A common approach to demands for lawful access to encrypted data is to allow a trusted third party (TTP) to gain access to private data. However, there is no way to verify that this trust is well placed as the TTP may open all messages indiscriminately. Moreover, existing approaches do not scale well when, in addition to the content of the conversation, one wishes to hide one’s identity. Given the importance of metadata this is a major problem. We propose a new approach in which users can retroactively verify cryptographically whether they were wiretapped. As a case study, we propose a new signature scheme that can act as an accountable replacement for group signatures, accountable forward and backward tracing signatures.
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