Computers talking to each other over a network need a shared secret. Just like two Spartan generals requiring the exact same code to exchange orders, devices using symmetric key encryption must possess identical keys to secure their data.
This method is straightforward. One computer takes a packet of information and scrambles it using a secret key before sending it across the network. The recipient computer, holding that same key, unscrambles the data. If you don’t know who will be communicating, you can’t pre-install the key. The entire system relies on the assumption that the code stays between the two endpoints.
Think of it like a simple substitution cipher. You tell a friend that every letter in your message should be shifted two spots down the alphabet. “A” becomes “C.” “B” becomes “D.” You send the garbled text. Your friend shifts it back. To anyone else intercepting the packet, it is just noise.
Computers do this too. But instead of a two-letter shift, they use massive keys. The first major algorithm developed in the United States was the Data Encryption Standard, or DES, approved in the 1970s. It used a 56-bit key.
That was fine until processors got faster. Security experts now consider DES insecure. A 56-bit key offers 70 quadrillion combinations. That sounds like a lot, until you have a modern computer trying every single one in sequence. A brute force attack can crack DES in a short while. It is essentially solved.
DES has been replaced by the Advanced Encryption Standard, or AES. AES uses keys that are 128, 192, or 256 bits long. Most experts believe AES will remain sufficient for a very long time. A 128-bit key provides more than 300 undecillion possible combinations. Brute forcing that today is impossible. Tomorrow? Maybe. But not for a while.
This isn’t new. Julius Caesar used a similar substitution technique over two thousand years ago. He shifted letters three spots up the alphabet. To send “CROSSING THE RUBICON,” he wrote “FURVV LQJWK HUXEL FRQ.” He also broke the text into groups of even length to hide word sizes. Symmetric encryption is just Caesar’s cipher on a scale you can barely comprehend.
