What is a ProofMark?

A ProofMark is an intrinsic data-level tamper indicator embedded in or associated with a data set. It is a cryptographic time stamp that binds the unique digital fingerprint that particular data (a message digest, or "hash") with time sourced from a trusted National Timing Authority, such as NIST. Like tamper-evident seals on Tylenol bottles, ProofMarks don't actually prevent tampering. Instead they discourage tampering by making it very obvious if data modification occurs.