Remove deprecated config/secrets.yml

Rails' `config/secrets.yml` is deprecated and is not be
supported in Rails 7.2. Thus, we need to change how we
handle the `secret_key_base`.

After this commit, the application will continue to use
the environment variable `SECRET_KEY_BASE` when present
(this is how the `staging` and `production` sites already work).
When the environment variable is not set, the preset value in
the file `tmp/local_secret.txt` is used.

This *appears* to check in a private key and thus would be vulnerable,
but it's not a vulnerability in this case. The rationale for this
assertion is documented in the assurance case (as part of this
commit), as well as more details about why it's been done this way.

Signed-off-by: David A. Wheeler <[email protected]>

docs/assurance-case.md

@@ -1884,9 +1884,11 @@ as of 2015-12-14:
    because of their wide support and efficiency.
    We use the default Rails CookieStore mechanism to store sessions;
    it is both simple and much faster than alternatives.
-   Rails implements an automatic authentication mechanism (using a
-   secret key) to ensure that clients cannot undetectably change
-   these cookies; a changed value is thrown away.
+   Rails implements an automatic cookie authentication mechanism (using a
+   secret key referred to as `secret_key_base`)
+   to ensure that clients cannot undetectably change
+   these cookies; a changed value created without being re-authenticated
+   is thrown away.
    Logged-in users have their user id stored in this authenticated cookie
    (There is also a `session_id`, not currently used.)
    Session data is intentionally kept small, because of the limited
@@ -2512,6 +2514,45 @@ However, rails_best_practices is only run in development and test,
 where the environment and input data are trusted,
 so we believe this is not a real vulnerability.
 
+#### Checked-in `tmp/local_secret.txt` appears to have a secret
+
+The file `tmp/local_secret.txt` may appear to be a security vulnerability.
+That's because it's a key (it's the value of `secret_key_base` for
+development and test environments) whose value is checked into a public
+version-controlled repository.
+
+However, it is not a vulnerability. This value is *only* used during
+test and development. The fact that its value is public is
+irrelevant, since those systems are not publicly accessible, or don't
+have any real secrets, or both.
+
+We set this file for the following reasons;
+
+1. It's convenient to have a *constant* value of
+   `secret_key_base`, because it means it's much easier to halt and
+   restart during development and test. Having it change on every restart
+   makes development and test much harder.
+2. It eliminates occasional test failures in parallelized testing.
+   At the time of this writing, when Rails needs a value for `secret_key_base`
+   and cannot find it in the environment variable `SECRET_KEY_BASE` or a file,
+   it creates a new value for `secret_key_base` and stores it in the file
+   `tmp/local_secret.txt`.
+   [A race condition results](https://github.com/rails/rails/issues/53661),
+   because if tests are running in parallel, other tests may try to load the
+   file before it's been created by another process.
+   I've verified in the Rails code that if this file exists,
+   its content is used.
+
+In the staging and production environments we provide a truly secret value for
+the environment variable `SECRET_KEY_BASE`. When this environment variable
+is set, it is used instead. Their values are the ones
+we actually need to keep private. We do *not* check in their values.
+
+For more information:
+
+* <https://apidock.com/rails/Rails/Application/secret_key_base>
+* <https://apidock.com/rails/v7.1.3.2/Rails/Application/generate_local_secret>
+
 #### Information Exposure from actioncable
 
 The "actioncable" module was identified as potentially vulnerable to

tmp/local_secret.notes.md

@@ -0,0 +1,3 @@
+# The file `tmp/local_secret.txt` is NOT a vulnerability
+
+For more information, see the file `docs/assurance-case.md`.

tmp/local_secret.txt

@@ -0,0 +1 @@
+b49f4179a4f76042d3e44115f8e1bf424117f3389db00fb1dc31bcf1500bd0609f426412cc5af9871a40ade34887e65e61c895878d4973678682819098a513b7