To protect data transmissions against interception, various methods are in use.

The encryption process basically processes clear text with a key, with the transformed message then transmitted over the insecure media. At the receiving end, deciphering is achieved using the same key and the clear text message is recovered.

Encryption may be on-line or off-line. On-line or stream encryption takes place at transmission time, whereas off-line or block encryption often is done before transmission, usually in the form of coding the clear text message into five letter or five figures groups. This is done by a key sequence.

Bit inversion inverts logical zeroes of a codeword with logical ones and vice versa either in a static pattern, e.g., bit 3 and bit 5 or in a dynamic pattern depending on the value of the codeword. The result of bit inversion is character substitution – the original clear text character is substituted with another character.

Character transposition or permutation replaces characters in one position in the clear text with characters in another position. One of the most famous implementation of permutation transformations is the German Enigma cryptographic machine in use before and during the World War II.

Substitution and permutation of clear text messages is relatively easily broken using statistical methods and a knowledge of the frequency of characters in various languages.

Shift-register encryption, a stream encryption method, uses one or more shift-registers into which the clear language message is shifted and EXORed with a key or part of itself. Taps in various positions of the registers may feed bits back to the input to complicate decryption by interceptors. The shift-registers of the transmitting and receiving equipment must be initialized to the same value - the seed.

System security in conventional cryptographic systems depends on the safeguard of the key and on the length of the key. Generally speaking, the longer the key, the more difficult the work of the cryptanalyst will be. Unbreakable systems like the one-time pad will only use its key once and its key length is as long as the clear text message. Cryptographic messages are constructed by byte wise EXORing of the clear text message and the key. Kerckhoffs principle is the design criteria for modern cryptographic systems: “The security of a cryptographic system must not depend on keeping the algorithm secret. The security is only dependent on keeping the key secret.”

Modern cryptographic systems use the principles of public keys. Traditional systems use the same key for encryption and decryption, and safeguard this key. Public key systems use two different keys, one for encryption and the other for decryption. Public keys and their encryption algorithms are publicly available, but the decryption keys are secret and private; the transmitting party encrypts his message with the public key, and the recipient decrypts the message using his private key.