大多数基于口令的密钥协商协议都只考虑如何在客户与服务器之间协商密钥。 客户到客户口令认证密钥协商协议考虑如何在两个客户之间协商密钥，而这两个客户 只分别与各自的服务器共享口令。在[?]中, Jin Wook Byun 等提出了两个协议，分别称为跨域C2C-PAKE协议，和单服务器C2C-PAKE协议。最近 针对这两个协议，提出了一系列攻击和改进方案。在本文中，我们指出跨域C2C-PAKE 协议及其所有的改进方案都是不安全的。我们设计了一个新的跨域C2C-PAKE协议。 它可以抵抗已知的所有攻击。

Most password-based key exchange protocols consider how to exchange a session key between a client and a server. Client-to-Client password authenticated key exchange protocol considers the scenario where two clients want to establish a session key but they only share their passwords with their own servers. In [1], Jin Wook Byun et al. proposed two such protocols called cross-realm C2C-PAKE and single-server C2C-PAKE. Recently some flaws of these two protocols are found and some improvements are suggested. In this paper, we show that the cross-realm C2C-PAKE protocol and its all improved forms are still insecure. And we also present a new cross-realm C2C-PAKE protocol which is resistant to all known attacks.