The Emperor's Codes: The Breaking of Japan's Secret Ciphers

During the last weeks, I've read the book The Emperor's Codes: The Breaking of Japan's Secret Ciphers. As you can guess from the title, the book relates to the various UK, American and Australian code breaker teams working on breaking the encrypted communication of Japan during the second world war.

There have been plenty of books about the history of breaking Germany's Enigma ciphering machine, but information on how the Japanese codes were broken so far didn't seem to be as widespread - despite the resepective archives being opened up during the last decades.

It has been a most interesting reading. As you can imagine, at that time almost nobody had a sufficient understanding of the Japanese language, not even thinking about how to encode Japanese writing into morse code.

Nonetheless, all of the Japanese merchant, diplomatic, army and navy codes have been broken during the war. And surprisingly, the Japanese never really assumed something is wrong with their actual encryption method. All they did is to replace the codebook or the additive codebook.

Also, just like in today's GSM (A5/1) crypto attacks, even back then the importance of known plaintext could not be underestimated. The verbosity of Japanese soldiers addressing a superior officer and the stereotypical nature of reports on weather or troop movements gave the cryptographers plenty of known plaintext for many of their intercepted message.

What was also new to me is the fact that the British even back then demanded that Cable+Wireless provides copies of all telegraphs through their network. And that's some 70-80 years before data retention on communications networks becomes a big topic ;)

Overall, definitely a very interesting book. I can recommend it to anyone with an interest in security, secret services, WW2 history and/or cryptography.

Performance Enhancements in a Frequency Hopping GSM Network

Dieter Spaar had pointed out this book some months ago when I first raised some questions regarding frequency hopping and the orthogonal nature of hopping sequences with the same HSN but different MAIO.

Last week while David Burgess was with me, he also indicated that this book was great and he unfortunately didn't think of bringing it along with him.

Meanwhile, I have immediately ordered the book and am already at something like 30% completion. It is a most interesting book to read, approaching GSM from an advanced network planning angle, with a specific focus on the effects of frequency hopping, uplink/downlink power control and DTX on the overall system performance of a GSM network.

The theoretical foundations are always put in a GSM network simulator with detailed channel model, but also actually implemented in a real-world GSM network in Denmark.

Next to all the GSM specifications with their plethora of options and operator dependent settings, this book gives a detailed (but still very technical) background on how and why an Operator would configure his network to maximize the service quality offered to his subscribers.

From the results, you can for example very clearly see that

• frequency hopping over a cyclic sequence gives higher gain improvement than random hopping, especially if the number of channels in the mobile allocation is low
• frequency hopping gain is very dependent on the speed at which the MS moves. At 3kph, the gain when hopping over 8 channels can be 7dB, while at 50kph the same hopping will only provide 1.5dB
• MAIO management (using different MAIO but same HSN) for all sectors in a cell gives significant FER improvements
• handover algorithms differ quite a bit between non-frequency-hopping and frequency-hopping networks

In the end, it seems, network planning is never about allocating your channels in a way they don't overlap. That would limit the network capacity way too much. Network planning seems to only be about averaging out the interference that cells inevitably have with each other and ensure that the quality of the system only degrades with increasing load.