services.discourse.secretKeyBaseFile

The path to a file containing the secret_key_base secret.

Discourse uses secret_key_base to encrypt the cookie store, which contains session data, and to digest user auth tokens.

Needs to be a 64 byte long string of hexadecimal characters. You can generate one by running

openssl rand -hex 64 >/path/to/secret_key_base_file

This should be a string, not a nix path, since nix paths are copied into the world-readable nix store.