Compare public key cryptography and Symmetric key cryptography?

Answer 1

In public key cryptography there are two parts: 1) secret part 2) public part ,in order for p1 to send p2 a message ,p1 first need to obtain p2's public key and using this key encrypts the message then p2 using his secret key decrypts the message. And in private key cryptograhy there is only one key so in order for p1 to send a message to p2, p1 should firstly ask p2 for his key then encrypt the message using that key and then p2 uses the same key to decrypt the message.

Answer 2

Public key cryptography utilizes asymmetric encryption; a pair of algorithmically related keys are required to encrypt and decrypt messages. By design, one key is held "private" and the other is "public" i.e. available to anyone. Messages enrypted with the public key can only be decrypted using the private key; such messages can only be read by someone holding the private key. By this means messages can be sent securely to the holder of the private key. Messages encrypted with the private key can only be decrypted to an intelligible message using the public key. Certificate authorities can be used to confirm who the public key belongs to - thus providing authentication of the source of the private-key-signed message.

Messages Symmetric key cryptography utilizes the same key to encrypt and decrypt messages. Only parties with whom the key have been shared can encrypt and decrypt the messages sent using the key. This mainly provides for confidentiality.

Note that in some schemes public key cryptography is used to facilitate the exchange or establishment of shared symmetric keys for a communications session. That's how SSL/TLS works to establish secure channels - the ones that are supposed to give you a little picture of a padlock telling you your session is secure. Schemes that utilize public key encryption to exchange a temporary symmetric key are referred to as "hybrid cryptosystems".