Basic Cryptography: Symmetric Key Encryption

Abstract

Symmetric Key (or “secret key”) cryptography is one of the two main branches of cryptography (the other being Asymmetric Key (or “public/private key”) cryptography. Symmetric Key cryptography scrambles “plaintext” (human-readable) information via one of various algorithms, along with a single cryptographic key, into the garbled form (“ciphertext”). Decryption with symmetric key recovers the original plaintext from the ciphertext through use of the same algorithm and key that was used to encrypt the information. Use of any other key in decryption will result in gibberish. This moves the need for secrecy from the algorithm, which can be published and peer reviewed, to the key. Symmetric Key cryptography is very fast and well suited to encrypting large amount of information. It is however, very poorly suited to key management. Asymmetric Key cryptography on the other hand is very slow, and suitable only for encrypting small amounts of information (e.g. one 256-bit key) but is ideally suited to key management. Most real-world cryptographic systems use both kinds of cryptography. We will build more and more complex structures on these simple concepts until we reach a full Public Key Infrastructure (PKI).