A. The DES algorithm is based on the idea of the Feistel cipher. Explain how the Feistel cipher structure achieves substitution and permutation. b. A secure block cipher must be able to achieve strong confusion and diffusion in order to be able to viably resist cryptoanalysis. Explain what these concepts mean and why they are important.

a. Feistel Cipher is basically a design model used in many block encryption algorithms one of them is DES. The Feistel structure has many rounds of substitution followed by permutation. In each round the plaintext is processed.

A Feistel Cipher comprise of;

1. Divide plaintext block into two parts

2. Perform multiple rounds of

a. Substitution on left half

b. Permutation by swapping halves

See attachment for the diagram of a feistel cipher.

B.

The concept of confusion and diffusion were identified by Claude Shannon in his paper Communication Theory of Secrecy Systems, published in 1949.

According to Shannon's definitions,

-Confusion means, making the relationship between the ciphertext and the key as complex and involved as possible.

It basically hides the relationship between the cipher text and the key. This property makes it difficult to find the key even tough there are many plaintext cipher text pairs created using the same key.

Diffusion is property that the non-uniformity which is much harder to detect. The distribution of the individual and pairs of neighbouring letters in the plaintext should be redistributed into the non-uniformity in the distribution of much larger structures of the ciphertext. The concept behind diffusion is to hide the relationship between the plaintext and the ciphertext. A cipher with good diffusion, should make sure that even if one bit in plaintext changes, then the ciphertext should change completely, in a manner such that its unpredictable.

Both Confusion and diffusion prevents the deduction of encryption key difficult. As a result this protects the original message or plaintext. Confusion and diffusion can be achieved using substitution and permutation.

Encryption methods use the principles of confusion and diffusion.

Confusion does the work of changing the data drastically from input to output by many ways like translate the data using a non-linear table created from the key.

Diffusion is used in encryption to make every bit of input affect every bit of the output which in tun makes the analysis difficult.