You're right and indeed the promo code generator uses a hardened version of the proposed NIST standard for FPE, called BPS. It's the result of vast academic res

You're right and indeed the promo code generator uses a hardened version of the proposed NIST standard for FPE, called BPS. It's the result of vast academic research since 1950s, on how to shuffle, generate, unique random permutations within an arbitrary set.