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The GSMK CryptoPhone technology is based on published and well researched algorithms for both encryption and voice processing. It uses very long keys, resulting in a product that provides peace of mind today and in the future. All calls are encrypted with 256-bit keys using AES and Twofish running as counter mode stream ciphers. Using both AES and Twofish provides a much stronger design then 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 that provides a "fall back" inside the crypto-system design. The design goal was to provide not only "tactical security" that lasts for a few months or years, but to design for security against future developments in cryptanalysis in the next decades. The key used for each call is generated using a 4096-bit Diffie-Hellman shared secret exchange, hashing the resulting 4096 bits to the 256 bit session key by means of SHA256. To prevent man-in-the-middle attacks, a six-letter hash is generated from the Diffie-Hellman result and displayed to the user. The user then reads three letters over the encrypted line to the communication partner and verifies the three letters the communication partner reads to him. If there were a discrepancy in the six letters, a man-in-the-middle attack has been detected. The random material required for the Diffie-Hellman exchange is generated by using the least significant bit from the microphone signal (not during calls of course) and enhancing this entropy with the Fortuna algorithm. This scheme ensures that each encrypted call is performed with a completely new and random key. All key material is securely erased immediately after the call ends. The graphic below shows the encryption flow from key exchange to data encryption. For more in-detail explanation of the GSMK CryptoPhone, please refer to the Q&A-section.
Further References Cryptanalysis
Literature References COMINT/SIGINT
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