I can't understand HMAC enough to know whether it solves it, but there seems to be a trade-off between keeping it secure and introducing randomness and making people do lookups on the other end (which would have been super slow then, and I don't know how feasable hash tables are when dealing with punchcards)
http://en.wikipedia.org/wiki/German_tank_problem#Countermeas... http://en.wikipedia.org/wiki/Known-plaintext_attack
I can't understand HMAC enough to know whether it solves it, but there seems to be a trade-off between keeping it secure and introducing randomness and making people do lookups on the other end (which would have been super slow then, and I don't know how feasable hash tables are when dealing with punchcards)