def hash_block
      hash_string = [index,timestamp,data,prev_hash].join
      sha = Digest::SHA256.new
      sha.update(hash_string)
      sha.hexdigest
    end
def hash_block
      hash_string = [index,timestamp,data,prev_hash].join
      sha = Digest::SHA256.new
      sha.update(hash_string)
      sha.hexdigest
    end
Create the blocks hash by encrypting all the blocks data using SHA256