American Express credit cards have the interesting property that 3 out of the last 4 digits are almost always ‘100’. Because many UIs only show the last 4 digi

American Express credit cards have the interesting property that 3 out of the last 4 digits are almost always ‘100’. Because many UIs only show the last 4 digits, and because of the birthday paradox, it only takes 4 American Express cards saved to a website before you have a 50% chance of 2 of them being indistinguishable from one another. https://www.doctorofcredit.com/last-four-digits-american-express-card-numbers-birthday-problem/ https://www.doctorofcredit.com/last-four-digits-american-exp...