On Wed, Dec 21, 2016 at 5:13 PM, George Spelvin <linux@xxxxxxxxxxxxxxxxxxx> wrote: > As a separate message, to disentangle the threads, I'd like to > talk about get_random_long(). > > After some thinking, I still like the "state-preserving" construct > that's equivalent to the current MD5 code. Yes, we could just do > siphash(current_cpu || per_cpu_counter, global_key), but it's nice to > preserve a bit more. > > It requires library support from the SipHash code to return the full > SipHash state, but I hope that's a fair thing to ask for. I don't even think it needs that. This is just adding a non-destructive final operation, right? > > Here's my current straw man design for comment. It's very similar to > the current MD5-based design, but feeds all the seed material in the > "correct" way, as opposed to Xring directly into the MD5 state. > > * Each CPU has a (Half)SipHash state vector, > "unsigned long get_random_int_hash[4]". Unlike the current > MD5 code, we take care to initialize it to an asymmetric state. > > * There's a global 256-bit random_int_secret (which we could > reseed periodically). > > To generate a random number: > * If get_random_int_hash is all-zero, seed it with fresh a half-sized > SipHash key and the appropriate XOR constants. > * Generate three words of random_get_entropy(), jiffies, and current->pid. > (This is arbitary seed material, copied from the current code.) > * Crank through that with (Half)SipHash-1-0. > * Crank through the random_int_secret with (Half)SipHash-1-0. > * Return v1 ^ v3. Just to clarify, if we replace SipHash with a black box, I think this effectively means, where "entropy" is random_get_entropy() || jiffies || current->pid: The first call returns H(random seed || entropy_0 || secret). The second call returns H(random seed || entropy_0 || secret || entropy_1 || secret). Etc. If not, then I have a fairly strong preference to keep whatever construction we come up with consistent with something that could actually happen with invocations of unmodified SipHash -- then all the security analysis on SipHash goes through. Anyway, I have mixed thoughts about the construction. It manages to have a wide state at essentially no cost, which buys us quite a bit of work factor to break it. Even with full knowledge of the state, an output doesn't reveal the entropy except to the extent that it can be brute-force (this is just whatever the appropriate extended version of first preimage resistance gives us). The one thing I don't like is that I don't see how to prove that you can't run it backwards if you manage to acquire a memory dump. In fact, I that that there exist, at least in theory, hash functions that are secure in the random oracle model but that *can* be run backwards given the full state. From memory, SHA-3 has exactly that property, and it would be a bit sad for a CSPRNG to be reversible. We could also periodically mix in a big (128-bit?) chunk of fresh urandom output to keep the bad guys guessing. (P.S. This kind of resembles the duplex sponge construction. If hardware SHA-3 ever shows up, a duplex sponge RNG might nice indeed.) -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
