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Date: Sun, 9 Dec 2012 11:45:41 +0530
From: Sayantan Datta <std2048@...il.com>
To: john-dev@...ts.openwall.com
Subject: Re: bitslice DES on GPU
Hi Alexander,
On Sun, Dec 9, 2012 at 7:19 AM, Solar Designer <solar@...nwall.com> wrote:
> On the other hand, 0x0000000f is small
> > enough to fit in an immediate offset field or even in a register number
> > field (GCN allows for encoding of values in the -16 to +64 range in that
> > way, via impossible register numbers).
>
Since the real world offset values would never exceed 8 bits , I think we
should focus on that. That way we could also eliminate *endianness issue. *
*
*
Your best bet to find them is really to compare several builds, for
different hard-coded offsets. Then for the locations that differ across
the builds, you can calculate the deltas - and match those against
deltas between your hard-coded offsets, with varying shift amounts
added (since the immediate offset field does not have to start right at
instruction boundary, nor at a byte boundary).* *
Okay. So searching the exact values is not the way to go. So basically
we've to diff two binaries and find the locations where they differ. And
since the operands are not at byte boundaries , the operand might be spread
over two consecutive bytes(considering nonzero 8 bit operands). Also taking
in account a shift of 0bit and 4bit should suffice , because hopefully the
size of operands and opcodes are always 4bit multiples. Are there any
opcode or operand field which is not a multiple of 4bits ? Then find the
deltas in their values and verify it is the same the source
kernels. Negative numbers may be a problem though , because it would
require the knowledge of how they are being encoded. For example the 2's
complement of a small 12bit number say 000000000101 is a very large number
111111111011 .
Regards,
Sayantan
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