Triple DES uses three different keys and three executions of DES algorithm. The algorithm is Cipher text = E k3 [D k2 [E k1 [Plain Text]]] where E k [X] = DES Encryption of X using key K and D k [X] = DES Decryption of X using key K Remember that in DES Decryption of encrypted plain text with a different key is almost same as another encryption. This is true as encryption and decryption use the same algorithm. To decrypt cipher text we reverse the operations. Plain text = D k1 [E k2 [D k3 [Cipher Text]]] Block Diagram of Triple DES: Using DES thrice is equivalent to having a DES key length of 168 bits. Brute force method to break triple DES with 106 decrypts per micro second will take 5.9 X 10 30 years! Even at 1012 fold increase in computer speed will make triple DES secure against brute force attacks to break code The only reason D is used as middle step in triple DES is to allow decryption of data encrypted using single DES hardware. In this case K