From: Brian Candler <b.candler@...>
Date: 2008-10-16T22:25:39+09:00
Subject: Re: Big endian convention in Ruby

Zangief Ief wrote:
> Thanks you for you answer.
> Actually I would like to rewrite the SHA1 algorithm 
> (http://en.wikipedia.org/wiki/SHA_hash_functions#SHA-1_pseudocode) in a 
> pure ruby implementation. And in this way, I would need the ability to 
> accomplish the "Pre-processing" step by converting the input as a 64-bit 
> big-endian integer. I believe that's could be more simple to do in Ruby 
> then in an other language such as in C. But I am not really sure about 
> the way to do so.

ri String#unpack

Unfortunately, the q/Q conversion character seems to use native ordering 
and I don't think there's a network-order equivalent:

irb(main):002:0> "\000\000\000\000\000\000\000\001".unpack("Q")
=> [72057594037927936]

If all you're concerned about is this step:

"append length of message (before pre-processing), in bits, as 64-bit 
big-endian integer"

then you could do it by converting to hex first:

  buff << [("%016X" % len)].pack("H*")

BTW, I presume you're doing this as an academic exercise. After all, 
there's already:

require 'digest/sha1'
puts Digest::SHA1.hexdigest("hello world")

HTH,

Brian.
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