biflipper.rb

# Cipher text is a bytestring
# bi is the block index, or which block to edit.  Flipping a bit in this block will result in a one bit edit in the bi+1 plaintext block
# i is the targeted bit's index
# z is the array of decoded bytes.  
# The idea here is to target bit i, but also flip the bits of previously discovered bytes. 
def flip_bit(cipher_text,bi,i,z)
  bsize=16
  i.times do |j| 
    tb=(z[j-1]==nil) ? 0 : z[j-1]
    cb=cipher_text.bytes()[bi*bsize-j-1]
    cipher_text[bi*bsize-j-1]=(cb^tb^i).chr
  end
  return cipher_text