--------------------------------[Psion5 PDA Weak Encryption System]--------->

Author: evilcry
e-mail: evilcry@virgilio.it / evilcry@gmail.com
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-------------------[The Analisys]--------->

Psion 5 PDA implements a very weak encryption system, here i'll demonstrate how we can obtain a fully clear plaintext from an encrypted block (a Cipher Only Attack).

The algorithm used is a simple Binary Addition:

Encryption phase: The key entered is added character by character (byte by byte) to the data to be encrypted. Here addition modulo 256 is used. The key is applied cyclically, once all bytes of the key have been used, the algorithm reverts to the first character until the text has been completely encrypted.

Decryption phase: The characters of the key have to be subtracted from the encrypted text modulo 256.

As previously said a Cipher Only Attack can be performed, we need to know: 

1)Statistical Frequency: The characters which occur most frequently), but if we don't know this parameter, there is no problem, because we can use statistical tables, or at least a small bruteforce it's range is truly small, only 26 as the number of letters, more if we are working with not common languages (xenographyc tables are useful in this case).

2)The key length: We can determine the keylenght using Autocorrelation (which is an index of similarity for differents portions of text). 
Each character had not an equal probability occurs, but the probability of two identical characters occurring is relatively large if they are separated by a multiple of the key length. After we have found the keylenght with Autocorrelation test, throughout the Histogram analisys related to the character which occurs most frequently, we can found the encryption key.