Short Message Service (sms) security solution for mobile devices

3. One 
Time 
Pad 
One Time Pad (OTP) is a unique form of cryptography because it is the 
only unbreakable cipher, even in theory. OTP operation is straightforward and 
simple. The key is a string of random numbers that is as long as the message 
itself. Each key is used for only one message; the key is never reused and 
assumed to be perfectly random. A random key sequence added to a non-
random plaintext message produces a completely random cipher text [28]. 
Therefore every plain text message is possible and there is no way for the 
cryptanalyst to determine which plaintext is the correct one. 
The biggest challenges in using OTP are the generation of truly random 
key sequences and the key exchange. Similar to symmetric encryption, all 
parties in the secure conversation must have the same key. Furthermore, the key

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is now as large as the amount of data that is to be exchanged in the whole 
network. Furthermore, the use of keys must be synchronized among users such 
that the keys are never reused. 
The use of OTP may have great potential for SMS encryption because of 
the recent advances in memory technology. Mobile devices are now equipped 
with memory card slots for external memory devices such as Secure Digital (SD) 
cards. Memory capacities for SD cards have grown and the prices have dropped 
tremendously. A four gigabyte SD card may be purchased online at a price of 
less than US$100. Assuming that a 160 byte key sequence is used to encrypt 
every SMS message, 25 million messages can be encrypted using OTP. If 10 
messages are exchanged every minute in a network 24 hours a day, a four 
gigabyte key bank is enough to sustain 4.7 years of usage without the need for a 
key change. Therefore, the use of OTP may have great potential in applications 
where the content of the SMS is highly confidential.