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