usecase: proof of a file's existence?

[monkeypants]

I want to prove a file existed (on IPFS) on or before an approximate point in time. So, I send some dust to an address representing the semantics of my message. I include an IPFS hash of the file using the OP_RETURN code. What am I missing here?

The OP_RETURN can be maximum of 80 bytes. Will the IPFS hash ever by more than 80 bytes?

Is it a problem to reuse OP_RETURN addresses? For example, say we agree to the convention that some specific address mapped to the FOO semantic (payer asserts FOO about the file referenced at OP_RETURN, by paying to the conventional FOO_ADDRESS). I know addresses should only be used once for actual payments, but if it's an OP_RETURN payment, can we safely reuse that address in that way?

[jbenet]

Will the IPFS hash ever by more than 80 bytes?

It could, with very long hash functions. Not realistic any time soon.

rest SGTM

[monkeypants]

I send some dust to an address representing the semantics of my message

No good. OP_RETURN scripts don't have a recipient address (it's like /dev/null address). http://bitcoin.stackexchange.com/questions/48594/is-it-safe-to-reuse-payee-address-for-op-return-payments

proofofexistence.com uses a prefix in the hash (0x444f4350524f4f46, 'DOCPROOF') to denote the semantics of the message.