Secure Anonymous Acknowledgments in a Delay-Tolerant Network

TCP and many other protocols use acknowledgments to provide reliable transmission of data over unreliable media. Secure acknowledgments offer a cryptographic guarantee that valid acknowledgments for a given message can only be issued by the intended receiver. In the context of an ad-hoc network, anonymous acknowledgments make it hard for an attacker to determine which device issued a particular acknowledgment. And unlike TCP, the acknowledgments described here work well even for connectionless communications. This acknowledgment mechanism assumes that message data is protected by secure encryption. The sender of a data message includes in the encrypted part of the message a randomly-generated acknowledgment. Only the intended receiver can decrypt the message and issue the acknowledgment. The acknowledgment is issued by sending it out to its peers, who will forward it until it reaches the sender of the data being acknowledged. Such randomly-generated acknowledgments in no way identify senders and receivers, providing a degree of anonymity. This paper describes the use of such acknowledgments in both ad-hoc networks and Delay-Tolerant Networks. In such networks every peer participates in forwarding data, including both the routing and the end-host functionalities of more conventional networks. In a Delay-Tolerant Network, peers may cache messages and deliver them to other peers at a later time, supporting end-to-end delivery even when peers are only connected intermittently. Caches have limited size, so peers must selectively remove cached messages when the cache is full. As an additional aid to selecting messages to be removed from a cache, peers can remove messages for which they have received a matching ack. This can be done while preserving both security and anonymity, by including in every message, unencrypted, a message ID computed as the hash of the message ack sent encrypted with the message. A peer seeing a new ack can then hash it and discard any cached message whose message ID matches the hash of the ack.