As stated earlier in the chapter, symmetric algorithms use a single key. The two symmetric algorithms I want to discuss are DES (Data Encryption Standard) and IDEA (International Data Encryption Algorithm).
DES
DES has been the encryption standard for the U.S. Government since 1976. IBM first developed it with the name Lucifer in 1974. I don’t want to get too deep into how DES works, but let’s take a quick look at some of the particu- lars of the algorithm. DES is a block cipher, meaning that it works on blocks of data. The DES key is 64 bits in length; however, only 56 bits are actually used, and are called the active key. The other 8 bits are used for parity. DES uses two different techniques, substitution and transposition (also known as confu- sion and diffusion, respectively), for 16 “rounds” in order to create the cipher- text. During each “round,” data is XOR’ed (Exclusive OR’ed) with a subkey and then that result is run through eight S-boxes (substitution boxes) and then through a P-box (permutation box). How I remember the purpose of S-boxes is that they are for (S)ecurity.
DES has been reaffirmed as the encryption standard for the U.S.
Government every five years since 1976, and has actually held up well consid- ering it is over 20 years old. But as time marches forward, DES will not be able to protect data as it once could, so the search is on for DES’s replacement that will be called AES (Advanced Encryption Standard). See the AES sidebar for more information.
In the interim, several variations of DES have been created in order to help protect the integrity of the ciphertext. Two variations are 3DES (Triple DES) and DESX. 3DES uses multiple keys, and DESX uses 64 bits of additional key material. More information on these algorithms can be found at:
3DES
www.iks-jena.de/mitarb/lutz/security/cryptfaq/q72.html DESX
AES
A search has been on since 1997 for a replacement for the aging DES algorithm. As stated earlier in the chapter, DES has been the official U.S. cryptographic standard for many years—too many years, in fact. It was still in use for an unknown number of years after it became practical (affordable) to build a special-purpose brute force DES cracking machine. If the EFF (Electronic Frontier Foundation) could do it in 1998 for less than $250K (U.S.), then certainly there must have been a few governments willing to spend several million for one a few years prior to that.
During the period of Jan 1997–July 1998 (Pre-Round 1), the National Institute of Standards and Technology (NIST) initiated a call for algorithms, and nearly all the top-name cryptographers or teams submitted something for consideration. These people have written algorithms on which the secu- rity world relies. This speaks for how hard good crypto is; essentially, a couple of the algorithms were broken right away by the participants.
During Round 1 (August 1998–April 1999), NIST announced 15 algo- rithms that would be considered for AES. Round 2 (August 1999–May 2000) narrowed the field of algorithms from 15 to 5:
• MARS • RC6 • Rijndael • Serpent • Twofish
AES is the ultimate hacking contest; however, it’s a hacking contest done right. There’s no cash prize (the prize is prestige). They’re taking several years to review the submissions. They’ve got the attention of the world’s top experts who are trying hard to break all the candidates.
The world could go on using triple DES or DESX forever, but the AES pro- cess factors in performance. There have been numerous studies done on the various candidates to see how they perform in all kinds of environments. These range from memory-limited 8-bit smart cards, to standard, high-speed 32-bit computers. The AES candidates are more flexible than DES in most respects. They are required to deal with a variety of block and key sizes, and most of them have time/storage tradeoffs that implementers can pick from to optimize for the environment they will run on.
IDEA
The International Data Encryption Algorithm was invented by Dr. X. Lai and Professor J. Massey in a combined research project between Ascom and the Swiss Federal Institute of Technology. It operates on a 64-bit plaintext block and uses a 128-bit key. IDEA uses a total of eight rounds in which it XOR’s, adds and multiplies four sub-blocks with each other, as well as six 16-bit sub- blocks of key material. More in-depth technical specifications of this algorithm can be found at:
www.ascom.ch/infosec/idea/techspecs.html
There are several different symmetric algorithms available for implementa- tion that I have not covered such as blowfish, RC2, RC4, CAST (named for Carlisle Adams and Stafford Tavares), and many more. If you have an interest in cryptography, you may want to explore these algorithms in-depth.
Note that PGP v2.0 and higher have used several different symmetric algo- rithms, including IDEA, 3DES, and most recently, CAST.