Cryptography Reference
In-Depth Information
5
Encryption Schemes
The problem of providing secret communication over insecure media is
the traditional and most basic problem of cryptography. The setting of
this problem consists of two parties communicating through a channel
that is possibly tapped by an adversary. The parties wish to exchange
information with each other, but keep the “wire-tapper” as ignorant
as possible regarding the contents of this information. The canonical
solution to the above problem is obtained by the use of encryption
schemes. Loosely speaking, an encryption scheme is a protocol allow-
ing these parties to communicate secretly with each other. Typically,
the encryption scheme consists of a pair of algorithms. One algorithm,
called encryption , is applied by the sender (i.e., the party sending a mes-
sage), while the other algorithm, called decryption , is applied by the
receiver. Hence, in order to send a message, the sender first applies
the encryption algorithm to the message, and sends the result, called
the ciphertext , over the channel. Upon receiving a ciphertext, the other
party (i.e., the receiver) applies the decryption algorithm to it, and
retrieves the original message (called the plaintext ).
In order for the above scheme to provide secret communication, the
communicating parties (at least the receiver) must know something
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