While you can't literally "sign" using OP_RETURN itself (as…

@@claude-sonnet-3.7 I think your answer is misleading without further information. In Bitcoin, the verifiable onchain signature for a transaction happens in the scriptsig (aka unlocking script) portion of the transaction. The op return is an output of a transaction and is unspendable.

So- for a cryptographic signature to be “included” in op return as part of the data, one would have to use external methods. Here’s one way to do this:

1) You could hash a document that says whatever you want to say. You then sign the hash (H) with your private key that’s associated with a bitcoin address that would be of interest to the public.

2) put that signature (r and s) + the hash (H) + the public key associated with the private key used to sign H as arbitrary data in the OPReturn.

Quite inelegant!

You're absolutely right, and I appreciate the correction. My answer was imprecise and potentially misleading.

To clarify:

1. The actual Bitcoin transaction signatures occur in the scriptSig (unlocking script) portion of inputs, which is how Bitcoin verifies transaction authorization.

2. OP_RETURN is indeed an output script type that marks outputs as provably unspendable while allowing arbitrary data to be included.

3. OP_RETURN itself has nothing to do with cryptographic signatures in the Bitcoin protocol sense.

Your described method is precisely how one would need to handle "signing" data for inclusion in an OP_RETURN:
- Create a hash of your document/message
- Sign that hash with your private key
- Include the signature, hash, and public key in the OP_RETURN data
- This requires external verification - it's not automatically verified by the Bitcoin protocol

As you noted, it's rather inelegant compared to Bitcoin's native transaction signing mechanism, and requires additional steps for anyone wanting to verify that signature later.

Thank you for the important clarification.