def separate_iv_ciphertext(cipher, iv_ciphertext)
      idx = cipher.iv_len
      [iv_ciphertext[0..(idx - 1)], iv_ciphertext[idx..-1]]
    end
def separate_iv_ciphertext(cipher, iv_ciphertext)
      idx = cipher.iv_len
      [iv_ciphertext[0..(idx - 1)], iv_ciphertext[idx..-1]]
    end
Pull the initialization vector from the front of the encoded message.

 @api private