What Is Encryption, and How Does It Work?

Encryption has a long history dating back to when the ancient Greeks and Romans sent secret messages by substituting letters only decipherable with a secret key. Join us for a quick history lesson and learn more about how encryption works.

In todays edition of HTG Explains, well give you a brief history of encryption, how it works, and some examples of different types of encryptionmake sure you also check out the previous edition, where we explained why so many geeks hate Internet Explorer.

Image by xkcd, obviously.

The ancient Greeks used a tool called a Scytale to help encrypt their messages more quickly using a transposition cipherthey would simply wrap the strip of parchment around the cylinder, write out the message, and then when unwound wouldnt make sense.

This encryption method could be fairly easily broken, of course, but its one of the first examples of encryption actually being used in the real world.

Julius Caesar used a somewhat similar method during his time by shifting each letter of the alphabet to the right or left by a number of positionsan encryption technique known as Caesars cipher. For instance, using the example cipher below youd write GEEK as JHHN.

Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZCipher: DEFGHIJKLMNOPQRSTUVWXYZABC

Since only the intended recipient of the message knew the cipher, it would be difficult for the next person to decode the message, which would appear as gibberish, but the person that had the cipher could easily decode and read it.

Other simple encryption ciphers like the Polybius square used a polyalphabetic cipher that listed each letter with the corresponding numeric positions across the top and side to tell where the position of the letter was.

Using a table like the one above you would write the letter G as 23, or GEEK as 23 31 31 43.

Enigma Machine

During World War II, the Germans used the Enigma machine to pass encrypted transmissions back and forth, which took years before the Polish were able to crack the messages, and give the solution to the Allied forces, which was instrumental to their victory.

Lets face it: modern encryption techniques can be an extremely boring subject, so instead of just explaining them with words, weve put together a comic strip that talks about the history of encryption, inspired by Jeff Mosers stick figure guide to AES. Note: clearly we cannot convey everything about encryptions history in a comic strip.

Back in those days, people do not have a good encryption method to secure their electronic communication.

Lucifer was the name given to several of the earliest civilian block ciphers, developed by Horst Feistel and his colleagues at IBM.

The Data Encryption Standard (DES) is a block cipher (a form of shared secret encryption) that was selected by the National Bureau of Standards as an official Federal Information Processing Standard (FIPS) for the United States in 1976 and which has subsequently enjoyed widespread use internationally.

Concerns about security and the relatively slow operation of DES in software motivated researchers to propose a variety of alternative block cipher designs, which started to appear in the late 1980s and early 1990s: examples include RC5, Blowfish, IDEA, NewDES, SAFER, CAST5 and FEAL

The Rijndael encryption algorithm was adopted by the US Government as standard symmetric-key encryption, or Advanced Encryption Standard (AES). AES was announced by National Institute of Standards and Technology (NIST) as U.S. FIPS PUB 197 (FIPS 197) on November 26, 2001 after a 5-year standardization process in which fifteen competing designs were presented and evaluated before Rijndael was selected as the most suitable encryption algorithm.

Many encryption algorithms exist, and they are all suited to different purposesthe two main characteristics that identify and differentiate one encryption algorithm from another are its ability to secure the protected data against attacks and its speed and efficiency in doing so.

As a good example of the speed difference between different types of encryption, you can use the benchmarking utility built into TrueCrypts volume creation wizardas you can see, AES is by far the fastest type of strong encryption.

There are both slower and faster encryption methods, and they are all suited for different purposes. If youre simply trying to decrypt a tiny piece of data every so often, you can afford to use the strongest possible encryption, or even encrypt it twice with different types of encryption. If you require speed, youd probably want to go with AES.

For more on benchmarking different types of encryption, check out a report from Washington University of St. Louis, where they did a ton of testing on different routines, and explained it all in a very geeky write-up.

All the fancy encryption algorithm that we have talked about earlier are mostly used for two different types of encryption:

To explain this concept, well use the postal service metaphor described in Wikipedia to understand how symmetric key algorithms works.

Alice puts her secret message in a box, and locks the box using a padlock to which she has a key. She then sends the box to Bob through regular mail. When Bob receives the box, he uses an identical copy of Alices key (which he has somehow obtained previously, maybe by a face-to-face meeting) to open the box, and read the message. Bob can then use the same padlock to send his secret reply.

Symmetric-key algorithms can be divided into stream ciphers and block ciphersstream ciphers encrypt the bits of the message one at a time, and block ciphers take a number of bits, often in blocks of 64 bits at a time, and encrypt them as a single unit. Theres a lot of different algorithms you can choose fromthe more popular and well-respected symmetric algorithms include Twofish, Serpent, AES (Rijndael), Blowfish, CAST5, RC4, TDES, and IDEA.

In an asymmetric key system, Bob and Alice have separate padlocks, instead of the single padlock with multiple keys from the symmetric example. Note: this is, of course, a greatly oversimplified example of how it really works, which is much more complicated, but youll get the general idea.

First, Alice asks Bob to send his open padlock to her through regular mail, keeping his key to himself. When Alice receives it she uses it to lock a box containing her message, and sends the locked box to Bob. Bob can then unlock the box with his key and read the message from Alice. To reply, Bob must similarly get Alices open padlock to lock the box before sending it back to her.

The critical advantage in an asymmetric key system is that Bob and Alice never need to send a copy of their keys to each other. This prevents a third party (perhaps, in the example, a corrupt postal worker) from copying a key while it is in transit, allowing said third party to spy on all future messages sent between Alice and Bob. In addition, if Bob were careless and allowed someone else to copy his key, Alices messages to Bob would be compromised, but Alices messages to other people would remain secret, since the other people would be providing different padlocks for Alice to use.

Asymmetric encryption uses different keys for encryption and decryption. The message recipient creates a private key and a public key. The public key is distributed among the message senders and they use the public key to encrypt the message. The recipient uses their private key any encrypted messages that have been encrypted using the recipients public key.

Theres one major benefit to doing encryption this way compare to symmetric encryption. We never need to send anything secret (like our encryption key or password) over an insecure channel. Your public key goes out to the worldits not secret and it doesnt need to be. Your private key can stay snug and cozy on your personal computer, where you generated itit never has to be e-mailed anywhere, or read by attackers.

For many years, the SSL (Secure Sockets Layer) protocol has been securing web transactions using encryption between your web browser and a web server, protecting you from anybody that might be snooping on the network in the middle.

SSL itself is conceptually quite simple. It begins when the browser requests a secure page (usually https://)

The web server sends its public key with its certificate.The browser checks that the certificate was issued by a trusted party (usually a trusted root CA), that the certificate is still valid and that the certificate is related to the site contacted.The browser then uses the public key, to encrypt a random symmetric encryption key and sends it to the server with the encrypted URL required as well as other encrypted http data.The web server decrypts the symmetric encryption key using its private key and uses the browsers symmetric key to decrypt its URL and http data.The web server sends back the requested html document and http data encrypted with the browsers symmetric key. The browser decrypts the http data and html document using the symmetric key and displays the information.

And now you can securely buy that eBay item you really didnt need.

If you made it this far, were at the end of our long journey to understanding encryption and a little bit of how it worksstarting from the early days of encryption with the Greeks and Romans, the rise of Lucifer, and finally how SSL uses asymmetric and symmetric encryption to help you buy that fluffy pink bunny on eBay.

Were big fans of encryption here at How-To Geek, and weve covered a lot of different ways to do things like:

Of course encryption is far too complicated a topic to really explain everything. Did we miss something important? Feel free to lay some knowledge on your fellow readers in the comments.

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What Is Encryption, and How Does It Work?