user1137: Does the Bitcoin protocol provide means to add some freely definable data to transactions such that a small transaction could actually be used to timestamp arbitrary data?

user4193: Yes, but you only get a few bytes(not sure exactly how much, i think around 30, not even near a kilobyte for sure) per transaction, so the best way to go about it would be to hash your data first and then use that instead.

user5731: Yes, the bitcoin network can be used as a timestamping service
Here is an article about it
https://www.strongcoin.com/en/blog/using_the_blockchain_as_a_trusted_timestamping_service
Here are additional instructions:
https://bitcointalk.org/index.php?topic=52715.0

1137: <p>Does the Bitcoin protocol provide means to add some freely definable data to transactions such that a small transaction could actually be used to timestamp arbitrary data?</p>
 4193: <p>Yes, but you only get a few bytes(not sure exactly how much, i think around 30, not even near a kilobyte for sure) per transaction, so the best way to go about it would be to hash your data first and then use that instead.</p>
 5731: <p>Yes, the bitcoin network can be used as a timestamping service</p>

<p>Here is an article about it
<a href="https://www.strongcoin.com/en/blog/using_the_blockchain_as_a_trusted_timestamping_service" rel="nofollow">https://www.strongcoin.com/en/blog/using_the_blockchain_as_a_trusted_timestamping_service</a></p>

<p>Here are additional instructions:<br>
<a href="https://bitcointalk.org/index.php?topic=52715.0" rel="nofollow">https://bitcointalk.org/index.php?topic=52715.0</a></p>
