The single key must be kept secret. System has been used since the early 1970s. Only the two systems containing the secret key can encrypt and decrypt messages. Less secure, secret key must be transmitted and revealed to the decryption system. Cannot provide digital signatures that cannot be repudiated.
Systems are faster.
System is not vulnerable to impersonation. Utilizes different modes of enciphering. Utilizes two keys, one for encryption, one for decryption. Introduced in 1976 by Whitfield Diffie and Martin Hellman of Stanford University. Many systems can encrypt and one system can decrypt or vice versa.