What Is Encryption?

This section describes what is encryption - A bijection function that uses a key, encryption key, to compute the image.

Encryption Function - A bijection function that uses a key, encryption key, to compute the image.

Clear Text Space - The domain of an encryption function.

Cipher Text Space - The codomain of an encryption function.

Decryption Function - The inversion function of an encryption function. Decryption function also uses a key, decryption key, to compute the image.

Encryption Scheme - An algorithm that defines a cleartext space, a ciphertext space, and a set of encryption keys. For each encryption key, the scheme also defines an encryption function, a decryption key, and a decryption function. An encryption scheme is also called a cipher.

Symmetric Key Encryption - An encryption scheme that "the decryption function uses the same key as its encryption function". Symmetric key encryption is also called one key, or secret key encryption.

Asymmetric Key Encryption - An encryption scheme that "the decryption function uses needs a different key than the key used in its encryption function, and it is almost impossible to compute one from another". Asymmetric key encryption is also called public key encryption.

Block Cipher - An encryption scheme that "cleartext is broken up into blocks of fixed length, and encrypted one block at a time".

Stream Cipher - An encryption scheme that "cleartext is encrypted as a continuous stream alphabets".

Simple Substitution Cipher - An encryption scheme that "Each alphabet in the cleartext is substituted by a single alphabet defined by the key".

Homophonic Substitution Cipher - An encryption scheme that "Each alphabet in the cleartext is substituted by an alphabetic string randomly selected from a set of strings defined by the key".

Transposition Cipher - A block cipher that "Alphabets in a block is permuted in an order defined by the key".

Product Cipher - An encryption scheme that "uses multiple ciphers in which the ciphertext of one cipher is used as the cleartext of the next cipher". Usually, substitution ciphers and transposition ciphers are used alternatively to construct a product cipher.

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