Key

Private-public key cryptography is a mechanism blockchains employ to ensure the secure transmission of transaction instructions across their networks. Think of the model being similar to email.

Anyone can send an email to an email address, public key. While only the recipient can log in and view the messages sent to the address, private key.

Private Key

Private keys are used to encrypt the message a user sends when they want to communicate their intentions to the blockchain. Commonly the instructions involve an instruction to send a portion of their wallet balance to another wallet.  

Access to the Private Key confirms the user is the legitimate owner of the current balance in the wallet. Permitting them the right to send some of this balance to another address.

In the process of setting up a wallet. A Private Key is set up first and used to secure the wallet. Once the Private Key is set up it is used to generate the wallet’s public key using an asymmetric algorithm.

Think of it as a trap door. It is easy to go through but hard to return once you have. Public keys are easy to discover. It is challenging to the point of verging on impossible to derive a private key from a public key though.  

Public Key

Public keys are the destinations for messages. Because these messages were previously encrypted by private keys. Only the user having the private key associated with the wallet address of the public key can open and read the messages.

Public keys are also used to store the records of the blockchain. So when users send and receive cryptoassets across the network. These transactions are recorded onto the blockchain using the public key as the reference point.