Disk encryption software
Disk encryption generally refers to wholesale encryption that operates on an entire volume mostly transparently to the user, the system, and applications. This is generally distinguished from file-level encryption that operates by user invocation on a single file or group of files, and which requires the user to decide which specific files should be encrypted. Disk encryption usually includes all aspects of the disk, including directories, so that an adversary cannot determine content, name or size of any file. It is well suited to portable devices such as laptop computers and thumb drives which are particularly susceptible to being lost or stolen. If used properly, someone finding a lost device cannot penetrate actual data, or even know what files might be present.
Affine cipher
In the affine cipher the letters of an alphabet of size are first mapped to the integers in the range . It then uses modular arithmetic to transform the integer that each plaintext letter corresponds to into another integer that correspond to a ciphertext letter.
The encryption function for a single letter is 
where modulus is the size of the alphabet and and are the key of the cipher. The value must be chosen such that and are coprime. The decryption function is 
where is the modular multiplicative inverse of modulo . I.e., it satisfies the equation 
The multiplicative inverse of only exists if and are coprime. Hence without the restriction on , decryption might not be possible.
It can be shown as follows that decryption function is the inverse of the encryption function,