The Card
The plastic, the chip that is a computer, the cryptogram it signs, the terminal that reads it, and the token that stands in for it.
What Is Physically on a Card
You will be able to name the six physical forms in which one card carries one piece of information, say which three are obsolete, and explain why all six…
8,593 words/37 min read12The Number
You will be able to verify a card number by hand, on paper, in about ninety seconds, and to build the check digit rather than merely test it.
8,398 words/37 min read13The Four Things Called the Security Code
You will be able to name the four values that a card carries under the heading "security code" — CVV1, CVV2, iCVV and dCVV — and say which of the four…
8,327 words/36 min read14The Chip Is a Computer
You will be able to state precisely what separates a chip from a magnetic stripe: a stripe is a recording and a chip is a correspondent, so a stripe…
9,192 words/40 min read15The Cryptogram
You will be able to explain why a criminal holding a perfect byte-for-byte record of everything a card said last Tuesday cannot use that record to buy…
8,387 words/36 min read16Online and Offline
You will be able to answer the question the previous chapter skipped — whether anybody actually phones the bank — and say what happens on the occasions…
9,136 words/40 min read17Proving It Is You
You will be able to explain why the Application Cryptogram is a proof and the three bytes of tag 9F34 are only a claim, and why that difference is where…
8,484 words/37 min read18Contactless
You will be able to explain where a card with no battery gets its power, and why the honest description is mutual inductance in the near field rather…
8,373 words/36 min read19The Token in Your Phone
You will be able to explain why the last four digits printed on a phone receipt differ from the last four on your plastic, and prove it to yourself by…
8,069 words/35 min read20The Terminal
You will be able to explain why a card machine wipes its own keys the instant its case is opened, and why the result is a device that will never take…
8,301 words/36 min read21Point-to-Point Encryption
You will be able to explain why a shop can take card payments for a decade and never once hold a readable card number, and why that is a design decision…
8,436 words/37 min read22What the Terminal Actually Sends
You will be able to take any piece of a payment message and say which of the four writers — the card, the terminal, the acquirer or the issuer — put it…
8,309 words/36 min read