Schemes of anonymous communication enable entities to send or receive their messages without disclosing their identities to others including managers of communication systems and receivers or senders of the messages. Among various existing schemes this chapter introduces Crowds, DC net, Mix-net, ESEBM (Enhanced Symmetric key Encryption Based Mix-Net), and Onion Routing. Mechanisms to protect anonymous communication systems from malicious entities are also discussed.
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Identities of message senders or receivers are sometimes as sensitive as messages themselves. For example, a company may acquire highly confidential information about its rival companies from identities of their customers and suppliers. Therefore, the importance of anonymous communication is increasing as more people are being involved in network based communication. Anonymous communication mechanisms enable entity S that sends message M or entity R that receives M, to exchange M without disclosing their identities to others. According to subjects to be made anonymous, there are 3 types of anonymous communication schemes, i.e. when S can send M without disclosing its identity, the mechanism is called sender anonymous, when R can receive M without disclosing its identity, it is called receiver anonymous, and when both S and R can exchange M without disclosing their identities it is called mutually anonymous. This chapter mainly discusses sender anonymous communication mechanisms because receiver and mutually anonymous ones can be developed based on them, and except in the last part of this chapter, the word anonymous communication is used for representing the sender anonymous communication.
Now, (sender) anonymous communication mechanisms enable entity S to send message M to its receiver R without disclosing the identity of S to any entity except S including R and managers of the communication channel. Here, message M usually reaches R from S while being relayed by multiple entities as shown in Figure 1, because when M reaches R directly from S, R can easily know the source S. In the remainder, entities that send messages and receive messages are called senders and receivers, respectively. Also entities that relay messages are called relay servers or simply servers.
The requirements for sender anonymous communication are summarized as follows, i.e.
- 1.
No one except sender S itself can identify the sender of massage M,
- 2.
No one except sender S itself can know that messages M1 and M2 sent from S are the messages sent from the same sender,
- 3.
Sender S can confirm the successful delivery of its message M to receiver R without disclosing its identity to others,
- 4.
Sender S can receive reply messages to its message M from receiver R without disclosing its identity to any other entity, and
- 5.
Communication channels can protect themselves from accesses from unauthorized entities.