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Symmetric Cryptography
  Chapter 1: Basic Terminology
Encryption is the process of turning a clear-text message (Plaintext) into a data stream which looks like a meaningless and random sequence of bits (ciphertext). The process of turning ciphertext back into plaintext is called decryption.

Cryptography deals with making communications secure. Cryptoanalysis deals with breaking ciphertext, that is, recovering plaintext without knowing the key. Cryptology is a branch of mathematics which deals with both cryptography and cryptoanalysis.

A cryptographic algorithm, also known as a cipher, is a mathematical function which uses plaintext as the input and produces ciphertext as the output and vice versa.

All modern ciphers use keys together with plaintext as the input to produce ciphertext. The same or a different key is supplied to the decryption function to recover plaintext from ciphertext. The details of a cryptographic algorithm are usually made public. It is the key that the security of a modern cipher lies in, not the details of the cipher.

Symmetric algorithms use the same key for encryption and decryption. These algorithms require that both the sender and receiver agree on a key before they can exchange messages securely.

Some symmetric algorithms operate on 1 bit (or sometimes 1 byte) of plaintext at a time. They are called stream ciphers. Other algorithms operate on blocks of bits at a time. They are called block ciphers. Most modern block ciphers use the block size of 64 bits.

Public-key algorithms (also known as asymmetric algorithms) use two different keys (a key pair) for encryption and decryption. The keys in a key pair are mathematically related, but it is computationally infeasible to deduce one key from the other. These algorithms are called "public-key" because the encryption key can be made public. Anyone can use the public key to encrypt a message, but only the owner of the corresponding private key can decrypt it.

Some public-key algorithms such as RSA allow the process to work in the opposite direction as well: a message can be encrypted with a private key and decrypted with the corresponding public key. If Alice (or anyone else) can decrypt a message with Bob's public key she knows that the message must have come from Bob because no one else has Bob's private key. Digital signatures work this way.

Symmetric Cryptography

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