Earlier quoted context omitted.
> Though I suppose the ATM company could combat this by encoding the machine's ID or a nonce with the message that the card's smart chip responds to, so it only works when replayed on that first machine - is this what happens in practice? Yes, more or less. The chip and contactless flows defined by EMV both require the card to generate a nonce for the transaction. The terminal also generates its own nonce[1]. [1]: ht…
Unfortunately "random" is a headache for a certain type of bureaucrat. They can't really do "random" or "unpredictable" and it needs somebody who actually understands what is going on to sit down and walk through how they can check whether to tick this box. For the Web PKI we had this problem extensively. No, if you set the top bit of this 128-bit integer, that's not 128 random bits. That's 127 random bits, the top o…
There’s ~4e23 _grains of sand_ on Earth. There are more possible values in 128 bits than grains of sand on Earth.
Take it further. There are 10^11 stars in our galaxy. If every star in the Milky Way had a planet identical to Earth orbiting it, there would be ~4e35 grains of sand on all the Earths orbiting all the stars of the Milky Way.[1]
If you assigned each of those grains to its own value, we’d only need 0.11% of the possible values of 2^128.[2]
I think it’s safe to say those bureaucrats are wrong :)
1: https://www.wolframalpha.com/input?i=%28number+of+stars+in+t...