Basic Concepts
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(Continued from previous part...)
Encryption
Encryption Function  A bijection 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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