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METHOD TO ENCRYPT TG ID, ALGORITHM ID, AND KEY ID OF A PROJECT 25 TRANSMISSION IN AN ENCRYPTED CONTROL CHANNEL SYSTEM

IP.com Disclosure Number: IPCOM000007906D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-03
Document File: 1 page(s) / 54K

Publishing Venue

Motorola

Related People

Hans Christopher Sowa: AUTHOR

Abstract

The system is a Project 25 land mobile trunked radio system using an encrypted control channel (CC). The CC is encrypted and uses a message indi- cator (Ml) for the inbound CC and another Ml for the outbound CC. The control channel MIS are incremented on every microslot and are only sent on the CC periodically to save control channel bandwidth. The subscriber unit must increment the CC MIS in order to keep track of them. Voice com- munications are also encrypted and use a different Ml than the CC MIS. The voice Ml is sent as embedded information with the voice.

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Technical Developments

METHOD TO ENCRYPT TG ID, ALGORITHM ID, AND KEY ID OF A PROJECT 25 TRANSMISSION IN AN ENCRYPTED CONTROL CHANNEL SYSTEM

by Hans Christopher Sowa

SYSTEM BACKGROUND:

rithm ID and the key ID. An adversary could moni- tor the TG ID in the voice header and use it as a plain text attack on the encrypted TG ID informa- tion on the CC. Also, an adversary could monitor the algorithm ID and key ID to determine which encryption key is used most often in the system for voice trafhc, and attack that key first.

  The system is a Project 25 land mobile trunked radio system using an encrypted control channel (CC). The CC is encrypted and uses a message indi- cator (Ml) for the inbound CC and another Ml for the outbound CC. The control channel MIS are incremented on every microslot and are only sent on the CC periodically to save control channel bandwidth. The subscriber unit must increment the CC MIS in order to keep track of them. Voice com- munications are also encrypted and use a different Ml than the CC MIS. The voice Ml is sent as embedded information with the voice.

THE SOLUTION:

THE PROBLEM:

  An adversary is monitoring traffic on the sys- tem. The control channel (CC) signaling is encrypted to prevent traffic analysis. However, the voice trans- mission has the following information unencrypted; the talk group (TG) ID (in the header), the algo-

  The key used to encrypt the control channel is also used encrypt the voice transmission TG ID, algorit...