Re: [dev] [sbase] [patch v2] Add md5sum

From: sin <sin_AT_2f30.org>
Date: Wed, 19 Jun 2013 14:55:12 +0300

On Wed, Jun 19, 2013 at 12:51:33PM +0200, Szabolcs Nagy wrote:
> * stateless <stateless_AT_archlinux.us> [2013-06-19 11:38:00 +0100]:
> > This is a version of md5sum(1) using the md5 routines from 9base - slightly
> > adapted to compile.
> >
>
> be careful with integer arithmetics in crypto code
>
> your code invokes undefined behaviour because of
> signed int overflow:
>
> unsigned f(unsigned char c) { return c<<24; }
>
> c is promoted to int not unsigned in the left shift
>
> it will work in practice (usually) but that's only
> by accident
>
> you should cast c to the right type

You are right. I will send some incremental patches
to fix this (and for 9base too).
I presume this is in the utils/md5*.c code which I have not
looked at in anger yet.

Integer promotion rules are nasty! I think something like
the following would still be ok?

unsigned f(unsigned int c) { return c<<24U; }

Thanks,
stateless
Received on Wed Jun 19 2013 - 13:55:12 CEST