In symmetric-key schemes, the encryption and decryption keys are the same. Communicating parties must have the same key in order to achieve secure communication. An example of a symmetric key is the German military's Enigma Machine. There were key settings for each day. When the Allies figured out how the machine worked, they were able to decipher the information encoded within the messages as soon as they could discover the encryption key for a given day's transmissions.
In public-key encryption schemes, the encryption key is published for anyone to use and encrypt messages. However, only the receiving party has access to the decryption key that enables messages to be read. Public-key encryption was first described in a secret document in 1973; before then all encryption schemes were symmetric-key (also called private-key).:478 Although published subsequently, the work of Diffie and Hellman, was published in a journal with a large readership, and the value of the methodology was explicitly described
and the method became known as the Diffie Hellman key exchange.
A publicly available public key encryption application called Pretty Good Privacy (PGP) was written in 1991 by Phil Zimmermann, and distributed free of charge with source code; it was purchased by Symantec in 2010 and is regularly updated.