user6885: I know you can use the bitcoin client to sign messages (for which you own the private key) and verify messages from other public addresses. But can you actually embed a signed message into a transaction so its on the blockchain for anyone to verify? When I say "signed message" i'm referring to an arbitrary message like in the attached example.


user3697: No. This is not possible, although, technically it could be possible by adding n child transaction each formed as a tx out of you original transaction. The child transaction then could each have a OP_RETURN (data only) output. With a OP_RETURN output you can store up to 80 bytes (mind 5286). A DER signature will be around 72 Bytes (71.5 in avg)
... so not much to left besides the signature (~8 bytes per OP_RETURN tx as tx out of the original tx). So maybe use OP_RETURN txout 0 to n-1 for the text/data and n for the signature. Obviously this will increase your transaction fee a little because you need at least 2 additional transactions.
Conclusion:
Not possible unless your are happy to experiment with forging your own custom transactions.

6885: <p>I know you can use the bitcoin client to sign messages (for which you own the private key) and verify messages from other public addresses. But can you actually embed a signed message into a transaction so its on the blockchain for anyone to verify? When I say "signed message" i'm referring to an arbitrary message like in the attached example.</p>

<p><a href="https://i.stack.imgur.com/HT1Ux.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HT1Ux.png" alt="enter image description here"></a></p>
 3697: <p>No. This is not possible, although, technically it could be possible by adding <code>n</code> child transaction each formed as a tx out of you original transaction. The child transaction then could each have a OP_RETURN (data only) output. With a OP_RETURN output you can store up to 80 bytes (mind <a href="https://github.com/bitcoin/bitcoin/pull/5286" rel="nofollow">5286</a>). A DER signature will be around <a href="https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki#der-encoding" rel="nofollow">72 Bytes (71.5 in avg)</a></p>

<p>... so not much to left besides the signature (~8 bytes per OP_RETURN tx as tx out of the original tx). So maybe use OP_RETURN txout <code>0</code> to <code>n-1</code> for the text/data and <code>n</code> for the signature. Obviously this will increase your transaction fee a little because you need at least 2 additional transactions.</p>

<p><strong>Conclusion:</strong>
Not possible unless your are happy to experiment with forging your own custom transactions.</p>
