For example, say Joe wants to send Mary some secure files or messages. When Mary receives the message, she uses her private key to decrypt the message, which was encrypted with her public key. Similarly, Mary uses Joe's public key to encrypt her reply before sending it. To assure Joe that she sent the answer and that it was not forged, Mary signs this message with her private key, which generates a digital signature block that Joe can verify using Mary's public key. Digital certificate authorities issue digital signatures and verify the user's identity much the same way a DMV verifies an identity and issues a driver's license.
Symmetric cryptography uses a single key to encrypt and decrypt messages.
Its weakness is that, to transmit an encoded message, users must also send the private key, which means a secure distribution route is needed. No matter how securely the doors are locked, a persistent intruder can find a way through. While no encryption program is 100 percent uncrackable, most intruders lack the time or skill to bypass or dismantle such security tools. One primary indicator of encryption strength is the key's bit rate, which is the number of bits in a key. The amount of time required to decode depends on the length of the decryption key.