Re: [PATCH v7 3/6] random: use SipHash in place of MD5

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Dec 21, 2016 at 6:07 PM, Hannes Frederic Sowa
<hannes@stressinduktion.org> wrote:
> On 22.12.2016 00:42, Andy Lutomirski wrote:
>> On Wed, Dec 21, 2016 at 3:02 PM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>>>  unsigned int get_random_int(void)
>>>  {
>>> -       __u32 *hash;
>>> -       unsigned int ret;
>>> -
>>> -       if (arch_get_random_int(&ret))
>>> -               return ret;
>>> -
>>> -       hash = get_cpu_var(get_random_int_hash);
>>> -
>>> -       hash[0] += current->pid + jiffies + random_get_entropy();
>>> -       md5_transform(hash, random_int_secret);
>>> -       ret = hash[0];
>>> -       put_cpu_var(get_random_int_hash);
>>> -
>>> -       return ret;
>>> +       unsigned int arch_result;
>>> +       u64 result;
>>> +       struct random_int_secret *secret;
>>> +
>>> +       if (arch_get_random_int(&arch_result))
>>> +               return arch_result;
>>> +
>>> +       secret = get_random_int_secret();
>>> +       result = siphash_3u64(secret->chaining, jiffies,
>>> +                             (u64)random_get_entropy() + current->pid,
>>> +                             secret->secret);
>>> +       secret->chaining += result;
>>> +       put_cpu_var(secret);
>>> +       return result;
>>>  }
>>>  EXPORT_SYMBOL(get_random_int);
>>
>> Hmm.  I haven't tried to prove anything for real.  But here goes (in
>> the random oracle model):
>>
>> Suppose I'm an attacker and I don't know the secret or the chaining
>> value.  Then, regardless of what the entropy is, I can't predict the
>> numbers.
>>
>> Now suppose I do know the secret and the chaining value due to some
>> leak.  If I want to deduce prior outputs, I think I'm stuck: I'd need
>> to find a value "result" such that prev_chaining + result = chaining
>> and result = H(prev_chaining, ..., secret);.  I don't think this can
>> be done efficiently in the random oracle model regardless of what the
>> "..." is.
>>
>> But, if I know the secret and chaining value, I can predict the next
>> output assuming I can guess the entropy.  What's worse is that, even
>> if I can't guess the entropy, if I *observe* the next output then I
>> can calculate the next chaining value.
>>
>> So this is probably good enough, and making it better is hard.  Changing it to:
>>
>> u64 entropy = (u64)random_get_entropy() + current->pid;
>> result = siphash(..., entropy, ...);
>> secret->chaining += result + entropy;
>>
>> would reduce this problem by forcing an attacker to brute-force the
>> entropy on each iteration, which is probably an improvement.
>>
>> To fully fix it, something like "catastrophic reseeding" would be
>> needed, but that's hard to get right.
>
> I wonder if Ted's proposal was analyzed further in terms of performance
> if get_random_int should provide cprng alike properties?
>
> For reference: https://lkml.org/lkml/2016/12/14/351
>
> The proposal made sense to me and would completely solve the above
> mentioned problem on the cost of repeatedly reseeding from the crng.
>

Unless I've misunderstood it, Ted's proposal causes get_random_int()
to return bytes straight from urandom (effectively), which should make
it very strong.  And if urandom is competitively fast now, I don't see
the problem.  ChaCha20 is designed for speed, after all.