We investigate how the behaviors of malicious trusted parties affect participants of optimistic non-repudiation protocols. We introduce a notion of risk balance for exchange protocols. Intuitively, risk balance refers to fairness in the amount of protection a protocol offers to the participants against malicious trustees. We explore how risk balance relates to the notions of accountable trustees and transparent trustees previously introduced by Asokan and Micali, respectively. As a case study,

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... e investigate the consequences of malicious behaviors of trusted parties in the context of two fair non-repudiation protocols, proposed by Gürgens, Rudolph and Vogt (2005) . We discover a number of security issues in these protocols and propose simple solutions for fixing them.