Best Effort and Practice Activation Codes

Activation Codes are used in many different digital services and known by many different names including voucher, e-coupon and discount code. In this paper we focus on a specific class of ACs that are short, human-readable, fixed-length and represent value. Even though this class of codes is extensively used there are no general guidelines for the design of Activation Code schemes. We discuss different methods that are used in practice and propose BEPAC, a new Activation Code scheme that provides both authenticity and confidentiality. The small message space of activation codes introduces some problems that are illustrated by an adaptive chosen-plaintext attack (CPA-2) on a general 3-round Feistel network of size 2. This attack recovers the complete permutation from at most 2 plaintext-ciphertext pairs. For this reason, BEPAC is designed in such a way that authenticity and confidentiality are independent properties, i.e. loss of confidentiality does not imply loss of authenticity.