Key Clustering

In cryptography, key clustering is said to occur when two different keys generate the same ciphertext from the same plaintext, using the same cipher algorithm. A good cipher algorithm, using different keys on the same plaintext, should generate a different ciphertext, irrespective of the key length.

Assume that there is a plaintext P, two different keys, K1 and K2, and an algorithm A. Ciphertexts C1 and C2 with the two keys are generated as follows:

P → A(K1) → C1

P → A(K2) → C2

C1 should not equal C2, if they do then key clustering has occurred.