You're seeing just the callback crypto_config, go back to Oasis.Token module for more information.
Link to this callback

crypto_config(conn, opts)

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crypto_config(conn :: Plug.Conn.t(), opts :: Keyword.t()) ::

Avoid using the application enviroment as the configuration mechanism for this library, and make crypto-related key information configurable when use bearer authentication.

The Oasis.Plug.BearerAuth module invokes this callback function to fetch a predefined Oasis.Token.Crypto struct, and then use it to verify the bearer token of the request.