Peer-to-peer systems provide a distributed way of sharing and storing information. Each peer stores some information and responds to queries. In some applications, protecting anonymity of a responding peer is important to increase availability of information. This paper presents a cryptographic protocol on Chord to protect anonymity of peers when responding to queries. In this protocol, peers form anonymity groups and generate responses inside groups. Responder of a query has k-anonymity protection against an adversary who can sniff all communication on the network. Validity of an anonymous reply can be verified so fake replies of malicious peers are prevented. The proposed approach can be adapted to other DHT structures to protect responder anonymity.