Re: Detecting/parsing a hex string

[David Fifield <david () bamsoftware com>]

On Wed, Oct 08, 2008 at 11:41:36AM -0600, Patrick Donnelly wrote:
> On Wed, Oct 8, 2008 at 5:25 AM, Ron <ron () skullsecurity net> wrote:
> > Hi all,
> >
> > I need to detect whether or not a string is an ascii representation of a hex
> > string (eg, "12ab3d1a3badabeea1...."), and, if it is, to convert it to a
> > binary string. Is there an easy way to do that (maybe with
> > bin.pack/bin.unpack)?
> >
> > Thanks,
> > Ron
>
> a = "414243"
> if string.find(a, "^%x+$") then
>   a = string.gsub(a, "%x%x", function(a) return
> string.char(tonumber(a, 16)) end) --> a = "ABC"
> end

I suspect this won't work when the string is long enough to overflow the
capacity of a double, which a 128-bit key will be.

Better than checking first whether the operation will produce an error
is just to do the operation and catch any error produced. However (from
reading the source code), bin.unpack appears to ignore non-hex
characters in a hex conversion:

      for (ii = 0; ii < l; ii++) {
        if (isxdigit(a[ii])) {
          ...
        } else if (isspace(a[ii])) {
          /* ignore */
        } else {
          /* err ... ignore too*/
        }
      }

I think that's a bug that should be fixed.

David Fifield

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