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POString to BigNum and back again (in Ruby) to allow circular shift
 

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    <p>As a personal challenge I'm trying to implement the SIMON block cipher in Ruby. I'm running into some issues finding the best way to work with the data. The full code related to this question is located at: <a href="https://github.com/Rami114/Personal/blob/master/Simon/Simon.rb" rel="nofollow">https://github.com/Rami114/Personal/blob/master/Simon/Simon.rb</a></p> <p>SIMON requires both XOR, shift and circular shift operations, the last of which is forcing me to work with BigNums so I can perform the left circular shift with math rather than a more complex/slower double loop on byte arrays.</p> <p>Is there a better way to convert a string to a BigNum and back again. </p> <p>String -> BigNum (where N is 64 and pt is a string of plaintext)</p> <pre><code> pt = pt.chars.each_slice(N/8).map {|x| x.join.unpack('b*')[0].to_i(2)}.to_a </code></pre> <p>So I break the string into individual characters, slice into N-sized arrays (the word size in SIMON) and unpack each set into a BigNum. That appears to work fine and I can convert it back.</p> <p>Now my SIMON code is currently broken, but that's more the math I think/hope and not the code. The conversion back is (where ct is an array of bignums representing the ciphertext):</p> <pre><code>ct.map { |x| [x.to_s(2).rjust(128,'0')].pack('b*') }.join </code></pre> <p>I seem to have to right-justify pad the string as bignums are of undefined width so I have no leading 0s. Unfortunately the pack requires the defined with to have sensible output.</p> <p>Is this a valid method of conversion? Is there a better way? I'm not sure on either count and hoping someone here can help out.</p> <p>E: For @torimus, the circular shift implementation I'm using (From link above)</p> <pre><code>def self.lcs (bytes, block_size, shift) ((bytes &lt;&lt; shift) | (bytes &gt;&gt; (block_size - shift))) &amp; ((1&lt;&lt; block_size)-1) end </code></pre>
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    1. COShould that be `.rjust(64,'0')` in the second list, or is the padding with extra nulls (that I get running a 64 char string through both your lines of code) part of the cipher algorithm?
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    2. COConcerning the bit rotation operation following [SO answer](http://stackoverflow.com/a/3477091/2352780) may come handy.
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    3. CO@NeilSlater the rjust is on 128 bits because the SIMON rounds work on 2 N-sized words together to form a 128 bit block.
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