IORCC Deobfuscation: Primes less than 1000 by Jim Lawless

Let's start by examining the raw code in one big piece:

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   $e=""

def a()
   $a=$a+1
end

def b()
   $a=$a+5
end

def c()
   $e=$e+$a.chr
end

def d()
   $a=10
end

def e()
   $a=$a+16
end

d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;e;b;b;
a;c;d;c;d;e;e;e;e;e;e;b;b;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;c;d;e;e;e;e;
e;b;b;a;a;a;a;a;c;d;e;e;e;e;e;e;a;a;c;d;e;e;e;e;e;b;b;a;c;d;e;b;a;c;
d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;e;e;a;a;c;d;e;e;b;a;a;
c;d;e;e;b;a;c;d;e;e;b;a;c;d;e;e;b;a;c;d;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;b;
b;a;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;e;b;a;a;a;c;d;c;d;e;b;a;a;a;a;a;c;d;e;
e;e;e;e;b;b;a;a;c;d;e;e;e;e;e;e;a;a;a;a;a;c;d;e;e;e;e;e;e;b;b;a;c;
d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;
e;b;a;c;d;c;d;e;e;e;e;e;e;b;b;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;c;d;e;e;
e;e;e;b;b;a;a;a;a;a;c;d;e;e;e;e;e;e;a;a;c;d;e;e;e;e;e;b;b;a;c;d;e;b;
a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;a;c;d;e;e;e;a;a;c;d;e;b;
a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;b;a;c;d;c;d;e;e;e;e;e;b;b;a;
a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;c;d;e;b;b;a;a;a;a;c;d;e;b;a;c;d;e;b;b;a;
a;a;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;e;a;a;a;a;c;
d;e;e;e;e;e;e;a;a;a;c;d;e;e;e;e;e;e;a;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;
a;a;c;d;e;e;e;e;e;e;b;b;a;c;d;e;e;e;e;e;e;a;a;c;d;e;e;e;e;e;e;a;a;a;
a;a;c;d;e;b;b;a;a;a;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;a;
c;d;e;b;b;a;a;a;a;a;c;d;e;b;b;a;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;e;e;a;a;
a;c;d;e;e;b;a;c;d;e;b;b;a;a;a;a;a;c;d;e;b;a;c;d;c;d;e;b;a;a;a;a;a;c;d;e;e;
e;e;e;b;b;a;a;c;d;e;e;e;e;e;e;a;a;a;a;a;c;d;e;e;e;e;e;e;b;b;a;c;d;e;
e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;e;
b;a;a;c;d;c;d;e;e;e;e;e;b;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;c;d;e;e;e;e;e;
b;b;a;c;d;e;e;e;e;e;b;a;a;c;d;e;e;e;e;e;e;a;c;d;c;d;e;e;e;e;e;b;b;a;c;
d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;c;d;e;b;a;a;a;a;a;
c;d;e;e;e;e;e;b;b;a;a;a;a;a;c;d;e;e;e;a;a;a;c;d;e;b;a;a;a;a;a;c;d;e;e;
e;e;e;b;b;a;a;a;a;a;c;d;e;e;a;c;d;e;e;b;a;a;c;d;c;d;e;e;e;e;e;b;b;a;a;
a;a;a;c;d;e;e;e;e;e;b;b;a;a;c;d;e;b;a;c;d;e;b;b;a;a;a;a;c;d;e;b;b;a;a;
a;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;a;c;d;e;b;b;b;a;c;d;e;
b;a;a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;a;a;a;c;d;e;b;b;a;a;a;a;a;c;d;e;e;
e;a;a;a;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;b;b;a;a;
a;a;a;c;d;c;d;e;e;e;e;e;b;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;c;d;e;e;e;e;e;
b;b;a;c;d;e;e;e;e;e;b;a;a;c;d;e;e;e;e;e;e;a;c;d;c;d;e;e;e;e;e;b;b;a;c;
d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;c;d;e;e;e;e;e;b;b;
a;c;d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;c;d;e;e;e;e;e;
b;b;a;a;a;a;a;c;d;e;e;e;e;e;b;b;a;a;c;d;e;b;b;a;a;a;a;c;d;e;b;a;a;a;
a;a;c;d;e;e;e;e;e;b;b;a;a;c;d;e;e;e;e;e;e;a;a;a;a;a;c;d;e;e;e;e;e;e;
b;b;a;c;d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;e;e;e;a;
a;a;c;d;e;e;e;a;a;a;c;d;e;e;b;a;c;d;e;b;b;a;a;a;a;a;c;d;e;b;a;c;d;c;d;e;e;
e;e;e;e;b;a;c;d;e;e;e;e;e;e;b;b;a;c;d;e;e;e;e;e;e;b;a;a;a;a;a;c;d;e;
e;e;e;e;e;b;a;a;a;a;c;d;e;b;a;c;d;e;b;a;a;a;c;d;e;b;a;a;a;a;c;d;e;e;e;
e;e;e;e;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;e;e;e;e;
e;e;a;a;a;c;d;e;b;a;c;d;e;e;e;e;e;b;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;
a;a;a;c;d;e;b;a;c;d;e;e;e;e;e;e;b;a;c;d;e;e;e;e;e;e;b;a;a;a;c;d;e;e;e;
e;e;b;b;a;a;a;a;a;c;d;e;e;e;e;e;e;a;a;a;c;d;e;e;e;e;e;b;b;a;c;d;e;b;
a;a;a;c;d;c;d;e;e;e;e;e;b;b;a;c;d;e;e;e;e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;
a;a;a;a;a;c;d;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;d;e;b;a;c;
d;e;e;e;a;a;a;c;d;e;b;a;c;d;e;b;a;a;a;a;a;c;d;e;e;e;e;e;e;b;a;a;a;a;c;
d;e;b;a;c;d;e;e;a;c;d;e;b;a;c;d;e;e;b;a;a;c;d;c;d;e;e;e;e;e;b;b;a;c;d;e;e;e;
e;e;e;a;a;a;a;c;d;e;e;e;e;e;b;a;a;a;a;a;c;d;c;d;e;b;a;c;d;e;b;a;c;d;e;b;
a;c;d;e;b;a;c;d;e;b;a;c;d;e;b;a;c;d;e;b;a;c;eval $e

