Hello,
I'm setting up a new server and, again, seek for a decently secure (from
a security specialist's POV) way to store and verify user passwords in a
database. Additionally now, GDPR requires me to use a solid
state-of-the-art solution.
My OS is Ubuntu 20.04, Dovecot version 2.3.7, database backend with
PostgreSQL 12.
Obviously, storing the plaintext password is a terrible idea. SHA-based
methods aren't suitable either. bcrypt has been recommended often [1].
PBKDF2 was preferred over bcrypt even more [2]. I'm managing all
database contents with an ASP.NET Core application that implements the
management user frontend. It's a bit hard to find bcrypt support for
.NET (there are a few NuGet packages of unknown quality [3]).
.NET does however implement, use and recommend PBKDF2 for its own user
management. If this is by far the best way to go, I'm already covered on
that side. Now the problem is, once again*, how I can use this in
applications to make them as secure.
I need a solution for Dovecot and Exim. Exim seems to be able to ask
Dovecot (IMAP) for user authentication, so I might try that and only
need to solve the problem in Dovecot alone.
Dovecot documentation says that PBKDF2 is somewhat possible [4]. It
requires the hash in the format "$1$salt$rounds$hash". I guess that
"salt", "rounds" and "hash" are the parameters here. But what is their
format?
The .NET implementation [5] describes its format as "{ 0x01, prf
(UInt32), iter count (UInt32), salt length (UInt32), salt, subkey }"
with big-edian integers. The result is base64-encoded. Prf is an enum
value, describing the HMAC SHA-256. Subkey is the hash value part.
I might find a way to convert .NET's format into what Dovecot
understands. The hmac is SHA-256, the rounds is 10000. But I wasn't able
to get it working.
My test password is: simplepassword
The .NET hash:
AQAAAAEAACcQAAAAEG0Anzb9vKOqsAKxLyhXedCTJoHrP381hKiKBHuPHhMdkjqW8Bks8RFOQZLssJ2grQ==
The converted hash for Dovecot:
$1$bQCfNv28o6qwArEvKFd50A==$10000$kyaB6z9/NYSoigR7jx4THZI6lvAZLPERTkGS7LCdoK0=
I've also found the source code in Dovecot that should verify the hash
[6]. It gives some more hints about the expected format that are sadly
missing from the documentation, making it almost useless. I also tried
with the "{PBKDF2}" prefix, with the base64 padding "=" removed and with
the hash part converted from base64 to hex. Nothing works. The source
mentions "SHA1" somewhere. Is that all it can accept? No up-to-date SHA-256?
So what have I done wrong here? Why can't I authenticate? The Dovecot
log isn't helpful, it doesn't even mention the user name I tried to log
in with from Thunderbird, most of the time (it's unpredictable).
What is the correct usage of Dovecot's PBKDF2 feature? Is it functional
at all? There's a test case for it [7] but that's not helpful to me.
If Dovecot's PBKDF2 support is not functional or not compatible with
ASP.NET Core's parameters, what options do I have? Can I build my own
authentication service that Dovecot can communicate with, to fill the
gap of missing crypto support?
Yves
*) During my research today, I find myself finding my own questions from
6 years ago when I did this the last time. The situation hasn't changed
much since then. Secure password hashing is still impossible or
complicated in server applications like Exim or Dovecot.
[1] https://codahale.com/how-to-safely-store-a-password/ (linked from
the Dovecot documentation)
[2]
https://www.reddit.com/r/node/comments/4u1jcn/is_bcrypt_the_best_possible_password_hashing/
[3] https://nugetmusthaves.com/Tag/bcrypt
[4]
https://doc.dovecot.org/configuration_manual/authentication/password_schemes/#other-schemes
[5]
https://github.com/dotnet/AspNetCore/blob/master/src/Identity/Extensions.Core/src/PasswordHasher.cs
[6]
https://github.com/dovecot/core/blob/81b5b188c478ec36bea8bda8fcad1e5f32ac612b/src/auth/password-scheme-pbkdf2.c#L50
[7]
https://github.com/dovecot/core/blob/ff5305136ae747867b6f6af9a1737188ae7b3b5a/src/auth/test-libpassword.c#L113
