phc-discussions - Re: [PHC] Specification of a modular crypt format (2)

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Message-ID: <CAGiyFddmjRbUEhdOhrpHGn1JeM5jBOTYh3JPN+AMP0NHfmgRug@mail.gmail.com>
Date: Tue, 29 Sep 2015 15:01:02 +0000
From: Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format (2)

@Thomas: let's copy the updated source code on a new shared doc
https://docs.google.com/document/d/13bv-3HarFpQOD1PayJ2hMvRtVIs5QPu1FZ5XonEv57c/edit?usp=sharing
 ?

On Tue, Sep 29, 2015 at 2:56 PM Thomas Pornin <pornin@...et.org> wrote:

> On Tue, Sep 29, 2015 at 03:05:59AM +0300, Alexander Cherepanov wrote:
> > AIUI the decoder tries to be strict. Then perhaps it's worth to
> > check that 'str' doesn't start with ' ' or '-' here and try to catch
> > overflows.
>
> Strictly speaking, the decoder tries to be strict when strictness is
> cheap. If we want a strict decoder of numerical values, strtoul()
> won't be enough; we would need something like this:
>
> /* ------------------------------------------------------------------- */
> #include <limits.h>
>
> /*
>  * Decode decimal integer from 'str'; the value is written in '*v'.
>  * Returned value is a pointer to the next non-decimal character in the
>  * string. If there is no digit at all, or the value encoding is not
>  * minimal (extra leading zeros), or the value does not fit in an
>  * 'unsigned long', then NULL is returned.
>  */
> static const char *
> decode_decimal(const char *str, unsigned long *v)
> {
>         const char *orig;
>         unsigned long acc;
>
>         orig = str;
>         acc = 0;
>         for (orig = str;; str ++) {
>                 int c;
>
>                 c = *str;
>                 if (c < '0' || c > '9') {
>                         break;
>                 }
>                 c -= '0';
>                 if (acc > (ULONG_MAX / 10)) {
>                         return NULL;
>                 }
>                 acc *= 10;
>                 if ((unsigned long)c > (ULONG_MAX - acc)) {
>                         return NULL;
>                 }
>                 acc += (unsigned long)c;
>         }
>         if (str == orig || (*orig == '0' && str != (orig + 1))) {
>                 return NULL;
>         }
>         *v = acc;
>         return str;
> }
> /* ------------------------------------------------------------------- */
>
> (This code is completely untested; I just wrote it in this email.)
>
> I am not sure this is worth the effort. It is not very complex to do
> (less than 40 lines of C code).
>
>
> > >     if (pp->m < 1 || (pp->m >> 30) > 3) {
> >
> > Why not just compare with a constant here?
>
> This is to avoid a spurious warning. If I write this:
>
>         if (pp->m < 1 || pp->m > 0xFFFFFFFF)
>
> then the compiler, on a 32-bit machine, will complain that the
> comparison result is "always true". I don't want spurious warnings: they
> train developers to ignore warnings.
>
>
> > >     if (pp->p < 1 || pp->p > 64) {
> >
> > 255 here?
>
> Oh yes. Indeed. I fixed the specification, but forgot to change the code
> accordingly. Thanks !
>
>
>         --Thomas
>

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