Yup, that's 72 lines of code of which 49 are one-letter method calls. Quite a lot, isn't it?

Anyway, all those method calls are only there for building code to the $e variable which is then passed to Ruby via eval() at the very end.

I won't actually go through explaining the actual logic used for constructing the code — it's fairly simple and readable via the method definitions at the beginning anyway. We'll just use the short-cut and let Ruby give us the executed code directly:

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$s=5
while $s<1000
$i=2
$found=0
while $i<$s 
if( ($s.modulo($i))==0) 
$found=1
break
end
$i=$i+1
if (($i*$i)>$s)
break
end
end
if($found==0) 
puts "#{$s} is prime"
end
$s = $s + 1
end

That doesn't look too complex, but it's probably a good idea to reformat it anyway. I'll change globals into local variables, drop superfluous parentheses and use short-hand notations like += and % — while you might argue that that changes the code quite a lot I think it is way more rubyish that way:

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s = 5

while s < 1000
  i = 2
  found = 0

  while i < s 
    if s % i == 0 
      found = 1
      break
    end

    i += 1

    if i * i > s
      break
    end
  end

  if found == 0 
    puts "#{s} is prime"
  end

  s += 1
end

This is basically an outer loop for iterating over numbers between 5 to 1000 and an inner one that tests whether that number can be divided without remainder by any of the numbers from 2 to Math.sqrt(number) — this is a fairly primitive algorithm for finding prime numbers. Wikipedia has more information about algorithms for finding prime numbers.

Personally, I don't like this entry all that much as it does not try too hard to be clever or obscure. It also does not generate all primes and the code under the initial hiding layer is fairly simple. It also misses the prime numbers 2 and 3 and could be written both in simpler and more obscure ways.

— Florian Groß