Authentication with Partial Passwords and processing the shared secret with PHP   October 14th, 2015

Recently at, we wanted to even better secure the authentication to a web application that we provide to our customers.

There are several options to extend authentication:

We decided for partial passwords: You have your username and your first password, you login with that, and if that worked, you are asked about some specific letters from your second password: Please give us the third and 5th character of your second password!

See this paper called “Give Me Letters 2, 3 and 6!”: Partial Password Implementations & Attacks that describes Partial Password implementations and attacks in a detailed study.

The next question is, how to store the information about the partial password: you cannot hash it, because you will have to verify single characters. The solution is to use a secret sharing scheme, as described in Partial Passwords – How. It uses the Shamir’s Secret Sharing algorithm.

I have now implemented this idea, processing partial passwords with Shamir’s secret sharing scheme, in PHP. You can find the code licensed under the MIT at: There is an example included.

The Wikipedia article on Lagrange polynomial has been helpful as well in properly implementing the algorithm in PHP.

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