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MS to bundle 'broken' random number tool in Vista SP1

Developers urged to avoid built-in backdoor

Microsoft plans to bundle a cryptographically flawed pseudo random number generator in its upcoming service pack for Windows Vista.

Cryptographers have expressed concern about a possible backdoor in a standard for random number generators approved by the National Institute of Standards and Technology (NIST) this year.

The cryptographically weak Dual_EC_DRBG approach, which is based on the mathematics of elliptic curves, was one of four "deterministic random bit generators", approved by the NIST in March.

Flaws in the approach (Dual_EC_DRBG) first emerged in August at the Crypto 2007 conference when cryptographers Dan Shumow and Niels Ferguson demonstrated that two constants in the standard used to define the algorithm's elliptic curve have a relationship with a second, secret set of numbers.

Anyone who had access to the second set of numbers would have a kind of skeleton key able to unlock any instance of Dual_EC_DRBG. Suspicions that this weakness might be used as a backdoor have been fueled by the NSA's support of Dual_EC_DRBG in the standards-setting process.

Random number generators are important because the correct operation of SSL and other protocols relies on their randomness. Standards set in this area by NIST are significant because they are likely to be followed by hardware and software suppliers in much the same way that the Advanced Encryption Standard (AES), which was also approved under the auspices of the NIST, has become widely adopted.

Crypto guru Bruce Schneier, who previously described the weakness as a backdoor, notes that the Dual_EC_DRBG approach will be implemented in Windows Vista SP1.

Although the technology will not be applied by default, that leaves users reliant on the good sense of developers in avoiding the cryptographically weak approach. The default random number generator in Vista SP1 will be CTR_DRBG, technology based on the AES standard that's reckoned to be far more robust than Dual_EC_DRBG.

Schneier's latest warning on the issue has sparked a lively discussion on his blog with participants expressing concern that the flawed Dual_EC_DRBG random number generator could appear more prominently in either the IE or .NET developer framework further down the road. ®

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