Refer to cheat sheet on securely storing keys rather than using envir…

…onment variables (#1440)

cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.md

@@ -81,7 +81,7 @@ To generate HMAC CSRF tokens (with a session-dependent user value), the system m
 - **A session-dependent value that changes with each login session**. This value should only be valid for the entirety of the users authenticated session. Avoid using static values like the user's email or ID, as they are not secure ([1](https://stackoverflow.com/a/8656417) | [2](https://stackoverflow.com/a/30539335) | [3](https://security.stackexchange.com/a/22936)). It's worth noting that updating the CSRF token too frequently, such as for each request, is a misconception that assumes it adds substantial security while actually harming the user experience ([1](https://security.stackexchange.com/a/22936)). For example, you could choose one of the following session-dependent values:
  - The server-side session ID (e.g. [PHP](https://www.php.net/manual/en/function.session-start.php) or [ASP.NET](<https://learn.microsoft.com/en-us/previous-versions/aspnet/ms178581(v=vs.100)>)).
  - A random value (e.g. UUID) within a JWT that changes every time a JWT is created.
-- **A secret cryptographic key** Not to confuse with the random value from the naive implementation. This value is used to generate the HMAC hash. Ideally, store this key as an environment variable.
+- **A secret cryptographic key** Not to be confused with the random value from the naive implementation. This value is used to generate the HMAC hash. Ideally, store this key as discussed in the [Cryptographic Storage page](https://cheatsheetseries.owasp.org/cheatsheets/Cryptographic_Storage_Cheat_Sheet.html#key-storage).
 - **A random value for anti-collision purposes**. Generate a random value (preferably cryptographically random) to ensure that consecutive calls within the same second do not produce the same hash ([1](https://github.com/data-govt-nz/ckanext-security/issues/23#issuecomment-479752531)).
 
 **Should Timestamps be Included in CSRF Tokens for Expiration?**