diamond.rb

#!/usr/bin/env ruby
# Copyright 2013 Jesse Zilstorff

class Diamond
    def swap(stream, key, cipher, offset)
        last = key.first
        stream.bytes.with_index.map do |byte, index|
            key.unshift(last ^= (index+offset) ^ key.pop)
            byte ^ last ^ cipher.getbyte(index)
        end.pack("C*")
    end
    def encrypt(instream, outstream, key)
        blksize = instream.stat.blksize
        offset = key.pop
        buffer = 0x00.chr*blksize
        cipher = swap(buffer, key, buffer, offset)
        while instream.read(blksize, buffer)
            cipher = swap(buffer, key, cipher, offset)
            outstream.write(cipher)
        end
    end
    def decrypt(instream, outstream, key)
        blksize = instream.stat.blksize
        offset = key.pop
        buffer = 0x00.chr*blksize
        cipher = swap(buffer, key, buffer, offset)
        while instream.read(blksize, buffer)
            outstream.write(swap(buffer, key, cipher, offset))
            cipher.replace(buffer)
        end
    end
end

Diamond.new.send(
    ARGV.shift,
    File.new(ARGV.shift),
    File.new(ARGV.shift, IO::CREAT|IO::WRONLY),
    "PUT YOUR RANDOM-LOOKING KEY HERE".bytes.to_a
)