As of 2018-06-26
99% of the time when someone says let’s “Put That On the Blockchain” what that is transalted into is a timestamp and sha-256 hash placed onto the main Bitcoin or Ethereum blockchains.
The original proof of existence is https://proofofexistence.com
Since the first blockchain was the Bitcoin Blockchain, there is a limited amount of information that can be put onto each block. Ethereum does not have the same limit on blocksize, but a concept of “gas” used to keep down the insanity of putting increasingly larger sets of data onto “the blockchain”. Even the forked Bitcoin, BCH, has an increased block size to 32mb, but only 220 bytes in the OP_RETURN for placing a document.
Let’s consider messages as small documents, so when we are talking about Documents we are really talking about documents like PDF, Songs, Videos, Presentations, etc. So let’s consider documents 1 mb and larger.
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