It's just a hashing algorithm, mixing up the bits of data in a
deterministic but irreversible way.
If you notice that the "alphabet-distance" between the variable names on
each line is always the same,
my implementation might make more sense. Since the offsets are always the
same, I'm just rotating the
array and re-applying the same logic.
Here's the c code that will tell us what the result should be for mix i.8:
#include <stdio.h>
int main() {
int a,b,c,d,e,f,g,h;
a=0;b=1;c=2;d=3;e=4;f=5;g=6;h=7;
a^=b<<11; d+=a; b+=c;
b^=c>>2; e+=b; c+=d;
c^=d<<8; f+=c; d+=e;
d^=e>>16; g+=d; e+=f;
e^=f<<10; h+=e; f+=g;
f^=g>>4; a+=f; g+=h;
g^=h<<8; b+=g; h+=a;
h^=a>>9; c+=h; a+=b;
// a b c d e f g h
printf("%d %d %d %d %d %d %d %d\n", a,b,c,d,e,f,g,h);
}
Answer:
68112070 67580715 540296643 2058 539239185 529307 67580712 539769534
Here's the corrected code:
top=: <:2^32
SH =: top AND (34 b.)~ NB. 32 bit shift
MP =: top AND + NB. 32 bit addition ("modplus")
mix =: verb define
for_i. 11 _2 8 _16 10 _4 8 _9 do.
'a b c d e f g h' =. y
x =. a XOR b SH i
y =. 1 |. x, (b MP c), c, (d MP x), e, f, g, h
end.
)
assert (mix i.8) -: 68112070 67580715 540296643 2058 539239185 529307
67580712 539769534
On Fri, Sep 4, 2015 at 10:34 PM, Dan Bron <[email protected]> wrote:
> Michal Wallace wrote:
> > I don't know whether or not this produces the correct results because I
> > don't have any test data, but...
>
>
> Yeah, that is troublesome. Unfortunately, it’s the same catch-22 I’m in.
>
> I’m transliterating the C code here:
>
> http://rosettacode.org/wiki/The_ISAAC_Cipher#C <
> http://rosettacode.org/wiki/The_ISAAC_Cipher#C>
>
> as directly as possible into J, so that I can get a working program which
> produces the expected outputs for the given inputs.
>
> Once I have a working program that I can test, interrogate, and reason
> about, I’ll be in a much better position to refactor the code into
> idiomatic, and, hopefully, elegant J.
>
> But the very reason I have to do it this cart-before-horse way is because
> I don’t (yet) understand the algorithm on a conceptual level. So I’m
> starting from the code.
>
> I guess what I was asking for in my previous email was for someone who
> does or can easily grok the concepts underlying the code to express them in
> J (which is a language I speak, so such code would teach me those concepts).
>
> Barring that, someone who is confident enough in his C to trust in his
> translation of the macro would also suffice.
>
> (One big obstacle here, and I think more broadly to the lack of adoption
> of ISAAC the author of that article laments is the majority of
> easily-accessible artifacts dealing with it are code, rather than prose.)
>
> -Dan
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
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