Addendum to “The FFX Mode of Operation for Format-Preserving Encryption” A parameter collection for enciphering strings of arbitrary radix and length

Background. A scheme for format-preserving encryption (FPE) is supposed to do that which a conventional (possibly tweakable) blockcipher does—encipher messages within some message space X—except that message space, instead of being something like X = {0, 1}128, is more general [1, 3]. For example, the message space might be the set X = {0, 1, . . . , 9}16, in which case each 16-digit plaintext X ∈ X gets enciphered into a 16-digit ciphertext Y ∈ X . In a stringbased FPE scheme—the only type of FPE that we consider here—the message space is of the form X = {0, 1, . . . , radix− 1}n for some message length n and alphabet size radix.