When you create a file, sign a contract, or publish something online, how do you prove to a court that it existed at a given moment, unchanged, and that you authored it?The EU's eIDAS regulation gives two primitives for exactly this:- Qualified timestamps — a trusted authority cryptographically attests that the hash of your content existed at a specific time. Change one byte and the hash no longer matches.- Advanced / qualified e-signatures — bind a document to a signer with legal weight across the EU.Together they let you prove existence, integrity, and authorship of any digital content, in a form that holds up as court-admissible evidence.I've been building Instantproof to wrap these into a simple API, so you can timestamp and e-sign files, documents, and web pages without standing up your own PKI.A few practical notes for devs:- Hash client-side (SHA-256) and timestamp the hash, not the file — that keeps the content private while still proving it existed.- Store the RFC 3161 timestamp token alongside the file; verification then works fully offline.- Under eIDAS, "advanced" vs "qualified" signatures carry different legal weight — pick what your use case actually needs.How do you handle proof-of-existence in your own stack?