Programming Languages

Buy Signature

Let's imagine you want to have your testament notarized. What would you do?

You first write your testament You pay a notary to stamp it You store a copy for future reference

If you think about it, there are many interactions in the world that follow that same process:

• When your diploma is certified by your University.
• When you renew your driver's license, there is no test, just a renewal stamp.

We could generalize the process as:

A buyer (you in this case) Pays a seller Takes something, or better yet, a representation of the item (a digest) to a seller. The seller can now sign the digest. All parties can see that the digest was signed.

This is the interaction that the buy_sig.glow contract represents.

Visualizing the Buyer and Seller interactions

Glow code

1    #lang glow
2    @interaction([Buyer, Seller])
3    let payForSignature = (digest : Digest, price : Nat) => {
4      deposit! Buyer -> price;
5      @verifiably!(Seller) let signature = sign(digest);
6      publish! Seller -> signature;
7      verify! signature;
8      withdraw! Seller <- price;
9    }

• 2 Buyer and seller have agreed to the terms of this sale. They both know what the signature is about, and they want to conduct this sale.
• 3 The digest of the message to sign is a parameter of the interaction, as is the convened price.
• 4 The buyer deposits the money according to the price.
• 5 The seller signs, but it is private only to the seller.
• 6 The signature is made public for everyone to see.
• 7 The signature is verified by everyone in a way that the contract enforces.
• 8 Finally, the money is transferred to the seller.

There are several things to note:

• 1 The code looks a lot like the sequence diagram we created before.
• 2 The lines of code with @Seller annotation are private.
• 3 The language itself takes care of prerequisites; If the buyer never deposits, then the seller never will be able to sign.

Lessons learned

• Identify the participants of a contract with @interaction
• When an instruction is annotated with the participant's name, that value is private for the participant. Like @verifiably!(Seller)
• There are clear instructions for deposit! and withdraw!