Prioprepare for Chrome
More About Prioprepare
As simple as hashing your document and storing it in a bank transfer.
The idea is simple:
Compute a cryptographic hash value from a document and copy it to the text fields of a bank transfer. With such a hash value stored at a bank there is evidence of the existence of the content of a document at a certain point in time. This can be used to e.g. support a proof of priority.