Paper 2013/269

Chosen Ciphertext Secure (CCS): Symmetric Key CCA Encryption with Minimal Ciphertext Expansion

Jonathan Trostle

Abstract

In some wireless environments, minimizing the size of messages is paramount due to the resulting significant energy savings. We present a new symmetric encryption scheme: CCS or Chosen Ciphertext Secure scheme. CCS has the property that modications to the ciphertext randomizes the resulting plaintext. Using this property, we prove the scheme is CCA2 secure. Thus we obtain CCA2 encryption schemes with minimal ciphertext expansion which are applicable to resource constrained wireless environments. For protocols that send short messages, our scheme is similar to Counter with CBC-MAC (CCM) for computation but has much shorter messages (since we can use a smaller MAC tag) for a similar level of security. A key idea is that various protocol fields in the underlying plaintext act as an authentication tag given changes to the message ciphertext. To the best of our knowledge, CCS is the first scheme that achieves CCA2 security with only 2-3 bytes of ciphertext expansion.