From: Brian Candler <b.candler@...>
Date: 2008-10-23T03:46:29+09:00
Subject: Re: Big endian convention in Ruby

Ashrith Barthur wrote:
> 1. There is a bit wise operation that is required between 2 Hex values 
> and this will only occur if both of them are Integer-Hex or Integer 
> anything.

"Integer-Hex" doesn't really mean anything.

The specification says it works on 32-bit unsigned integer values, and 
that each 64-byte block of source data is treated as 16 x 32-bit words. 
You can get this via

   # data is a 64-byte string

   w = data.unpack("N16")
   # now w is an array of 16 Integers

> For example lets say the string is 'abc'
> a gives 61 when unpacked in hex. so now. if the array holding this is
> messageHex and the position of a is [0] then we will have to explicitly 
> say
> messageHex[0].hex.to_i this will ensure that it is a integer in hex.

String#hex will give you an Integer directly; the to_i is superfluous.

But in any case, the conversion into hex-ascii in the first place is 
superfluous. Unpack directly to Integers, as shown above.

> Next thing.. appending strings '0x80' or '0x00' is felt not to be 
> appropriate by me because.. if you were to use 0x80 or 0x00 then ruby 
> thinks that its an integer already and you dont need to do any explicit 
> type casting.

Sorry, but I am unable to make any sense of that sentence at all. The 
input data to SHA1 is an arbitary-sized string of bytes (*); the padding 
algorithm requires you to add more bytes (*) to the end, to achieve 
alignment into 64-byte blocks. So adding padding bytes is exactly what 
is required.

(*) actually an arbitary-sized string of *bits*, but most 
implementations assume that it's a whole number of bytes, i.e. n*8 bits.

> Also ruby does not explicitly give you a value in hex if you do any 
> mathematical or bitwise operation in hex, it always defaults to dec.

I think you may have lost the distinction between a number, and its 
external representation.

Doing a bitwise operation "in hex" or "in decimal" doesn't make any 
sense. The number is stored internally in binary - this is a digital 
computer, after all - and the bitwise operations are done on those bits. 
It is only converted into a hex or decimal representation at the point 
where you input or output the number.

  a = 20
  a.to_s        # converts to string "20"
  a.to_s(16)    # converts to string "14"

Anyway, maybe you would like to submit this as a ruby quiz, as you'd 
probably get some good implementations to look at.

Brian.
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