Why are PBKDF2-SHA256 and PBKDF2_SHA256 different in 389-ds?

Why are PBKDF2-SHA256 and PBKDF2_SHA256 different in 389-ds?

In a mailing list discussion recently it came up about what password hash format should you use in 389-ds. Confusingly we have two PBKDF2 SHA256 implementations, which has a bit of history.

Too Lazy, Didn't Read

Use PBKDF2-SHA256. (hyphen, not underscore).

What's PBKDF2 anyway?

Passwords are a shared-knowledge secret, so knowledge of the password allows you to authenticate as the person. When we store that secret, we don't want it stored in a form where a person can steal and use it. This is why we don't store passwords cleartext - A rogue admin or a database breach would leak your passwords (and people do love to re-use their passwords over many websites ...)

Because of this authentication experts recommend hashing your password. A one-way hash function given an input, will always produce the same output, but given the hash output, you can not derive the input.

However, this also isn't the full story. Simply hashing your password isn't enough because people have found many other attacks. These include things like rainbow tables which are a compressed and precomputed "lookup" of hash outputs to their inputs. You can also bruteforce dictionaries of common passwords to see if they match. All of these processes for an attacker use their CPU to generate these tables or bruteforce the passwords.

Most hashes though are designed to be fast and in many cases your CPU has hardware to accelerate and speed these up. All this does is mean that if you use a verification hash for storing passwords then an attacker just can attack your stored passwords even faster.

To combat this, what authentication experts truly recommend is key derivation functions. A key derivation function is similar to a hash where an input always yields the same output, but a KDF also intends to be resource consuming. This can be ram or cpu time for example. The intent is that an attacker bruteforcing your KDF hashed passwords should have to expend a large amount of CPU time and resources, while also producing far fewer results.

In 389-ds when I implemented PBKDF2_SHA256 I specifically chose to use the '_' (underscore) to not conflict to '-' which is the OpenLDAP PBKDF2 scheme. We have differences in how we store our PBKDF2_SHA256 in the userPassword value so they aren't compatible. At the time since I was writing the module in C it was easier to use our own internal format than to attempt C String manipulation which is a common source of vulnerabilities. I opted to use a binary array with fixed lengths and offsets so that we could do bound checks rather than attempting to split a string with delimiters (and yes, I accounted for endianness in the design).

The issue however is that after we implemented this we ran into a problem with NSS (the cryptographic library) that 389-ds uses. NSS PBKDF2 implementation is flawed, and is 4 times slower (I think, I can't find the original report from RedHat Bugzilla) than OpenSSL for the same result. This means that for 389-ds to compute a password hash in say ... 1 second, OpenSSL can do the same in 0.25 seconds. Since we have response time objectives we wish to meet, this forced 389-ds to use fewer PBKDF2 rounds, well below the NIST SP800-63b recommendations.

For a long time we accepted this because it was still a significant improvement over our previous salted sha256 single round implementation, and we hoped the NSS developers would resolve the issue. However, they did not fix it and did not accept it was a security issue.

Since then, we've added support for Rust into 389-ds and had a growing interest to migrate from OpenLDAP to 389-ds. To support this, I added support for OpenLDAP formatted password hashes to be directly imported to 389-ds with a Rust plugin called pwdchan that is now part of 389-ds by default. In this plugin we used the OpenSSL cryptographic provider which does not have the same limitations as NSS, meaning we can increase the number of PBKDF2 rounds to the NIST SP800-63b recommendations without sacrificing large amounts of (wasted) CPU time.


Use PBKDF2-SHA256.

  • It's written in Rust.
  • It meets NIST SP800-63b recommendations.