ASPECT Scripting Compiler Discovery
A while back I wrote a conversion of RFC 1321 (RSA Data Security’s Message-Digest Algorithm for C) to ASPECT script. During this conversion I discovered that the C code that was used to generate a rotate no carry bit-shift did not work as intended. (In the RFC they refer to it as circularly shifting.)
Back then Wikipedia didn’t have an article on bitwise operations and I did not have the access to mathematic books. I was doing some coding this last weekend and found an irregularity which I have still not successfully reproduced or explained, and while doing research on the matter, I came across the bitwise operation article at Wikipedia and discovered why ASPECT does not work as intended:
The reason for this is that C uses a logical shift whereas ASPECT uses an arithmetic shift. This plays havoc with RFC 1321 and as such I had to do some mathematical tinkering to get an arithmetic shift to behave like a logical shift, which allows for the creation of a rotate no carry shift.
This is the code that comes from RFC 1321
#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32-(n))))
This is the resultant code with the arithmetic to logical shift conversion code:
func ROTATE_LEFT : integer
param integer x, n
integer z
z = (x >> (32-n)) & ~(0×80000000 >> (31-n))
return ( (x << n) | z )
endfuncIn order to reduce compiler and stack overhead, this could probably be shorted to a #define that looks like this (I haven’t tested this):
#define ROTATE_LEFT(x,n) ( (x << n) | (x >> (32-n)) & ~(0×80000000 >> (31-n)) )
In summary you replace
((x) >> 32-(n))
with
(x >> (32-n)) & ~(0×80000000 >> (31-n))
This causes the arithmetic shift to operate like a logical shift if the sign bit is set to 1 (which is the only time an arithmetic and logical shift operate differently). I posted a bit of source to my script page which demonstrates this “bug” and the fix.
NOTE: A circular shift (or rotate no carry shift) is not inherent in either C source or ASPECT, hence the creation of the ROTATE_LEFT function in ASPECT source (or the compiler #define in C source)
Merry Christmas
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