The SCRAM-SHA-256 authentication protocol, defined by RFC 7677 and available since PostgreSQL 10, uses SCRAM secrets. There are used during authentication as equivalents of passwords and stored in pg_authid, shaped based on RFC 5803 for LDAP. This format can be described as a string made of:
<SCRAM mechanisms>$<iterations>:<salt>$<stored key>:<server key>
For all the details regarding that, feel free to look at the RFCs quoted above. When it comes to SCRAM-SHA-256 in PostgreSQL, the mechanism is simply saved as “SCRAM-SHA-256”, iterations are 4096, the salt is made of 16 random bytes. These are default values hardcoded in PostgreSQL, still the most internal routines (like scram_build_secret() in scram-common.c) and the protocol are able to work in a very flexible way, hence it is possible to have secrets with custom iteration values or salt lengths. The point is that secrets updated depending on these variables would be able to work out of the box. A few years ago, I have for example developed a small extension called scram_utils, able to customize SCRAM secrets and store them in the pg_authid catalog for a given role. PostgreSQL has never by itself offered the possibility to configure that in-core, which is what this post is about, but only for the number of iterations, following this commit that should land in Postgres 16:
commit: b577743000cd0974052af3a71770a23760423102 author: Daniel Gustafsson <email@example.com> date: Mon, 27 Mar 2023 09:46:29 +0200 Make SCRAM iteration count configurable Replace the hardcoded value with a GUC such that the iteration count can be raised in order to increase protection against brute-force attacks. [...] Reviewed-by: Michael Paquier <firstname.lastname@example.org> Reviewed-by: Jonathan S. Katz <email@example.com> Discussion: https://postgr.es/m/F72E7BC7-189F-4B17-BF47-9735EB72C364@yesql.se
The iteration number is a double-edged sword:
- A low value means less time spent computing the salted password during authentication, speeding up the process. However, passwords are more sensitive to brute-force attacks.
- A high value uses more computing power to generate the salted password and it makes authentication longer, still it offers more protection.
As the commit message quotes, RFC 7677 recommends a default of 15000 iterations, while 4096 is the Postgres default, and it has also been argued that even 4096 is still too expensive for some, which would mean extra costs required by the user just for the sake of being able to connect to a database and gain access to its data. So this really is a balance between what is thought as safe or affordable. And, depending on the needs and the requirements of an environment, honestly, satisfying either one of them is simple: just use a low or upper bound. Satisfying both, though, means sacrifying a portion of the other for a range found acceptable. The generation of the secret has two different costs:
- Its initial creation in CREATE ROLE or ALTER ROLE. This costs in computation resource in the backend if a password string is given in its clear form (perhaps not recommended), or some frontend, like libpq with psql’s \password.
- The connection attempt when computing the salted password in the frontend, after receiving the first message from the server during a SASL exchange for SCRAM.
Generating a secret can take some time with a high iteration number:
=# SET scram_iterations = 4096; SET =# CREATE ROLE scram_4096 password 'foo'; CREATE ROLE Time: 13.030 ms =# SET scram_iterations = 10000000; SET =# CREATE ROLE scram_10m password 'foo' LOGIN; CREATE ROLE Time: 5975.290 ms (00:05.975) =# SET scram_iterations = 1; SET =# CREATE ROLE scram_1 password 'foo' LOGIN; CREATE ROLE Time: 6.700 ms
A computation with 10M iterations took roughly 6s in a local environment, while of course 1 iteration was, well, fast. pg_authid stores the iteration number (barbaric regexp used here, so just look at the result):
=# SELECT rolname, regexp_replace(rolpassword, '(SCRAM-SHA-256)\$(\d+):([a-zA-Z0-9+/=]+)\$([a-zA-Z0-9+=/]+):([a-zA-Z0-9+/=]+)', '\1$\2:<salt>$<storedkey>:<serverkey>') AS rolpassword_masked FROM pg_authid where rolname ~ '^scram'; rolname | rolpassword_masked ------------+------------------------------------------------------- scram_4096 | SCRAM-SHA-256$4096:<salt>$<storedkey>:<serverkey> scram_10m | SCRAM-SHA-256$10000000:<salt>$<storedkey>:<serverkey> scram_1 | SCRAM-SHA-256$1:<salt>$<storedkey>:<serverkey> (3 rows)
As of the performance difference at authentication time, using a single psql command with a simple query to stress how long it would take to run the command is not fun. One way I have learnt to stress the connection code path of PostgreSQL without processing anything in the backend is to use pgbench with an custom empty script, like that:
$ cat /tmp/pgbench_empty.sql \set a 10
Combined with a fixed time defined by –time/-T, it is possible to quickly check how many connection attempts (HBA entries set to local) could be achieved in this given time frame (do not forget to set PGPASSWORD). Here are some quick numbers for each role defined previously (output has been cut a bit for clarity):
$ pgbench -n -T 30 -f /tmp/pgbench_empty.sql -C -U scram_4096 postgres number of transactions actually processed: 7099 latency average = 4.226 ms average connection time = 4.226 ms tps = 236.613828 (including reconnection times) $ pgbench -n -T 30 -f /tmp/pgbench_empty.sql -C -U scram_10m postgres number of transactions actually processed: 5 latency average = 6303.996 ms average connection time = 6303.993 ms tps = 0.158630 (including reconnection times) $ pgbench -n -T 30 -f /tmp/pgbench_empty.sql -C -U scram_1 postgres number of transactions actually processed: 18391 latency average = 1.631 ms average connection time = 1.631 ms tps = 613.005891 (including reconnection times)
The number of connections achieved for 10M iterations does not sound as a surprise, matching more or less with the number found when the secret was computed by CREATE ROLE. What is more surprising in this exercise is the difference between 1 iteration and the default of 4096, so even for short queries the default could really become a bottleneck. That can be leveraged with a connection pooler, as one workaround. At the end, users should make sure to study carefully what to use: security or speed.