On the Security of Key-Based Interval Splitting Arithmetic Coding With Respect to Message Indistinguishability

Key-based interval splitting arithmetic coding (KSAC) has been proposed to improve the security of traditional arithmetic coding (AC). Chosen-plaintext attacks have been proposed for KSAC when the same key is used to encrypt different messages. In this paper, we consider a stronger version of KSAC, where different keys are used to encrypt different messages. We then use message indistinguishability to prove that this version of KSAC is insecure under ciphertext-only attacks, a weaker form of attack than chosen-plaintext attacks. Indistinguishability in the presence of an eavesdropper is a security definition equivalent to semantic security. We prove the insecurity over the alphabet {A,B} with p_B=(1/2(1+2s)) and p_A=1-p_B where p_A and p_B are the probabilities of the source generating A and B, respectively, and s is the number of bits in each splitting key.