From: Brian Candler <b.candler@...>
Date: 2008-10-18T00:58:52+09:00
Subject: Re: Big endian convention in Ruby

Just to make this clearer: the padding operation just pads the message 
up to a multiple of 64 bytes (512 bits), where the last block consists 
of 56 bytes (448 bits) followed by 8 bytes of message length.

So assuming your message consists only of whole bytes, as your example 
implied, then I believe the padding operation is simply this:

message = "A message"
bits = message.size * 8
message << "\x80"
message << "\x00" while (message.size & 63) != 56
message << [("%016X" % bits)].pack("H*")

Now your message is exactly n * 64 bytes long, and you can proceed.
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