“Give Me Letters 2, 3 and 6!”: Partial Password Implementations and Attacks

A

partial password

is a query of a subset of characters from a full password, posed as a challenge such as

“Give me letters 2, 3 and 6 from your password”

. Partial passwords are commonly used in the consumer financial sector, both online and in telephone banking. They provide a cheap way of providing a varying challenge that prevents eavesdroppers or intermediate systems learning a shared secret in a single step. Yet, despite widespread adoption among millions of consumers, this mechanism has had little attention in the academic literature. Answers to obvious questions are not clear, for example, how many observations are needed for an attacker to learn the complete password, or to successfully answer the next challenge? In this paper we survey a number of online banking implementations of partial passwords, and investigate the security of the mechanism. In particular, we look at

guessing attacks

with a

projection dictionary

ranked by likelihood, and

recording attacks

which use previous information collected by an attacker. The combination of these techniques yields the best attack on partial passwords.