  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 Feis- tel network of size 2^(2n) . This attack recovers the
complete permutation from at most 2^(n+2) plaintext-ciphertext pairs. For this
reason, BEPAC is designed in such a way that authenticity and confidentiality
are in- dependent properties, i.e. loss of confidentiality does not imply loss
of authenticity.
