How to use Bitcoin blockchain to protect electronically signed agreements from changes

TL;DR: This tutorial demonstrates how to protect electronically signed agreement from changes using Bitcoin blockchain and its 1,100+ decentralized servers.

So you and you friends decided to form a startup to create a new software app that will change the world for the better. It is always good to have a simple written agreement before you start you actual work and you have decided to do this electronically using free e-signature service called This service takes an agreement file in Word, PDF, Text formats and then sends requests to parties to sign the document. To make sure the final agreement is legally binding, the site will track who and when viewed and electronically signed the document.

Ok, let’s upload the 2-page founder_agreement.docx :

The file will be uploaded in a second and you will be asked to select the mode. As you have 3 parties then select Sign by me and others:

The Invitations list to appear. Enter names and email addresses of your parties. In our case we will enter Kevin, Wendy and Salvador and their email addresses:

It will be much easier for parties if we pre-define where to put a signature for every signer.

Click Prepare and Assign Fields button and Add Fields editor will appear:

Click on Add Signature Field and the editor will create a signature field:

Move it to the proper place and select Kevin for this signature box:

Now repeat the same step for two other remaining parties and assign new signature fields to Wendy and Salvador accordingly:

Once you are done, click Proceed button at the right top corner:

The app will save your document and will send sign requests via email.

On the dashboard you may now see the detailed status for every participant:

In just a seconds Kevin, Wendy and Salvador receive signing requests to their email addreses. Signing requests look like this:

Kevin clicked the link and it opened with the document. He can see the box for his signature and clicks on it:

Once Kevin open the document, all parties receive the view notification:

In the meantime Kevin draws his signature and uses Insert Signature button:

Draw a signature, type it, upload or use a saved one

That’s all! His signature successfully inserted into the document:

He clicks Proceed button to save the document:

Every party gets another email notification that the document was just signed by Kevin:

The status and parties information is also updated for the document:

Now Wendy is also putting her signature into the document by clicking the link in sign request in her email box:

She clicks the field and types her signature:

Great! She can see her signature in the document now and clicks Proceed:

Salvador is also putting his signature into the document and clicks Proceed:

As all parties signed the document so the app automatically finalizing and locks the document. No more changes are allowed and the final PDF file is generated:

The final document Founders_agreement.pdf also includes the detailed log for all actions by all involved parties:

So the agreement signed by all three co-founders is available. But what if someone will open this document in PDF editor and will remove few lines? Or will create a new PDF based on this one?

This is where blockchain can really help. automatically calculates the digital fingerprint of the final document. This unique fingerprint is based on the SHA-256 algorithm which is represented as a string of symbols. What is also important, this fingerprint cannot be decrypted back into the document by design. But once you have calculated it for the document and someone will apply every a very small change to the original document, the fingerprint of a new document and the old one will not match anymore.

But how do we store this fingerprint? The Bitcoin blockchain with its network of decentralized 1,100+ servers around the world is a great fit! We will store the fingerprint inside a Bitcoin transaction that will be publicly stored in the blockchain. Important to note that the document itself will not be sent anywhere.

Click on the Store Proof in Bitcoin Blockchain button and it will display the information box along with the digital fingerprint.

Click on Proceed and it will ask you to send a small Bitcoin payment (2 mBTC ) to create a Bitcoin transaction that will act as a storage:

Send your payment to the given address and then check the current status by clicking on the Check Status button.

Once the transaction is processed by Bitcoin you will see the confirmation along with the information and the link to view it online:

Click the transaction information link and you will see that the digital fingerprint of the final document (5583a35e9ce8de694c1e2b91b6336c115266206ddc745e69b77e14e2bca31ea4) was succesfully stored inside the Bitcoin transaction.

Screenshot from Blocktrail web-site:

And the same transaction on as well:


As you may see, Bitcoin blockchain can be used to store the fingerprint of the document on 1,100+ Bitcoin servers decentralized network. Using free you may always verify the digital fingerprint of the document file and see if it matches the originally signed document

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