Ciphers Question:

For some time it has been common practice to protect and tansport a key for DES encryption with triple-DES. This means that the plaintext is, in effect, encrypted three times. There are, of course. a variety of ways of doing this; we will explore these ways below. See Question 85 for a discussion of multiple encryption in general.

A number of modes of triple-encryption have been proposed:

DES-EEE3: Three DES encryptions with three different keys.
DES-EDE3: Three DES operations in the sequence encrypt-decrypt-encrypt with three different keys.
DES-EEE2 and DES-EDE2: Same the previrous formats except that the first and third operations use the same key.

Attacks on two-key triple-DES have been proposed by Merkle and Hellman [MH81] and Van Oorschot and Wiener [VW91], but the data requirements of these attacks make them impractical.