There is a couple of differences. On the document certification side, Trustatom doesn't upload documents to the server unencrypted (it doesn't upload them at all at the moment, but there will be an encrypted vault functionality). Besides that, Trustatom makes use of private keys known only to the user so that only the user can prove their submission at a later time. Lastly, there is also a private fingerprint mode that helps avoiding putting file's fingerprint to the blockchain as is (for others to discover). What Trustatom lacks comparing to BlockSign right now is the ability to add a visual signature to the file.
On the grander scale of things, Trustatom's functionality is not limited to certification. It already has joint escrow functionality (escrow without a third party) and there will be more coming.
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