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METHOD TO DERIVE A UNIQUE ENCRYPTION KEY FOR A PRIVATE CALL WITHOUT USING A KEY EXCHANGE

IP.com Disclosure Number: IPCOM000007910D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-03
Document File: 2 page(s) / 133K

Publishing Venue

Motorola

Related People

Hans Christopher Sowa: AUTHOR

Abstract

The system is a land mobile a 2-way radio con- ventional system using encryption, or it is an tmnking system using encryption. A radio in the system is allowed to private call (PC) another radio in the sys- tem such that only the two radios are involved in the call.

This text was extracted from a PDF file.
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0 M

MO7VROLA Technical Developments

MEfHOD TO DERIVE A UNIQUE ENCRYPTION KEY FOR A PRIVATE CALL WITHOUT USING A KEY EXCHANGE

by Hans Christopher Sowa

SYSTEM BACKGROUND:

any additional data bandwidth. The UTEK used for the PC should not be derivable by any other users not involved in the PC.

  The system is a land mobile a 2-way radio con- ventional system using encryption, or it is an tmnking system using encryption. A radio in the system is allowed to private call (PC) another radio in the sys- tem such that only the two radios are involved in the call.

  Over-The-Air Rekeying (OTAR) is also used in the system by having a Key Management Facility (KMF) send trahic encryption keys (TEKs) to the radios. The TEKs OTARed to the users are sent encrypted by the radios unique key encryption key (UKEK). Each radio in the system has a different UKEK.

THE SOLUTION:

Configuration l-Supervisor radio to users

  Using the following invention we can derive a UTEK for use between a radio user and supervisor. All radios in the system will have the same secret PC key that is in protected key memory. The super- visor radio will also contain a hash (one way func- tion) of the UKEK of the radio users that he can PC the users. The PC between the user1 and the super- visor will be encrypted with the hash of UKEKl exclusive ORed with the PC secret key, using nota- tion this is H(UKEK1) EOR (PC Secret key). Since every user has a different UKEKn the HO]KEKn) will be different so only the user involved with PC with the supervisor will be able to derive the key for the PC. The hashes ofthe UKEKs are OTARed (sent encrypted over the air) to the supervisor radio. The user radios need the hashing algorithm and the super- visor radio does not need the hashing algorithm.

  Below is a table of the radio storage for the super- visor and 50 users. Unprotected information is stored unencrypted in radio EEprom and protected infor- mation is stored under tamper protection.

UTEKn derived for sup PC call H(UKEK1) EOR (PC secret key) H(UKEK2) EOR (PC secret key) I, I, ,, ,, ,, I, ,, ,, ,, n n I I I, n n n I I n ,, n ,, I, I, I, I, ,, n c c c I I, n

H(UKEK50) EOR (PC secret key)

UTEKn derived for sup PC call H(UKEKn) EOR (PC secret key)

THE PROBLEM:

A user would like to make a private call (PC) with another user in the system. Unfortunately, to
.make key management simpler, every user in the system has the same PC key. This allows other users to be able to go into conventional scan and hear the PC since they have the PC key. Additionally, we do not want to use additional data bandwidth and time to do a key exchange between the two units or get a unique TEK from a Key Management Facility.

We need the radios in the private call to derive a unique t&c encryption key (UIEK) without using

Supervisor Radio:

Unprotected H(UKEK1) H(UKEK2) nn,,,,,,nn, lnlnlln

Protected PC secret key

H(UKEKS0)

User n radio: Unprotected None

Protected UKEKn PC secret key

0 Motorola. Inc. 19% 118 November 199...