Reliably reproduce in C# a legacy password hashing method implemented in PHP

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We are migrating a PHP application that runs on Linux to our new Single Sign-On (SSO) infrastructure implemented in C# and running on Windows.

As part of the migration process, we need our C# SSO infrastructure to be able to hash passwords in the exact same way as the PHP application.

While the PHP application uses a rather reasonable password hashing algorithm, in addition to the password and salt the string that gets hashed unfortunately also contains the cosine of the salt value (interpreted as an integer)... A rather unusual decision, to put it midly.

Unsurprisingly, it turns out that computing the cosine of a large integer in PHP and in C# leads to slightly different results. This means that we probably cannot reliably reimplement the legacy password hashing algorithm in our new SSO infrastructure.

One solution we thought about is running the PHP password hashing function in an AWS Lambda and query that Lambda from our SSO infrastructure.

Can you think of other options?

hash passwords php c#
3 Answers

Presumably, the salts are stored along with each password. You could use the PHP code to calculate that cosine, and store that also with the password. I would then also add a password version number and default all those older passwords to be version 1. Then, in your C# code, for any new passwords, you implement a new hashing algorithm, and store those password hashes as passwords version 2. For any version 1 passwords, to authenticate, you do not have to calculate the cosine, you simply use the one stored along with the password hash and the salt.

The programmer of that PHP code was probably wanting to do a clever version of pepper. By storing that cosine, or pepper along with the salt and the password hashes, you basically change that pepper into a salt2. So, another versionless way of doing this would be to use two salts in your C# hashing code. For new passwords you could leave the second salt blank or assign it some other way. For old passwords, it would be that cosine, but it is already calculated.

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