On 22.12.2016 00:42, Andy Lutomirski wrote:
> On Wed, Dec 21, 2016 at 3:02 PM, Jason A. Donenfeld <Jason@xxxxxxxxx> 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
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