Re: [viff-devel] VIFF and random numbers
It is not good to use the wrong kind of PRG, it should be fixed as soon as possible. But do we know that os.urandom will be OK on any platform, or is this OS -dependent at the end of the day? - Ivan On 06/07/2010, at 15.22, Thomas P Jakobsen wrote: VIFF itself as well as most protocols implemented in VIFF uses the viff.util package for random number generation. This package in turn uses the random package in the Python standard library. This means that random numbers are generated using a Mersenne twister. As far as I can see, this is a problem, since Mersenne twister PRNGs are generally not suited for cryptographic usage. E.g. it is not known to pass the next-bit test and withstand the state compromise extensions, see http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator. One solution would be to use the os.urandom() function instead. This has specifically been designed to produce cryptographically secure random numbers. (We should probably keep the old random generator, too. It is probably faster and not all random numbers used in VIFF and VIFF programs need to be cryptographically secure.) Let me know what you think about this. Kind regards, Thomas ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk
Re: [viff-devel] VIFF and random numbers
Indeed it should satisfy those properties. Say if you Shamir share something, the adversary might get t shares in order. If it can guess the next bit with non-negligible advantage, this will completely break our claim that the adversary has no information on the secret. Luckily it should not be hard to replace. I think we knew about this earlier but just forgot, actually. By the way, I am not sure we use any random numbers that should NOT secure in this way. Citat af Thomas P Jakobsen thomas@gmail.com: VIFF itself as well as most protocols implemented in VIFF uses the viff.util package for random number generation. This package in turn uses the random package in the Python standard library. This means that random numbers are generated using a Mersenne twister. As far as I can see, this is a problem, since Mersenne twister PRNGs are generally not suited for cryptographic usage. E.g. it is not known to pass the next-bit test and withstand the state compromise extensions, see http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator. One solution would be to use the os.urandom() function instead. This has specifically been designed to produce cryptographically secure random numbers. (We should probably keep the old random generator, too. It is probably faster and not all random numbers used in VIFF and VIFF programs need to be cryptographically secure.) Let me know what you think about this. Kind regards, Thomas ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk
Re: [viff-devel] VIFF and random numbers
Thomas P Jakobsen wrote: The urandom is os-specific: This function returns random bytes from an OS-specific randomness source. The returned data should be unpredictable enough for cryptographic applications, though its exact quality depends on the OS implementation. On a UNIX-like system this will query /dev/urandom, and on Windows it will use CryptGenRandom. I don't know whether this will be good enough. There is a paper describing various flaws: http://www.pinkas.net/PAPERS/gpr06.pdf If not, I guess we'll have to use some external package (openssl?) or implement our own algorithm. viff.util.rand is used to make all randomness replayable, which already helped me to find bugs triggered by certain randomness. I would like to have this feature also in the future, therefore I would vote against a random number generator using noise not only to generate a seed. If the environment variable VIFF_SEED is set to the empty string, VIFF already uses /dev/urandom by using random.SystemRandom for random number generation. This possibility is not documented however. Best regards, Marcel On Tue, Jul 6, 2010 at 15:40, Ivan Bjerre Damgård i...@cs.au.dk wrote: It is not good to use the wrong kind of PRG, it should be fixed as soon as possible. But do we know that os.urandom will be OK on any platform, or is this OS -dependent at the end of the day? - Ivan On 06/07/2010, at 15.22, Thomas P Jakobsen wrote: VIFF itself as well as most protocols implemented in VIFF uses the viff.util package for random number generation. This package in turn uses the random package in the Python standard library. This means that random numbers are generated using a Mersenne twister. As far as I can see, this is a problem, since Mersenne twister PRNGs are generally not suited for cryptographic usage. E.g. it is not known to pass the next-bit test and withstand the state compromise extensions, see http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator. One solution would be to use the os.urandom() function instead. This has specifically been designed to produce cryptographically secure random numbers. (We should probably keep the old random generator, too. It is probably faster and not all random numbers used in VIFF and VIFF programs need to be cryptographically secure.) Let me know what you think about this. Kind regards, Thomas ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk
Re: [viff-devel] VIFF and random numbers
I agree that tests should be reproducible. But it is also very important to use a cryptographically secure PRNG. I don't know whether these two requirements can be satisfied by the same number generator. If not, the best solution is to have two modes of operation: - A test mode where the execution can be reproduced and - a secure mode using a cryptographically secure PRNG Regards, Thomas ___ viff-devel mailing list (http://viff.dk/) viff-devel@viff.dk http://lists.viff.dk/listinfo.cgi/viff-devel-viff.dk
