The figure above describes a block cipher LDES (“linear” DES).
LDES is a mini example of a block cipher that has 2 rounds in the Feistel network. It operates
on 4-bit block and 2-bit key. The important feature of LDES is that the S-box has been replaced
with a linear operation which makes the whole cipher linear. The purpose of this exercise is to
help you understand the importance of S-box in DES. Without the S-box, the cipher becomes
linear and can be totally broken. This exercise will guide you step by step to break the linear
cipher LDES.
1. Based on the encryption in the figure above, draw a diagram for the corresponding decryption. (1 mark)
2. Implement the encryption / decryption algorithms using C++ or Java. (2 marks)
3. For each key = 00, 01, 10, 11, calculate E(0000), E(1000), E(0100), E(0010), E(0001), E(1100),
E(1010), E(1001), E(0110), E(0101), E(0011), E(0111), E(1011), E(1101), E(1110), E(1111).
Verify that, for every key, E(1100) = E(1000) + E(0100) + E(0000).
Write similar equations for E(1010), E(1001), E(0110), E(0101), E(0011), E(0111), E(1011),
E(1101), E(1110), E(1111) in terms of E(0000), E(1000), E(0100), E(0010), E(0001). (1 mark)
This demonstrates that if an adversary knows E(0000), E(1000), E(0100), E(0010), E(0001), he can
encrypt any message. In general, for a linear cipher with block size n, if an adversary knows n + 1 pairs of
plaintext/ciphertext, he can encrypt/decrypt any messages without even knowing the key
The post LDES is a mini example of a block cipher appeared first on My Assignment Online.