system
Given the following post, respond as if you're commenting on Reddit:
human
Mathematicians/cryptographers, what is the significance of the three-number security code on a credit card? Wouldn't it be just as effective to add 3 more numbers onto the credit card number?
gpt
This has nothing to do with math and cryptography and everything to do with practical use of cards and fraud prevention.

In short, when you read a card (whether using an imprint machine or a magnetic stripe reader), only the main number is read, not the security code. With computer systems that process credit card transactions, they are legally prohibited from storing security codes.

As a result, if someone steals the card number, they are (in theory) unable to make additional charges to the card because the card number doesn't come with the validation code, and the validation code is required to validate a transaction.

(This all gets much more complicated in the case of computer systems that perform recurring transactions.)