Content providers often face legal or economic pressures to censor or remove objectionable or infringing content they host. While decentralized providers can enable censorship-resistant storage, centralized content providers remain popular for performance and usability reasons. But centralized content providers can always choose not to respond to requests for a specific file, making it difficult to prevent censorship. If it is not possible to prevent, is it possible to detect and punish censorship on a centralized service? A natural approach is to periodically audit the service provider by downloading the file. However, failure to download a file is not a proof of censorship. First, the provider could claim benign failure. Second, the proof is non-transferable: verifying censorship requires third parties to individually request the censored file. Moreover, a content provider may only selectively deny access to particular users or only for a short time frame. As such, checking by downloading does not work even for third parties who are online and willing to make queries. In this paper, we introduce proof of censorship, whereby a content provider cannot delete or otherwise selectively remove content from their service without creating transferable cryptographic proof of their misdeed. Even if the provider restores the file at a later date, the proof remains valid, allowing the reputation of a content provider's commitment to censorship resistance to be based on the existence (or absence) of such proofs.
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