ANALOG DTMF ENCRYPTION

Publishing Venue

Motorola

Related People

Abstract

In current analog trunking systems, Motorola provides a secure 12kb option. This option will give a user secure voice communications. A current prob- lem with many 12kb secure systems is that the DTMF tones can not be encoded into I2kb secure. The analog systems today solve this problem by "fall- ing back" to non-secure mode to transmit the DTMF tones. Anyone "listening" to the communications can "pick" the DTMF tones up and decode them with off the shelf equipment into numbers. These numbers can be phone numbers, credit card num- bers, calling card numbers, or access numbers. With no way to secure this information, users who think A mobile can still fallback to non-secure mode to transmit the DTMF digits, but the radio will encode the DTMF digitswhile in a secure call. The mobile can use the same encoding algorithm and key that is used for the voice encoding, but the DTMF tones are encoded across a DTMF alpha- bet. The fixed end will then decode the DTMF tones using the same algorithm and key. The algorithm used can be any known or new algorithm, as long as a DTMF alphabet is used.

0 M ^MO-LA

Technical Developments Volume 24 March 1995

ANALOG DTMF ENCRYPTION

by Kristin Cramer, Mike Wirtjes and Jeff Jacobs

they may be transmitting their access numbers securely are not.

BACKGROUND

  In current analog trunking systems, Motorola provides a secure 12kb option. This option will give a user secure voice communications. A current prob- lem with many 12kb secure systems is that the DTMF tones can not be encoded into I2kb secure. The analog systems today solve this problem by "fall- ing back" to non-secure mode to transmit the DTMF tones. Anyone "listening" to the communications can "pick" the DTMF tones up and decode them with off the shelf equipment into numbers. These numbers can be phone numbers, credit card num- bers, calling card numbers, or access numbers. With no way to secure this information, users who think

  A mobile can still fallback to non-secure mode to transmit the DTMF digits, but the radio will encode the DTMF digitswhile in a secure call. The mobile can use the same encoding algorithm and key that is used for the voice encoding, but the DTMF tones are encoded across a DTMF alpha- bet. The fixed end will then decode the DTMF tones using the same algorithm and key. The algorithm used can be any known or new algorithm, as long as a DTMF alphabet is used.

SOLUTION

KEY w

011134432# I) ;:;;;y;;,n I) 21'94#435095926

DTMF Tones El

KEY a

Radio

From Radio

68 0 Motorola. 1°C 1995