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

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

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.

Bye,
Hannes