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.
> 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.
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