Let $k$ and $k'$ be two keys of symmetric encryption such that for some $m$ we have $\operatorname{Enc}_k(m)=\operatorname{Enc}_{k'}(m)$. Is it possible to exist a plain text $m'$ such that $\operatorname{Enc}_k(m') \neq \operatorname{Enc}_{k'}(m')$.

In fact, is there exist a key which be equivalent only for special plain text?


1 Answer 1


Yes. Actually it is quite likely that there are quite a few such keys for block ciphers. That said, finding one takes a collision search which may take a lot of time, especially for ciphers with a large block size.

Proving that there are no other messages that permute to the same ciphertext may even be computationally infeasible.

It would not be a good property for a block cipher to have keys that are fully equivalent. It may not directly destroy all trust in the cipher but it would certainly raise a few eyebrows.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.