Header hopping and packet mixers

We describe a steganographic protocol for packet switched networks that is based on the dynamic rewriting of packet headers. The protocol provides confidentiality without encryption based on Rivest´s “chaffing and winnowing” technique (“Chaffing and winnowing”, MIT Lab for Computer Science, March 1998). We identify new places in computer networks where this method can be applied and describe an alternative chaffing method that relies on unpredictable number sequences rather than the use of message authentication codes. Our approach requires far less chaffing bandwidth overhead. Moreover, thanks to the underlying active network, we can place packet stream repeaters and filters on intermediate (and untrusted) nodes. Header hopping is closely related to frequency hopping in wireless communications; we also link it to Chaum´s work (Communications of the ACM, Feb. 1981) on anonymous routing