Use a 32-byte version of db_key_base for web hooks

AES-256-GCM cipher mode requires a key that is exactly 32 bytes long.
We already handle the case when the key is too long, by truncating, but
the key can also be too short in some installations. Switching to a key
that is always exactly the right length (by virtue of right-padding
ASCII 0 characters) allows encryption to proceed, without breaking
backward compatibility.

When the key is too short, encryption fails with an `ArgumentError`,
causing the web hooks functionality to be unusable. As a result, zero
rows can exist with values encrypted with the too-short key.

When the key is too long, it is silently truncated. In this case, the
key is unchanged, so values encrypted with the new too-long key will
still be successfully decrypted.

app/models/hooks/web_hook.rb

@@ -6,12 +6,12 @@ class WebHook < ActiveRecord::Base
   attr_encrypted :token,
                  mode:      :per_attribute_iv,
                  algorithm: 'aes-256-gcm',
-                 key:       Settings.attr_encrypted_db_key_base_truncated
+                 key:       Settings.attr_encrypted_db_key_base_32
 
   attr_encrypted :url,
                  mode:      :per_attribute_iv,
                  algorithm: 'aes-256-gcm',
-                 key:       Settings.attr_encrypted_db_key_base_truncated
+                 key:       Settings.attr_encrypted_db_key_base_32
 
   has_many :web_hook_logs, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 

changelogs/unreleased/53659-use-padded-key-for-gcm-ciphers.yml

+---
+title: Fix web hook functionality when the database encryption key is too short
+merge_request: 23573
+author:
+type: fixed

lib/gitlab/background_migration/models/encrypt_columns/web_hook.rb

@@ -15,12 +15,12 @@ module Gitlab
           attr_encrypted :token,
                          mode:      :per_attribute_iv,
                          algorithm: 'aes-256-gcm',
-                         key:       ::Settings.attr_encrypted_db_key_base_truncated
+                         key:       ::Settings.attr_encrypted_db_key_base_32
 
           attr_encrypted :url,
                          mode:      :per_attribute_iv,
                          algorithm: 'aes-256-gcm',
-                         key:       ::Settings.attr_encrypted_db_key_base_truncated
+                         key:       ::Settings.attr_encrypted_db_key_base_32
         end
       end
     end