It bases on the pseudo random generator "Mersenne Twister" developed in 1996/1997 by Makoto Matsumoto and Takuji Nishimura. It is proved that the period of that generator is 2^19937-1 and a 623-dimensional equidistribution property is assured.
Nevertheless, please keep in mind that this random generator has been developed for Monte-Carlo simulations and is not necessarly secure for encryption applications like Cipher.
To initialze the random generator you are ask to provide at least a 16 sign pass phrase where you should use at least small and capital letters. If somebody starts a brute force attack then he would need in worst case around 2.86x10^27 tries.
For the reason that Cipher simply uses an XOR operation to generate the encrypted file out of the original file and the pseudo random sequence, it is not recommended to use the same pass phrase for two different files you want to encode. In such a case an attack can be for example an XOR between the two encrypted files which could probably compromise parts of the pseudo random sequence.
How to use Cipher:
Cipher is a command line tool. Just start it without any parameter to get the syntax:
Error: Missing Argument!
Syntax: cipher < Input File> < Pass Phrase>
Example: cipher "Test 1.doc" "password"
Cipher does not touches the original file but it creates an encrypted copy with the ending .cph
For the reason that the program uses an XOR operation for encryption the decryption of a file works exactly in the same way as the encryption.
Please keep in mind that for each encryption/decryption step Cipher simply adds ".cph" to the file.