What is public key cryptography?

In a public key cryptography system, two keys are required in order for two parties to exchange information in a secure fashion: a public key and a private key. If one key is used to encrypt a message, then only the other key in the pair can be used to decrypt it. Although the keys of the public and private key pair are mathematically related, it is computationally infeasible to derive one key from the other, so the private key is protected from duplication or forgery even when someone knows the public key. Therefore, it is safe to openly distribute your public key for everyone to use, but it is essential that your private key remains closely guarded and secret. The public key can be used to verify a message signed with the private key or encrypt messages that can only be decrypted using the private key. If someone wants to send you an encrypted message, they encrypt the message with your public key and you, being the sole possessor of the corresponding private key of the pair, are the only one who can decrypt it. For more information, see RSA's excellent FAQs about Public Key Cryptography.