Virtual Card Numbers

Published 3 October 2026

Round 1 — Case Interview: Virtual Card Numbers

The case interview used Capital One’s virtual card number product as the business scenario. The first part asked me to analyze the benefits and challenges of virtual card numbers from both the customer perspective and the company perspective. The interviewer also followed up specifically on technical challenges the company might encounter when operating such a product.

The second part introduced encoded card-number attributes and a set of payment-validation rules. For example, one digit could indicate the card network, and different networks could have different eligibility requirements.

A scenario would look like:

First digit:
4 -> Network A
7 -> Network B

Example rule for Network A:
- Transaction must be online
- Amount must be below $80
- Virtual card must be associated with the merchant

I was then given a sequence of payments containing card numbers and transaction attributes and had to walk through them one by one, clearly explaining whether each transaction was valid under the supplied rules.

The final part provided code implementing those validation rules and asked me to debug it. The bugs included incorrect combinations of && and ||, boundary-condition mistakes such as >= 80 versus > 80, and incorrect extraction of individual digits from a card number.

There was also a brief object-oriented design discussion about separating responsibilities such as card-number parsing and payment validation.