Symmetric Cryptography

Symmetric ciphers are algorithms that use the same key both to encrypt and decrypt data. The goal is to use short secret keys to securely and efficiently send long messages.

We can split symmetric-key ciphers into two types, block ciphers and stream ciphers.

Block ciphers break up a plaintext into fixed-length blocks, and send each block through an encryption function together with a secret key.

Stream ciphers meanwhile encrypt one byte of plaintext at a time, by XORing a pseudo-random keystream with the data.

Block ciphers only specify how to encrypt and decrypt individual blocks, and a mode of operation must be used to apply the cipher to longer messages.