online payments and credit cards: I have questions

[cellio]

As I make the rounds doing year-end donations, I'm reminded of two things that have long puzzled me:

1. Some web sites auto-detect the type of credit card based on the number. Apparently all credit-card numbers that begin with "4" are Visa. (I don't know if the reverse is true: do all Visa numbers start with 4?) Being me, I've cycled through the other nine digits and nothing else produces a match based on a single digit. What are the patterns for other providers? And are all these sites using some standard library for this, or are programmers really coding that by hand?

2. Years ago, a three-digit code ("CCV") was added to cards to mitigate fraud. On a physical credit card, this number is stamped rather than embossed, so those old-style manual credit-card gadgets that took an imprint of your card (on actual paper, with a carbon!) couldn't record it. Um, that's fine I guess, but online, that number isn't any more secure than the card number itself. And someone who steals your physical card has the number; it's not a password. Does that number have another purpose?

Comments

[madfilkentist]

The one benefit I can think of for the CCV is that it makes brute-force guessing harder. Prefix digits narrow down the space for a card number from a specific issuer. I think the numbers include a checksum digit, because some sites immediately tell me I've got an invalid number if I mistype a digit, and that further narrows the number of valid numbers. Anything that increases the number of possibilities helps against brute-force attacks.