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16

The number of possible permutations of a block cipher are $2^n!$ where $n$ is the block size. A permutation maps all $2^n$ possible input blocks to $2^n$ possible output blocks. A key, with key space $2^k$ selects one of them. Although that's a huge number of keys, it is dwarfed by the amount of possible permutations. Now it's not by definition impossible ...


10

Apparently there's at least one real-life example of a block cipher with equivalent keys: TEA has a few weaknesses. Most notably, it suffers from equivalent keys—each key is equivalent to three others, which means that the effective key size is only 126 bits. As a result, TEA is especially bad as a cryptographic hash function. This weakness led to ...


1

I don't think there is a name for this special kind of property, but it is a clear hint for polyalphabetic substitution ciphers. It is a special kind of polyalphabetic substitution cipher. The first alphabet is a normal random key, while each successive alphabet is generated by a one-character right shift of the previous alphabet. As a result there are as ...


1

In cryptography it is common to reason about the probability of an event in the probability space of all the random choices made (i.e. the random bits generated) during an algorithm's execution. So, in this description, "over the random coins of HGD" means the probability is computed over the probability space defined by the random bits used during HGD ...



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