Computer Engineering Department
Cryptography & Network Security
Contemporary Symmetric Ciphers
(Double-DES, Triple-DES & Blowfish)
Dr Loái Tawalbeh
Eng. Sulaiman Al-Basheer
6.1 What is the triple encryption?
It is a three-stages encryption with three different keys, to avoid costly requirements arises from using three different keys with total length of 3x56 = 186 bits a triple encryption with two keys maybe used.
6.2 What is the meet in the middle attack?
It's that attack doesn't depend on any particular property of the DES, instead, it will work against any type of block ciphers.
For the double-DES cipher & a given (P,C) pair, this attack works as follow:
1. Encrypt the plaintext P with all possibilities of K1, store the results in a table, & sort that table by the value of X. 2. Decrypt C with all possible values of K2, check each resulted value with the entries in the table, in case of match, check these two keys against another known pair (P1,C1), if match, accept them as the correct keys.
6.4 Why is the middle portion of 3DES is a decryption rather than an encryption ?
1. It's a decryption process in order to change the traditional nature of the DES, if it's an encryption, it'll stay a DES but with longer key size. 2. In the cryptography, there is no significance of using the decryption in the middle stage, the only advantage of doing so is to allow users of 3DES to decrypt data encrypted by the users of the older single DES ( C = Ek1[Dk2[Ek1[P]]] = Ek1[P].
Suppose that the middle portion is decryption instead of encryption, ( C = Ek1[Ek2[Ek1[P]]]
The previous assumption in equation 1 will not be ever met. On the other hand 3. If an encryption process is done instead of decryption the meet-in-the-middle...