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Double AES Encryption

kr flag

Let's say we have the following encryption:

$AES^2_k(x) =AES_{k1}(AES_{k2}(x))$ with k = k1 || k2 and k1, k2 are 256 bit keys.

$AES_{k2}(x)$ means that we use AES encryption to encrypt message x with key k2. Apparently, there is a security vulnerability with this, since the double encryption should require $2^{511}$ exhaustive searches for a 50% of success, whereas there is a way to perform an attack to achieves at most $2^{257}$ evaluations with unlimited RAM.

How could such attack happens?

fgrieu avatar
ng flag
This question is closed because it asks for textbook [meet-in-the-middle-attack)](https://crypto.stackexchange.com/questions/tagged/meet-in-the-middle-attack.
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