boundary. Set to zero.
	Communications State Selector			1	0 = SOTDMA communication state follows 1 = ITDMA communication state follows
	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.
Total bits				464 – 976	3 – 5 Slot Binary Message

Attachment:

eais_idd_v5_4_8may17.pdf [1]