emerald.rb

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

class Emerald
    def scramble(stream, key, cipher, start)
        last = key.first
        stream.bytes.map.with_index do |byte, index|
            key.unshift(last ^= (index+start) ^ key.pop)
            byte ^ last ^ cipher.getbyte(index)
        end.pack("C*")
    end
    def encrypt(instream, outstream, key)
        blksize = instream.stat.blksize
        buffer = 0x00.chr*blksize
        cipher = scramble(buffer, key, buffer, key.pop)
        subkey = key.pop(key.size/2)
        start = subkey.last
        index = -1
        while instream.read(blksize, buffer)
            subkey.push(start ^= (index+=1) ^ subkey.shift)
            cipher = scramble(buffer, key, cipher, start)
            outstream.write(cipher)
        end
    end
    def decrypt(instream, outstream, key)
        blksize = instream.stat.blksize
        buffer = 0x00.chr*blksize
        cipher = scramble(buffer, key, buffer, key.pop)
        subkey = key.pop(key.size/2)
        start = subkey.last
        index = -1
        while instream.read(blksize, buffer)
            subkey.push(start ^= (index+=1) ^ subkey.shift)
            outstream.write(scramble(buffer, key, cipher, start))
            cipher.replace(buffer)
        end
    end
end

Emerald.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
)