At 10:32 PM 10/3/00 +0100, Ben wrote:
>Ulf Moeller wrote:
> >
> > On Tue, Oct 03, 2000, Tom Biggs wrote:
> >
> > > I have so many questions, but this one is most pressing -
> > > Is there a reasonable upper limit on the size in bits of a BN?
> > > For various HW reasons we were hoping we could cap BNs
> > > at 4096 bits for ModExp functions and the like. Is this possible?
> > > Or is there some operation that might require more? The BN
> > > docs imply that the software-implemented BNs can be any
> > > arbitrary size.
> >
> > As a matter of fact, BIGNUMs can't be larger than INT_MAX bytes.
> > In practice you'll rarely see numbers larger than 2048 bits, and
> > it would make sense to fall back to the software implementation at
> > say 2049 or 4097 bits.
Having it fall back to software is a good idea, thanks.
The sudden performance deterioration at 4097 bits would
be unfortunate, but I could raise an alert to notify the caller.
> > > format I need. It seems to me that using the BN_ULONG
> > > type gets me mired in all sorts of endian and other
> > > machine-dependent problems. Is that true or can the BN
> > > be treated as a byte array of bits? (yes, I know, read the
> > > source, study the generated code... I will... but sometimes
> > > a quick comment is worth hours of code study.)
> >
> > The BIGNUM type is supposed to allow efficient operations on the
> > host machine, it is not meant to be portable. See man BN_bn2bin
> > for the conversion functions, and man bn_internal for some information
> > on the BIGNUM format.
OK, I saw the BN_bn2bin functions in the ENGINE code, but I'm
trying to eliminate as many conversion operations as I can.
Ideally I'd like to DMA the number right out of the BN's array.
>I'll wager they're talking about programmable hardware, in which case,
>ordering isn't a major problem, so long as it is known what it is when
>the toughware (I'm searching for a term that's a bit firmer than firm,
>but not as hard as hard :-) is loaded. BIGNUMs behave in a way that is
>mostly pretty compatible with hardware apart from this.
Yes, we can deal with simple endian conversions of words up to
64 bits long. I'm going to go study the BN structure in depth now,
to see if it can be used directly. I thought I saw a separate byte
at the front used to indicate sign? Or was I imagining that?
Ben, it sounds like you're saying that I can probably use the
BIGNUMs directly as long as endianness is accounted for. True?
((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((
Tom Biggs
'89 FJ1200 DoD #1146
"The whole aim of practical politics is to keep the populace alarmed -
and hence clamorous to be led to safety - by menacing it with an endless
series of hobgoblins, all of them imaginary." -- H.L. Mencken
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