This document proposes a secure solution for landline telephone systems using encryption. It describes a hardware system that uses a microcontroller, RAM, and analog components to encrypt voice communications through time domain scrambling of speech blocks controlled by an algorithm and key. The system uses symmetric cryptography with keys up to 16 decimal digits long, providing over 10 quadrillion possible key combinations, making a brute force attack to break the key useless. Transposition tables are changed for every block, adding further security.

1. SECURE SOLUTION FOR LANDLINE TELEPHONE SYSTEM
KDU International Research Symposium - 2013
Lt Col(Dr) RM Monaragala , L Cdr DWS Lakmina, MR WLPK Wijesinghe, O/C
WMAGDP Weerakoon, O/C BU Edirisinghe and O/C AADS Indika
Department of Electrical and Electronic, Kotelawala Defence University, Kandawala Estate,
Ratmalana, Sri Lanka
<ravimonaragala@yahoo.com>, <weerakoonr@yahoo.com>, <umeshedirisinghe@gmail.com>,
<indike98@hotmail.com>
Abstract— Telecommunication service has become a most essential service in today’s world to
achieve various communication purposes as an example simply as sending massages one destination
to another. But there is an excessive risk sorting out due to the tendency of communication methods
to be tapped in general usage. Therefore it is required a secure way to communication being done
specially in military purposes.
Thus the human speech can gain significant cryptographic complexity and most redundancy being
concealed, by consequent application of time domain scrambling with long blocks divided into many
partitions controlled by a key algorithm that provides new transposition tables for every single
transposition block. Even if we take into account today's signal- and crypto-analytical capabilities, an
analogue transmission like this offers strong 'tactical security'.
The proposed hardware consists of a standard AVR-microcontroller. Specially used extended (S)RAM
for have high performance, latch register and only few analogue components. This constitutes for an
autonomous, transparent, experimentation-friendly and inexpensive platform.
In this implementation, we set up as a symmetrical cryptosystem whose initial keys are decimal
numbers of up to 16 digits. These symmetric keys will be entered directly to the device by using a
keypad. With 16 decimal places we have nearly 1016
different starting conditions (10 quadrillions) of
possible keys for the cryptosystem.
When we consider about possible attack (brute-force) to break off this key will be useless. It is
assumed that other conceivable attacking strategies (based on signal-analytical or phonetic pre-
processing techniques) were also too time-consuming for most realistic scenarios.
Of course, the transposition table is changed with every block in this system.