Practical Issues

Symmetric encryption is usually applied to blocks of data larger than 64 or 128 bits, this means that the plaintext must be split up into blocks of this size and processed as such.

Electronic Codebook (ECB) is the simplest approach to this, wherein each block of data is simply encrypted with the same key. While simple and easily paralleled, this approach can introduce regularities into the ciphertext, which can be exploited in Cryptanalysis.

Two solutions to this are cipher block chaining, and stream ciphers.