user27488: What I have in mind are proof of existence software, than are a kind a non-centralized and virtual notary service. They time-stamp digital information submitting a hash to the blockchain.
I know already that technically, the blockchain can be used for that. After all, it's basically a time-stamping system.
However, is this kind of service disruptive to the bitcoin community? Can it become disruptive when it reaches a certain level and be curtailed?
I also wonder whether the messages are prunable. Important are the transactions, not the associated messages. If the blockchain grow and grow, would the messages be still be kept?

user27169: There are already tons of services doing this, so probably worth focusing your creative efforts elsewhere. Here are the ones we know about on Coin Secrets:

Proof of Existence
BlockSign
Stampd
La Preuve
Bitproof
Stampery
ProveBit

As for pruning, assuming you use the same OP_RETURN mechanism as these tools, then they may be pruned by some nodes. But I imagine there will always be plenty of full archives of the blockchain available (even if this is eventually a paid-for service) so I doubt that your timestamped hashes will ever be truly lost.

27488: <p>What I have in mind are proof of existence software, than are a kind a non-centralized and virtual notary service. They time-stamp digital information submitting a hash to the blockchain.</p>

<p>I know already that technically, the blockchain can be used for that. After all, it's basically a time-stamping system.</p>

<p>However, is this kind of service disruptive to the bitcoin community? Can it become disruptive when it reaches a certain level and be curtailed?</p>

<p>I also wonder whether the messages are prunable. Important are the transactions, not the associated messages. If the blockchain grow and grow, would the messages be still be kept?</p>
 27169: <p>There are already tons of services doing this, so probably worth focusing your creative efforts elsewhere. Here are the ones we know about on <a href="http://coinsecrets.org/" rel="nofollow">Coin Secrets</a>:</p>

<ul>
<li><a href="http://www.proofofexistence.com/" rel="nofollow">Proof of Existence</a></li>
<li><a href="https://blocksign.com/" rel="nofollow">BlockSign</a></li>
<li><a href="http://stampd.io/" rel="nofollow">Stampd</a></li>
<li><a href="http://www.lapreuve.net/" rel="nofollow">La Preuve</a></li>
<li><a href="https://bitproof.io/" rel="nofollow">Bitproof</a></li>
<li><a href="https://stampery.co/" rel="nofollow">Stampery</a></li>
<li><a href="https://github.com/thereal1024/ProveBit" rel="nofollow">ProveBit</a></li>
</ul>

<p>As for pruning, assuming you use the same <code>OP_RETURN</code> mechanism as these tools, then they may be pruned by some nodes. But I imagine there will always be plenty of full archives of the blockchain available (even if this is eventually a paid-for service) so I doubt that your timestamped hashes will ever be truly lost.</p>
