Question

In the DES algorithm the Round Input is 32 bits, which is expanded to 48 bits via ____________

(a) Scaling of the existing bits

(b) Duplication of the existing bits

(c) Addition of zeros

(d) Addition of ones

The question was asked in an interview for internship.

The origin of the question is The Data Encryption Standard (DES) and It’s Strength topic in chapter Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

Answer 1

The correct choice is (a) Scaling of the existing bits

For explanation I would say: The round key is 48 bits. The input is 32 bits. This input is first expanded to 48 bits (permutation plus an expansion), that involves duplication of 16 of the bits.