Are Bitcoin Block Hashes Predictable?

Bitcoin: How can I leave a message in the block chain?

• A transaction has to be made to be included in the block chain, but how can I leave a message (encrypted or not) in order to certify I have certain information at a specific time. And how can I access to that message later? A service that lets you do something like this is http://www.proofofexistence.com/

• Answer:

  There is nothing in the blockchain that let's you directly add a message to it. The service you linked describes what they do: This is done by generating a valid bitcoin transaction to two specially crafted addresses which encode/contain the hash. So you can embed a message by creating a bitcoin address with the SHA-256 hash of the message, and then have another service that maps that hash to your original message. This answer provides some other hacks that could be used to do this: http://bitcoin.stackexchange.com/questions/2631/what-are-the-key-differences-between-different-ways-of-embedding-messages-in-the Basically, you have to use something in the block that accepts data, encode your message and then embed that data into the element. There aren't too many parts of a block that accept this easily.