> Huh? If you have worked on this stuff, surely DDA and CDA ring a bell? They're both based on asymmetric cryptography, and they absolutely allow the terminal to dynamically verify whether a given card is authentic or cloned, without having to go online.
Yes I was a little wrong here. My most recent experience in this area is dealing with messages on the issuer side. It’s been a while since I’ve done anything serious at the card config level, and don’t have a huge amount of experience with the handshakes that occur between card and terminal.
The missing nuance is that the transaction cryptogram is symmetrical encrypted, and that’s the only cryptographic blob sent over the card network to the issuer. As the asymmetric stuff only happens locally between card and terminal, and isn’t included in any data transmitted from terminal to issuer.
So the terminal can check that the card is a real card using asymmetric encryption, and that the produced transaction cryptogram was produced by that card. But none of that evidence is passed on to the issuer, only the symmetric cryptogram, which can’t be used for non-repudiation is sent to the issuer later.
Hence the experience dealing with forged transaction cryptograms, which were forged by an acquirer in a systematic manner to try and gain stronger chargeback protection because their sloppy transaction processing resulted in them processing a lot of fraudulent transactions, and then being hit with a lot of chargebacks. The process of “proving” to the network that this acquirer was forging cryptograms was sending a letter signed by legal team attesting to fact we had evidence of forgery, plus a much nastier letter sent to the acquirer in question to knock it off, and stop doing obviously illegal things. Ultimately it’s the acquirers legal counsel that acts as the enforcement mechanism here, and the obvious threat of lawsuit they’re clearly can’t win tends to be a very strong motivator for companies to tidy up their act.
> EMV is a cryptographically sound (if dated and very complex) scheme, and secrecy of implementation is actually much less of a security factor than you seem to claim, despite the industry's (largely historical) obsession with secrecy.
I may have overstated it a little. Bad habits from spending too much time dealing with PCI rubbish. The difficult thing to get across in these threads about payment networks is how much of the systems “security” really comes from clever legal contracts, and smart distribution of liability and risk. Effectively making in everyone’s interests to not do anything really silly, but the actual technical security is almost secondary to legal mechanisms that exist to ensure that participants are highly motivated to make sure that fraudulent transactions are kept to a minimum.