1. A stronger form of DES where the algorithm is applied three times in order to encrypt data. Triple DES became necessary in the later part of the 20th century because DES had become so weak (a $200k machine was able to decrypt a DES encrypted message in hours). At that time, the new AES replacement had not yet appeared. In order to leverage DES hardware/software products, it was decided just to use DES three times with multiple keys. Point: Triple DES runs in "E-D-E" mode where it encrypts the data with the first key, then decrypts it with the second key, then encrypts with the third key. The second "decryption" phase is really just an encryption step: it is only by convention that one direction is considered encryption and the other direction decryption. The reason this technique is chosen is that if the same key is supplied three times, the effect is the same as a single encryption step. Controversy: The NSA urged banks not to adopt Triple-DES as a standard, citing national security concerns. They instead urged banks to adopt the Clipper chip. The banks went with Triple-DES in ANSI X9.52. In fact, as of 1998, several countries mandated the use of Triple-DES (replacing DES) in financial transactions. Controversy: Simply tripling the encryption does not necessarily triple its strength. For example, there are many crypto algorithms whereby encrypting with two different keys simply means you can decrypt with a single third key. Many suspect that tripling DES only doubles is cryptographic strength. From Hacking-Lexicon