<DOC>
<DOCID>Cryptography_Basics.doc.txt</DOCID>
<AUTHOR>Doug Anger</AUTHOR>
<TEXT>
Cryptography (very basically) is the science of making secret information readable only to certain people. Most people use it on a daily basis without even knowing it. This article is a five-minute crash course in how crypto works, why the average person should care, and what you can do to keep your data secure.

Cryptography is a subject that appears in spy stories and science fiction movies, but it is also a real science that the average person uses nearly every day. When you swipe your credit card at the store, you are using cryptography. When you use an ATM, you are using cryptography. When you check your email over the web, you are (probably) using cryptography. Any time you see the little lock icon in your web browser, you are using cryptography. Cryptographic procedures secure your communications and financial information without requiring you to know anything about crypto or even that it is being used.

There are two basic types of cryptography: public key encryption, and secret key encryption. Secret key encryption requires the user to have some secret information called a secret key. As its name implies, this should not be shared with untrusted people. When a message (called a plaintext, because anyone can read it) is encrypted, some mathematical operation is performed on the plaintext and the secret key that results in a new piece of information called a ciphertext. The ciphertext cannot be read by anyone; it is indistinguishable from random text. The ciphertext can be decrypted, or turned back into the plaintext, by reversing the mathematical operation that was used for the encryption. This can only be done by someone who has the secret key.

A basic example of secret key cryptography would be as follows. Suppose we have a plaintext: The secret is forty two. We want to encrypt this so that no one else can read it, so we will shift all the letters to the right by three spaces. A becomes d, b becomes e, et cetera. (Three is the secret key and the mathematical operation is a letter shift. This is basically an addition because all letters are simply numbers in base 26.) So the ciphertext is Wkh vhfuhw lv iruwb wzr. This is pretty unreadable. To decrypt it, we have to know that the secret key was three and subtract that number. It is worth pointing out that in this system, there are only 26 possible keys, which makes deciphering the message relatively easy without the key. Real encryption systems will have billions (or more) possible keys.

Public key cryptography works in basically the same way except that the decryption key is different than the encryption key. This means that the encryption key can be made public (so that anyone can encrypt messages) while the decryption key is kept secret so that only the owner of the key can decrypt them.

There is a lot more to cryptography than simple encryption and decryption, but that is beyond the scope of this article.

There are several free programs that allow you to encrypt and decrypt things. For most users, the Gnu Privacy Guard, or GPG (http://www.gnupg.org/) is more than sufficient. Windows users can download GPG4Win (http://www.gpg4win.org/) that includes GPG and several other programs for generating keys in Windows and securing email. GPG can also be used to encrypt files.

TrueCrypt (http://www.truecrypt.org/) creates secure encrypted partitions (or virtual partitions) to keep files secure. Macintosh users can do this by creating a disk image in the Disk Utility and choosing AES encryption when saving it. The disk image file can be added to the login items under the Accounts panel in System Preferences.

Users of the command line can also use SECCURE, (http://point-at-infinity.org/seccure/) for secret key encryption of text, files, et cetera.
</TEXT></DOC>
