Hi.....
I'm currently working on my own implementation
of the IDEA block cypher and had a pretty hard
time of understanding (Ok,Ok I know there's a
bunch of publicly available implementations, but
I wanted to do it myself)
Anyway. After consulting a bunch of crypto books
I've stumbled about the idea_mul macro in
idea_lcl.h (openssl-0.9.6c) which is supposed
to do the modified multiplication mod 2^16+1
which looks like.
----------------------------------------------
#define idea_mul(r,a,b,ul) \
ul=(unsigned long)a*b; \
if (ul != 0) \
{ \
r=(ul&0xffff)-(ul>>16); \
r-=((r)>>16); \
} \
else \
r=(-(int)a-b+1);
---------------------------------------------
All other reference implementations of the
multiplication I could find look like:
---------------------------------------------
mul( a, b) {
int32_t p;
if (a)
{
if (b)
{
p = a * b;
b = p & 0xFFFF;
a = p >> 16;
return b - a + (b < a);
}
else
return (1 - a);
}
return (1 - b);
}
--------------------------------------------
And they definitely produce different results.
Is there any compensation within the crypto
code which I've missed (e.g. in the quite
complicated E_IDEA macro) or do we deal with
a modified IDEA implementation here ?
Since I'm afraid that Eric Young may not be
reachable via his e-mail at cryptsoft I ask
here.
Cheers
Sebastian Kloska
--
**********************************
Dr. Sebastian Kloska
Head of Bioinformatics
Scienion AG
Volmerstr. 7a
12489 Berlin
phone: +49-(30)-6392-1708
fax: +49-(30)-6392-1701
http://www.scienion.de
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