APRIL 19
Public Keys
The Key That Checks The Proof
The Signal
A public key lets Bitcoin check a digital signature without revealing the private key that created it.
The Reading
Reading
Yesterday we met the private key: a secret number that creates a digital signature, but here is the puzzle. If nobody should ever see your private key, how can the Bitcoin network know your signature is real?
That is the job of the public key. A public key is created mathematically from its matching private key. The maths works easily in that direction, but with today’s computers it is practically impossible to work backwards and discover the private key. Think of the public key as a mathematical checker. It can be shown to others because it does not give them the secret needed to create your signature.
When you send or spend bitcoin, your private key signs the transaction. Your private key stays hidden. The transaction, its signature and the information needed to check that signature can then be examined by Bitcoin nodes. The nodes use the public key to test the signature. If the signature matches the transaction and the public key, they have mathematical proof that the matching private key authorised the spend. If it does not match, the transaction fails that check.
The private key signs. The public key checks. One must stay secret. The other is designed to be shared or revealed when needed. Together they let Bitcoin answer a remarkable question: how can you prove you have the authority to spend without showing anyone the secret that gives you that authority?
First Principles
A private key creates a digital signature.
A public key helps verify that signature.
The public key can be revealed without revealing the private key.
Today’s Challenge
Find a padlock and its key somewhere at home. Give the padlock to someone else, but keep the key in your pocket.
They can see the lock. They can hold it. They can examine it. But seeing the lock does not give them your secret key.
Now take out the key and open the lock. The two belong together, but they have very different jobs. One can be shown. The other must be protected.
Bitcoin uses mathematics rather than a metal lock, but the idea helps: the public key can be seen and used to check, while the private key stays secret and does the signing. Which one would you leave on the table? Which one would you never hand over?
Tomorrow’s Signal
Wallets