Encryption

Last updated: September 12, 2008.

You've seen a rare book you want to buy online and it costs—wait for it—$500. It's on an online auction so you have to act fast. Fortunately, you bid in time to win and the book is yours. Happy with your success, you type in your credit card details to pay, without even thinking about it. Thanks to the wonders of e-commerce, one of the most valuable pieces of information you own (effectively the key to your entire bank account) whistles across the ether through merchants and banks and the seller receives your payment a few seconds later. Would you dream of sending $500 in cash this way? Passing it from person to person, through a long chain of people you've never met, with a little note attached: "Give this to Joe in Duluth"? Of course not! And yet you feel totally comfortable doing exactly the same thing online. The difference is that, when you pay electronically, your payment information is "scrambled" as it travels so only you and the person who receives the money (or their bank) ever get to see it. That's the brilliance of a mathematical technology called encryption (sometimes also referred to as cryptography). Let's take a closer look at how it works!

Photo: Military forces have always used encryption to ensure secret communications stay out of enemy hands. This is some of the complex encryption and radio communication equipment used by US forces in the 1980s. Today, all of us have encryption at our fingertips. Photo by Vincent Kitts courtesy of Defense Imagery.

What is encryption used for?

Encryption is another word for "coding," so when we talk about encrypting something we really just mean turning it into an indecipherable message using a secret code. We all like playing spies when we're kids, but why would we want to do that as adults? These days, the main reason is that we share so much information online. By its very nature, the Internet is a public medium. Every time you send an email or browse a Web page, the information your computer sends and receives has to pass through maybe a dozen or more other machines on its way to and from its ultimate destination. At every stage, that information could be intercepted by crooks or others of dubious intent. Encrypting information keeps it safe just long enough to make the journey. There's another reason you might want to use encryption: proving information really comes from you. Anyone can send an email pretending to be from someone else; you can use encryption to digitally "sign" your messages and verify your identity.

Photo: This cellphone is having an encryption chip installed in it to ensure secure communications. The chip uses a type of encryption called AES (Advanced Encryption Standard). Photo by Andrew Rodier courtesy of US Air Force and Defense Imagery.

How does secret-key cryptography work?

All codes are a bit like padlocks. You "lock" your message, the message travels to its destination, and then the recipient "unlocks" it and reads it. But not all codes work the same way.

Secret agents in spy movies use a method called secret-key cryptography. Suppose you're an agent working in Washington, DC and you need to send a message to another agent in Rome, Italy. The best way to do it is for the two of you to meet up in advance, in person, and agree on a method of locking and unlocking all the messages you'll send and receive in future. This method is called a secret key, because only the two of you will have access to it. The secret key could be something like "Replace every letter in the message with another letter three further on in the alphabet." So, to send the message "HELLO" to your contact in Rome, you simply move each letter three forward, which gives you "KHOOR." When the person at the other end gets the message, he simply has to move each letter back three positions in the alphabet to find out what you're really saying. In this case, the key isn't a piece of metal you poke in a lock: it's the method of cracking the code by shifting the letters. Real secret keys are obviously much more complex and sophisticated than this.

This way of securing information is also called PSK (pre-shared key) and in some circumstances it's very effective. It's widely used to secure wireless Internet networks, for example. When you set up a secure wireless network, you're asked to choose a secret key (effectively, a password) that's known to both your wireless router (your main local access point to the Internet) and to any portable computers that need to use it. When you're using wireless Internet, you may notice that your connection is encrypted with something called WPA-PSK (Wi-Fi Protected Access-Pre-Shared Key). If you try to log onto a new wireless network and you're asked for a password, what you're really supplying is a secret key that will be used to encrypt and decrypt all the messages that pass back and forth.

Photo: You don't need a padlock to secure information: you can do it with encryption.

Although secret (pre-shared) keys are effective and secure for things like this, they're not at all useful in other situations—like sending secure messages to people you've never met. That's because they rely on your knowing and meeting the person you're communicating with in advance to exchange the secret key. What if you can't do that? What if you want to exchange secure information with someone you've never met—someone who could be on the opposite side of the world? That's exactly the problem you have when you're paying for things online.

How does public-key encryption work?

In that case, you can use a different system called public-key cryptography, which is how online encryption works. The basic idea is simple. Each person has two keys, one called a public key and one called a secret key. Each "key" is actually a long, meaningless string of numbers—nothing like a metal key you'd use to open and close a door lock. The public key is something you can share with anyone, while the secret key is something you must keep private. Suppose you want to send a message to a friend using public key cryptography. You use their public key (which they've freely shared with the world) to encrypt the message and turn it into gibberish. You email the scrambled message to them over the Internet and when they receive it they use their secret key to decrypt (unscramble) and read it. That then is the essence of public-key crytography: anyone can encrypt a message and send it to you (using your public key), but only you can read it (using your secret key).

What's the trick?

It sounds like a trick! How can anyone encrypt a message but only you can decrypt it? Surely if one person can encrypt a message using a publicly available key, other people can decrypt it too using the same key? Not so! The answer lies in the two different keys and in the fact that some mathematical processes are much harder to do one way than the other.

Consider the two prime numbers 7901 and 7919 (prime numbers are ones that you can divide by no other numbers than one and themselves). Suppose you multiply them together to get 62568019. That's a pretty simple operation anyone can do in two seconds flat with a calculator. But what if I give you the number 62568019 and tell you to figure out the two numbers I multiplied together to make that number. You'd be there all day!

What if encrypting a message were as easy as multiplying two prime numbers but decrypting were as hard as figuring out what those numbers were? That's the basic idea behind public-key cryptography. When you secure a message with someone's public key, your computer performs an easy mathematical operation anyone could do. But once the message is encrypted, figuring out what information it contains is a very tough mathematical operation that would take you days, weeks, or months to complete (unless you happen to know the secret key).

You'll see from this that there is a basic flaw in public-key encryption. Given enough time and computing power, you could always figure out the secret key from the public key and decrypt the message. That's why public-key encryption relies on keys that are really big. The keys my computer uses, for example, are made up of 1024 bits (binary digits): a string of 1024 zeros or ones in a long line. The longer the keys you use (that is, the more bits they have), the tougher the encryption and the more secure your message will be. Secure Web pages typically use 128-bit or 256-bit encryption when they travel to and from your browser carrying banking information.

Types of public-key encryption

There are various different types of public-key encryption that you'll come across. The original idea was invented in the mid-1970s by two Stanford University mathematicians named Whitfield Diffie and Martin Hellman and systems that use their particular mathematical coding method (which is known as an algorithm) are usually called DH (Diffie-Hellman). Others include RSA (named for Ron Rivest, Adi Shamir, and Leonard Adleman), Elgamal (named for Taher Elgamal), Data Encryption Standard (DES) and Triple-DES, and the successor to DES, known as Advanced Encryption Standard (AES) or Rijndael. Web browsers and servers use encryption methods called SSL (Secure Sockets Layer) and TLS (Transport Layer Security), themselves based on algorithms such as RSA and DH, to protect information traveling back and forth over the Net. Some email programs have built-in encryption to make it easy to send and receive secure messages; there's also a popular web-based email system called Hushmail that has encryption built-in as standard. Many PCs use a widely available encryption program named PGP (Pretty Good Privacy) developed by American software engineer Philip Zimmerman in 1991 (Linux equivalents of PGP include KGPG and GnuPG).

Further reading

• How PGP works: A good, clear introduction to cryptography and the widely available PGP public cryptography tool.