3-DES
In cryptography, Triple DES is a block cipher formed from the Data Encryption Standard (DES) cipher by using it three times. When it was found that a 56-bit key of DES is not enough to guard against brute force attacks, TDES was chosen as a simple way to enlarge the key space without a need to switch to a new algorithm. The use of three steps is essential to prevent meet-in-the-middle attacks that are effective against double DES encryption. Note that DES is not a group; if it were one, the TDES construction would be equivalent to a single DES operation and no more secure.
