Cryptographers have broken the proprietary encryption used to prevent eavesdropping on more than 800 million cordless phones worldwide, demonstrating once again the risks of relying on obscure technologies to remain secure.

The attack is the first to crack the cipher at the heart of the DECT, or Digital Enhanced Cordless Telecommunications, standard, which encrypts radio signals as they travel between cordless phones in homes and businesses and corresponding base stations. A previous hack, by contrast, merely exploited weaknesses in the way the algorithm was implemented.

The fatal flaw in the DECT Standard Cipher is its insufficient amount of “pre-ciphering,” which is the encryption equivalent of shaking a cup of dice to make sure they generate unpredictable results. Because the algorithm discards only the first 40 or 80 bits during the encryption process, it’s possible to deduce the secret key after collecting and analyzing enough of the protected conversation.

“This standard, as with everything else we have broken, has been designed some 20 years ago, and it is proprietary encryption,” said Karsten Nohl, one of the cryptographers who helped devise the attack. “It relied on the fact that the encryption was unknown and hence could not be broken. This is a case where something that has some potential for being strong is broken by just this one design decision that in any public review would have been spotted immediately.”

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