Re: User functions for building SCRAM secrets

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: User functions for building SCRAM secrets
Date: 2022-11-01 00:56:37
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On Mon, Oct 31, 2022 at 04:27:08PM -0400, Jonathan S. Katz wrote:
> 1. password only -- this defers to the PG defaults for SCRAM
> 2. password + salt -- this is useful for the password history / dictionary
> case to allow for a predictable way to check a password.

Well, one could pass a salt based on something generated by random()
to emulate what we currently do in the default case, as well. The
salt length is an extra possibility, letting it be randomly generated
by pg_strong_random().

> 1. Location of the functions. I put them in
> src/backend/utils/adt/cryptohashfuncs.c as I wasn't sure where it would make
> sense to have them (and they could easily go into their own file).

As of adt/authfuncs.c? cryptohashfuncs.c does not strike me as a good

> Please let me know if you have any questions. I'll add a CF entry for this.

+{ oid => '8555', descr => 'Build a SCRAM secret',
+ proname => 'scram_build_secret_sha256', proleakproof => 't', prorettype => 'text',
+ proargtypes => 'text', prosrc => 'scram_build_secret_sha256_from_password' },
+{ oid => '8556', descr => 'Build a SCRAM secret',
+ proname => 'scram_build_secret_sha256', proleakproof => 't',
+ provolatile => 'i', prorettype => 'text',
+ proargtypes => 'text text', prosrc => 'scram_build_secret_sha256_from_password_and_salt' },
+{ oid => '8557', descr => 'Build a SCRAM secret',
+ proname => 'scram_build_secret_sha256', proleakproof => 't',
+ provolatile => 'i', prorettype => 'text',
+ proargtypes => 'text text int4', prosrc => 'scram_build_secret_sha256_from_password_and_salt_and_iterations' },

Keeping this approach as-is, I don't think that you should consume 3
OIDs, but 1 (with scram_build_secret_sha256_from_password_and_..
as prosrc) that has two defaults for the second argument (salt string,
default as NULL) and third argument (salt, default at 0), with the
defaults set up in system_functions.sql via a redefinition.

Note that you cannot pass down an expression for the password of
CREATE/ALTER USER, meaning that this would need a \gset at least if
done by a psql client for example, and passing down a password string
is not an encouraged practice, either. Another approach is to also
provide a role OID in input and store the newly-computed password in
pg_authid (as of [1]), so as one can store it easily.

Did you look at extending \password? Being able to extend
PQencryptPasswordConn() with custom parameters has been discussed in
the past, but this has gone nowhere. That's rather unrelated to what
you are looking for, just mentioning as we are playing with options to
have control the iteration number and the salt.


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