[Cryptography] ChaCha20 DRNG

[Stephan Mueller]

Am Donnerstag, 4. August 2016, 22:38:51 CEST schrieb Patrick:

Hi Patrick,

> Stephan Mueller wrote on 08/04/2016 08:22 AM:
> > As part of the development of my "Linux Random Number Generator -- a new
> > approach to the Linux /dev/random" project, I implemented a DRNG based on
> > ChaCha20.
> 
> I do have a question about the the syscall:
> 
>     do {
>         ret = syscall(__NR_getrandom, buf, buflen, 0);
>         if (0 < ret)
>             len += ret;
>     } while ((0 < ret || EINTR == errno || ERESTART == errno)
>          && buflen > len);
> 
> I haven't (yet) found any documentation on that, but it seems clear
> you're telling it to put at most buflen bytes into the buf and return
> the number of bytes it gave you in ret.
> 
> You're keeping a total of the number of bytes you've gotten in len.
> 
> However it seems to me that each syscall is going to start all over
> again at the beginning of the buf -- and yet you're tallying up the
> total len as if you're getting more bytes each time.
> 
> Should len be used as a running offset into buf, with (buflen-len) as
> the requested number of bytes?

Absolutely, this should be similar to the read(/dev/random) case:

syscall(__NR_getrandom, (buf + len), (buflen - len), 0);

Thanks for the bug report.
> 
> 
> Thanks,
> Patrick
> 
> P.S. I'm sure by far most of the time (ret == buflen) anyway, but still.

See getrandom(2) - there is a man page but no libc stub. It can break with 
EINTR and ERESTART.

Ciao
Stephan