How can I edit the wp_parse_auth_cookie function from the pluggable file in my functions?

Question

I need to modify that function to decrypt the username that I have encrypted.

Answer 1

Because it's a pluggable function, and surrounded by an if statement, you can define your own function by making the logic of the if statement evaluate to falsy. In this case, to change the if statement logic to falsy, you need only to define your own wp_parse_auth_cookie() function before the pluggable.php file is loaded. I recommend defining the function in an mu-plugin.