Triple DES with three distinct keys is the most secure form of triple-DES encryption. It can either be DES-EEE3 (encrypt-encrypt-encrypt) or DES-EDE3 (encrypt-decrypt-encrypt). DES-EDE1 is not defined and would mean using a single key to encrypt, decrypt and encrypt again, equivalent to single DES. DES-EEE4 is not defined and DES-EDE2 uses only 2 keys (encrypt with first key, decrypt with second key, encrypt with first key again). Source: DUPUIS, Cl?ment, CISSP Open Study Guide on domain 5, cryptography, April 1999.