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The Debian OpenSSL PRNG bug and DHE

Some colleagues (Hovav Shacham, Brendan Enright, Scott Yikel, Stefan Savage) and I have been tracking the aftermath of the Debian OpenSSL PRNG bug
Read more of the tracking here


To understand why, you need to understand the vulnerability better. Effectively, the vulnerability stopped any invocations of RAND_seed() from mixing data into the PRNG. The only time new seed data gets mixed in is when you get new randomness values via RAND_bytes(). Each time you call RAND_bytes() the current process ID gets mixed into the PRNG. So, for a given PID and a given sequence of invocations of RAND_bytes(), you always get the same string of random values. These values are (statistically) unique, but predictable: you can say “the nth value will always be one of the following 2^15 values depending on the PID”. However, it should be clear that even for a given PID, you can generate an arbitrary (well, almost) number of distinct values. So, if you had a process which generated a million DH keys in sequence, they'd all be different. Unfortunately for Bello's attack, this is exactly how many real Web servers work. For instance, Apache w/ Mod_SSL forks off a bunch of long-lived server processes which each handle many requests. Bello's attack would potentially work on the first connection, but the second connection would not be on the key list. You need another 2^15 values to handle the second connection. We've confirmed this by setting up a server, connecting to it, and pulling out more than 2^15 distinct public keys. So, you need to do something more complicated.

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