After that he explains why cryptography is essential for today’s internet communication: “cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.”

“Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication. There are, in general, three types of cryptographic schemes typically used to accomplish these goals: secret key (or symmetric) cryptography, public-key (or asymmetric) cryptography, and hash functions, each of which is described below. In all cases, the initial unencrypted data is referred to as plaintext. It is encrypted into ciphertext, which will in turn (usually) be decrypted into usable plaintext.”

After that interesting history lecture he starts with types of cryptographic algorithms over trust models to cryptographic algorithms in action. All supported by lots of figures that makes it easier to understand the concepts.

Take a look at gary kesslers site for the article.

Update: The developer is constantly adding new information to the website. You can now for instance find information about the Open Source encryption software True Crypt on the site, as well as match notes which may be interesting for programmers and math students alike.

Cryptographic algorithms in action furthermore look at the SSL “Family” of secure transaction protocols, IPsec protocol and RSA public key cryptography.

It is a long read that gets technical at times. You will also find lots of outgoing links for additional information about a concept or subject. The introduction nevertheless is one of the best that you will find about cryptography on today’s Internet.