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WIP: SCRAM authentication

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: WIP: SCRAM authentication
Date: 2015-03-30 10:52:46
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
There have been numerous threads on replacing our MD5 authentication 
method, so I started hacking on that to see what it might look like. 
Just to be clear, this is 9.6 material. Attached is a WIP patch series 
that adds support for SCRAM. There's no need to look at the details yet, 
but it demonstrates what the protocol changes and the code structure 
would be like.

I'm not wedded to SCRAM - SRP or JPAKE or something else might be 
better. But replacing the algorithm, or adding more of them, should be 
straightforward with this.

There is no negotiation of the authentication mechanism. SCRAM is just 
added as a new one, alongside all the existing ones. If the server 
requests SCRAM authentication, but the client doesn't support it, the 
attempt will fail. We might want to do something about that, to make the 
transition easier, but it's an orthogonal feature and not absolutely 

There are four patches in the series. The first two are just 
refactoring: moving the SHA-1 implementation from pgcrypto to 
src/common, and some refactoring in src/backend/auth.c that IMHO would 
make sense anyway.

Patches three and four are the interesting ones:

3. Allow storing multiple verifiers in pg_authid

Replace the pg_authid.rolpassword text field with an array, and rename 
it to 'rolverifiers'. This allows storing multiple password hashes: an 
MD5 hash for MD5 authentication, and a SCRAM salt and stored key for 
SCRAM authentication, etc. Each element in the array is a string that 
begins with the method's name. For example "md5:<MD5 hash>", or 

For dump/reload, and for clients that wish to create the hashes in the 
client-side, there is a new option to CREATE/ALTER USER commands: 
PASSWORD VERIFIERS '{ ... }', that allows replacing the array.

The old "ENCRYPTED/UNENCRYPTED PASSWORD 'foo'" options are still 
supported for backwards-compatibility, but it's not clear what it should 
mean now.


* Password-checking hook needs to be redesigned, to allow for more kinds 
of hashes.

* With "CREATE USER PASSWORD 'foo'", which hashes/verifiers should be 
generated by default? We currently have a boolean password_encryption 
setting for that. Needs to be a list.

4. Implement SCRAM

The protocol and the code is structured so that it would be fairly easy 
to add more built-in SASL mechanisms, or to use a SASL library to 
provide more. But for now I'm focusing on adding exactly one new 
built-in mechanism, to replace MD5 in the long term.

In the protocol, there is a new AuthenticationSASL message, alongside 
the existing AuthenticationMD5, AuthenticationSSPI etc. The 
AuthenticationSASL message contains the name of the SASL mechanism used 
("SCRAM-SHA-1"). Just like in the GSSAPI/SSPI authentication, a number 
of PasswordMessage and AuthenticationSASLContinue messages are exchanged 
after that, carrying the data specified by the SCRAM spec, until the 
authentication succeeds (or not).


* Per the SCRAM specification, the client sends the username in the 
handshake. But in the FE/BE protocol, we've already sent it in the 
startup packet. In the patch, libpq always sends an empty username in 
the SCRAM exchange, and the username from the startup packet is what 
matters. We could also require it to be the same, but in SCRAM the 
username to be UTF-8 encoded, while in PostgreSQL the username can be in 
any encoding. That is a source of annoyance in itself, as it's not 
well-defined in PostgreSQL which encoding to use when sending a username 
to the server. But I don't want to try fixing that in this patch, so it 
seems easiest to just require the username to be empty.

* Need a source of randomness in client, to generate random nonces used 
in the handshake. The SCRAM specification is not explicit about it, but 
I believe it doesn't need to be unpredictable, as long as a different 
nonce is used for each authentication.

* The client does not authenticate the server, even though the SCRAM 
protocol allows that. The client does verify the proof the server sends, 
but nothing stops a malicious server that's impersonating the real 
server from not requesting SCRAM authentication in the first place. It 
could just send AuthenticationOK without any authentication at all. To 
take advantage of the server authentication, we'll need to add something 
similar to the "sslmode=verify-ca" option in the client. In that mode, 
the client should refuse the connection if the server doesn't request 
SCRAM authentication (or some other future authentication mechanism that 
authenticates the server to the client).

* Channel binding is not implemented. Not essential, but would be nice 
to have. Together with the new client option mentioned in the previous 
point, it would allow the client to know that there is no 
man-in-the-middle, without having to verify the server's SSL certificate.

- Heikki

Attachment: 0001-Move-sha1.c-to-src-common.patch
Description: application/x-patch (26.0 KB)
Attachment: 0002-Refactor-sendAuthRequest.patch
Description: application/x-patch (5.5 KB)
Attachment: 0003-Add-support-for-multiple-verifiers.patch
Description: application/x-patch (22.6 KB)
Attachment: 0004-WIP-Implement-SCRAM-authentication.patch
Description: application/x-patch (60.6 KB)


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