TECHNICAL QUESTIONS

What kind of cryptography and what key length is used in the CryptoPhone?

All calls are encrypted with 256-bit keys using AES and Twofish as counter mode stream ciphers. Using both AES and Twofish results in much stronger encryption than using only one algorithm. For the highly unlikely case that a weakness is discovered in one of the algorithms, the use of the second algorithm provides still a sufficient margin of security. The use of the two very strong algorithms is a unique feature of the CryptoPhone. The key used is generated using a 4096-bit Diffie-Hellman shared secret exchange.

The crypto block diagram is shown below:

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What is the voice compression used in the CrypoPhone? How does it sound? Is there a delay?

The basic design of a secure GSM phone is to take the voice from the microphone, digitize it and run it through a compression algorithm, before encrypting it and sending it via a GSM data call to the other party. The compression algorithm is also called a codec and does with voice what mp3 does with music - making sure it takes up less data.

CryptoPhones use the CELP Codec running at 8kHz. The output stream of the codec is 4.8kbit/second, enabling it to be transported over a 9,6kbit GSM data call.

The speech and sound quality you can expect is comparable to transcontinental phone calls. You should note that the overall speech quality depends on the GSM signal quality, so degradation does happen in low coverage areas. While in unencrypted GSM the sound quality gets bad and you would experience dropouts as the phone moves out of coverage, with the CryptoPhone under the same circumstances the call delay would increase. Simple indicators on the CryptoPhone show GSM signal coverage and call quality / delay.

All calls made with the CryptoPhone are subject to a certain delay in the call, as if your call is routed over a satellite link. Most of the call delay originates from the way GSM networks handle the data calls. The CryptoPhone must use the GSM data call instead of the normal voice call mode to ensure a transparent communications channel between the two CryptoPhones. Because the delay is a side-effect of all GSM data calls there is nothing we can do about it. All available GSM encryption products on the market suffer equally from this delay. The CryptoPhone itself introduces comparatively little delay from the voice encoding and encryption.

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So what does the software architecture of the CryptoPhone look like?

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I noticed that your CryptoPhone is based on Windows CE / PocketPC. Isn't this a security risk?

CryptoPhone mobile phones run on top of a heavily modified and stripped-down Microsoft
Windows Mobile for Pocket PC/Smartphone ROM. The reason is that we wanted an affordable and well researched platform that offered sufficient performance for the speech encoding and crypto functions.A Pocket PC based system was chosen as the first platform for CryptoPhone because it was the only sufficiently fast device allowed us to do software integrity protection in ROM and the stripping of unnecessary functions.

The only commercially available alternative at the time of the necessary development decision was Symbian. Symbian is even more closed source (Windows CE is open source for developers in most parts) and was available only on a more expensive hardware platform. There was (and still is) no viable mass-market Embedded Linux based hardware with sufficient performance, stability, hardware integration and availability on the market at decision time, so we were not able to pursue this alternative.

We are aware that there are risks associated with using any Windows platform and we have taken a number of measures to mitigate these risks as best we could. We removed applications, communication stacks and system parts that are unnecessary for the CryptoPhone operation and which may cause potential security problems. You should not install third party software on the CryptoPhone to prevent software based attacks on the firmware integrity. The firmware update mechanism is cryptographically secured.

What other platforms do you plan to release the CryptoPhone on?

We plan to offer products based on 3G mobile phone networks (UMTS) and TCP/IP. The ultimate goal is to create a family of interoperable CryptoPhone products that offer the privilege of secure communication on all relevant networks. Watch our website for more specific products announcements.

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