New answers tagged format-preserving
Okay, here's the algorithm for "a fixed security level of 2128": If n ≤ 2^(2^128) then sometimes-recurse shuffle with 128 bits of security. If 2^(2^128) < n then encryption just outputs the plaintext and decryption just outputs the ciphertext. The identity function can trivially be computed in O(1) space, so that algorithm also uses only O(1) ...
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