Re: About git and the use of SHA-1

["Geoffrey Irving" <irving@naml.us>]

On Tue, Apr 29, 2008 at 7:58 PM, Martin Langhoff
<martin.langhoff@gmail.com> wrote:
> On Wed, Apr 30, 2008 at 8:31 AM, Geoffrey Irving <irving@naml.us> wrote:
>  >  I sincerely hope that pdf/postscript don't allow the internal
>  >  rendering code to branch based on the current date.  That would be an
>  >  absurd security hole, and would indeed make you entirely correct.  If
>
>  PS is Turing complete, and does know about dates. So yes, you can make
>  such conditionals.

I knew postscript was Turing complete, but had (naively) assumed it
executed sandboxed and deterministically and would therefore display
uniformly barring interpreter bugs.  Looking over the spec, I can't
find where it's possible to read the current date, but the
usertime/realtime variables are sufficient as long as the attacker
knows how fast the relevant machines are.

>  That original md5 paper with the 2 PDF files is mainly a good example
>  that you should trust binary blobs, that's all. The md5 trick is a
>  nice demo, but misses the point entirely.
>
>  I can't find it now, but someone had written a PDF file that printed
>  Pi computing in inside the PS VM. The tiny file would keep the printer
>  churning out paper until it ran out of memory. :-)

According to wikipedia, PDF doesn't have conditionals or loops of any
kind, so you probably mean a postscript file.

Geoffrey