An encryption method that uses a two-part key: one private; the other public. To send an encrypted message to someone, the recipient's public key is used, which can be published anywhere or sent openly via email. When the message arrives, the recipient uses his or her private key, which is always kept secret.
Public key cryptography differs from "secret-key cryptography," which uses the same key to encrypt and decrypt. The advantage of public key cryptography is that the public key can be published anywhere and transmitted in the open. The secret key method requires that both sides use the same key, which obviously cannot be freely published. However, public key cryptography is often used to send the secret key to the receiving end and then exchange data via secret keys. See cryptography
Digital Signatures Start With the Private Key
To create a digital signature that ensures the integrity of a message, document or other file, the signer's private key is used to encrypt the message digest. To verify the signature, the signer's public key is used to decrypt the signature back into the digest and compare it with a newly generated digest.
In the following example, the woman signs the message first and then entirely encrypts it before sending it. The man decrypts the message first and then verifies the signature. See digital signature