Question

For a fun project I want to support the SASL Mechanisms for authentication, especially PLAIN and DIGEST-MD5.

My question is: how can I store the users’ password securely if I need to support those two ways of authentication?

With only PLAIN auth it would be really easy, I just store the password with bcrypt and compare the user submitted password with the stored pw using the bcrypt_compare function.

But how can I store the password securely when also DIGEST-MD5 should be possible?
Should I store the whole calculated response and use that also for the PLAIN comparison?
Or is there some other way?

//Edit: Regarding the “fun”-project. At the moment it is a fun project but no one knows if it will be a non-fun project at some point. And I don’t want to decrease the security just because it’s a fun project..

Answer 1

The DIGEST-MD5 specification tells you what a server needs to store for that authentication method:

3.9 Storing passwords

Digest authentication requires that the authenticating agent
(usually the server) store some data derived from the user’s name
and password in a “password file” associated with a given realm.
Normally this might contain pairs consisting of username and H({
username-value, ":", realm-value, ":", passwd }), which is
adequate to compute H(A1) as described above without directly
exposing the user’s password.

…so all you need to store for DIGEST-MD5 is H({ username-value, ":", realm-value, ":", passwd }).

You could separately store a bcrypt hash to use for PLAIN authentication, or you could just use the DIGEST-MD5 value. If you stored separate values then you could allow your users to selectively turn off DIGEST-MD5 authentication, which would allow you to remove that (easily-bruteforced) information from the database for those users.