JN-25 was “10% Penetrated” whatever that means. The IJN changed the cipher in November 1941 AND utilized couriers & radio silence to minimize the traffic using the new JN-25B.
There simply was nothing to go on specific to Pearl Harbor.
Simon Singh
The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography
Reprint Edition
ISBN-13: 978-0385495325, ISBN-10: 0385495323