Lecture 19: Introduction to Encryption

Cryptography Basics

A message to be encrypted (known as plaintext) is transformed by the use of a function parameterised by a key, thus:

The security of the ciphertext depends on:

The nature of the encryption method, or algorithm. It is nowadays generally agreed that open publication of details of the algorithm is a Good Thing.
The secrecy of the key. Current opinion is that, given a suitably powerful encryption algorithm, the security of the system should depend entirely on:
1. keeping the key secret, and
2. the length (usually measured in bits) of the key itself, which is usually a very good indicator of the work factor required to crack the ciphertext by trying every possible key in turn.