Can't run digest() on PosrgreSQL 9.3 on RDS
The question was answered at the AWS forum by Shawn@AWS - The rds.extension list shows available, rather than installed, modules.

Thank you for using RDS Postgres. The &quot;rds.extension&quot; parameter shows you what extensions are available in RDS Postgres. You still need to execute the &quot;create extension&quot; command.
Using your test case:

postgres=&gt; show rds.extensions;
... ,pgcrypto, ...
postgres=&gt; create extension pgcrypto;
CREATE EXTENSION

postgres=&gt; SELECT digest('blah', 'sha1');
digest
\x5bf1fd927dfb8679496a2e6cf00cbe50c1c87145
(1 row)

Addendum
pg_available_extensions shows which extensions are available and which are already installed:
postgres=&gt; SELECT * FROM pg_available_extensions;

          name          | default_version | installed_version |                               comment
------------------------+-----------------+-------------------+---------------------------------------------------------------------
 chkpass                | 1.0             |                   | data type for auto-encrypted passwords
 xml2                   | 1.0             |                   | XPath querying and XSLT
 plpgsql                | 1.0             | 1.0               | PL/pgSQL procedural language
 pgcrypto               | 1.0             |                   | cryptographic functions
 postgres_fdw           | 1.0             |                   | foreign-data wrapper for remote PostgreSQL servers
...