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I think it's better to define the keyspace of an unkeyed function as having one element. Some advantages: Computing the key size as $log_2 1$ correctly tells you it's a 0 bit key For encryption you pick one key from the set. You can pick an element of a single element set, but can't pick from an empty set Functions with multiple inputs are often defined ...


It depends how you look at it. If you regard it as a Caesar cipher then the key is 13 (out of a key space of 26 for uppercase ASCII, although key 0 is a very weak key, resulting in the identify function). If you consider that 13 is part of the ROT13 cipher then it indeed has no key. Of course having a static key or no key does not make a difference in ...

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