The private key is used to encrypt or lock a message or transaction which is sent on the blockchain network. The sender can send a message using the public key of the receiver. On the other hand, the receiver can decrypt the message or the transaction using his private key. By using the private and public key, the communication or transaction is kept safe and tamper-proof.

In the context of blockchain technology:

Public Key:
- A public key is a cryptographic key that is shared openly and serves as an address to receive funds or messages.
- It is derived from the private key using a mathematical algorithm.
- While it can be freely distributed, it is computationally infeasible to reverse-engineer the private key from the public key.
- Anyone can use the public key to verify signatures or encrypt messages intended for the holder of the corresponding private key.
Private Key:
- A private key is a secret, known only to the owner, and is used for creating digital signatures and decrypting messages encrypted with the public key.
- It is crucial to keep the private key secure because whoever possesses it has control over the associated funds or data.
- Losing the private key can result in permanent loss of access to funds or information, and compromising it can lead to unauthorized access.

In summary, the public key is shared openly and used for verification or encryption, while the private key is kept confidential and used for signing transactions or decrypting messages. The security of blockchain systems relies on the secure management and protection of private keys.