Introduction

Recently at work I have been using the PyCrypto libraries quite a bit. The documentation is pretty good, but there are a few areas that took me a bit to figure out. In this post, I’ll be writing up a quick overview of the PyCrypto library and cover some general things to know when writing cryptographic code in general. I’ll go over symmetric, public-key, hybrid, and message authentication codes. Keep in mind this is a quick introduction and a lot of gross simplifications are made. For a more complete introduction to cryptography, take a look at the references at the end of this article. This article is just an appetite-whetter - if you have a real need for information security you should hire an expert. Real data security goes beyond this quick introduction (you wouldn’t trust the design and engineering of a bridge to a student with a quick introduction to civil engineering, would you?)

Some quick terminology: for those unfamiliar, I introduce the following terms:

• plaintext: the original message
• ciphertext: the message after cryptographic transformations are applied to obscure the original message.
• encrypt: producing ciphertext by applying cryptographic transformations to plaintext.
• decrypt: producing plaintext by applying cryptographic transformations to ciphertext.
• cipher: a particular set of cryptographic transformations providing means of both encryption and decryption.
• hash: a set of cryptographic transformations that take a large input and transform it to a unique (typically fixed-size) output. For hashes to be cryptographically secure, collisions should be practically nonexistent. It should be practically impossible to determine the input from the output.

Cryptography is an often misunderstood component of information security, so an overview of what it is and what role it plays is in order. There are four major roles that cryptography plays:

• confidentiality: ensuring that only the intended recipients receive the plaintext of the message.
• data integrity: the plaintext message arrives unaltered.
• entity authentication: the identity of the sender is verified. An entity may be a person or a machine.
• message authentication: the message is verified as having been unaltered.

Note that cryptography is used to obscure the contents of a message and verify its contents and source. It will not hide the fact that two entities are communicating.

There are two basic types of ciphers: symmetric and public-key ciphers. A symmetric key cipher employs the use of shared secret keys. They also tend to be much faster than public-key ciphers. A public-key cipher is so-called because each key consists of a private key which is used to generate a public key. Like their names imply, the private key is kept secret while the public key is passed around. First, I’ll take a look at a specific type of symmetric ciphers: block ciphers.