How do I prove a document has not been changed?

Take a cryptographic fingerprint (SHA-256 hash) of the document and timestamp it somewhere nobody can edit, such as Bitcoin. Later, anyone can hash the document again: if the fingerprint matches the timestamped one, the document is byte-for-byte the same as it was at that time. Change one character and the fingerprint changes completely.

How it works

1. Drop the file on sebbi.pro/bitcoin. It is hashed in your browser and never uploaded.
2. The fingerprint is sealed and timestamped in Bitcoin, free.
3. Keep the receipt code with the document.
4. To prove it later, anyone re-hashes the file and checks it at sebbi.pro/forever.