Public key cryptography is a type of encryption method that uses two keys - a public key and a private key - to secure data. In blockchain implementations, public key cryptography is used to create digital signatures that verify the authenticity of transactions. Each user in the blockchain network has a unique public key and private key pair. When a user initiates a transaction, they use their private key to create a digital signature that is attached to the transaction. The signature is then verified by other nodes in the network using the user's public key. This ensures that the transaction is legitimate and has not been tampered with.