In symmetric encryption, which of the following must be shared between parties?

In symmetric encryption, the essential characteristic is that the same key is used for both encryption and decryption processes. This shared key is known as the session key.

When two parties wish to communicate securely using symmetric encryption, they must both possess the same session key to encrypt messages before sending them and to decrypt received messages afterward. The security of symmetric encryption rests on the confidentiality of this shared key; if an unauthorized party gains access to the session key, they could decrypt the messages exchanged between the two parties.

The other elements mentioned—public key, private key, and encryption algorithm—are not shared similarly. In asymmetric encryption, for example, the public key is distributed while the private key remains confidential and unique to an individual. The encryption algorithm is a method used to process the data but does not require sharing in the same manner as keys in symmetric encryption. Consequently, the emphasis on the sharing of the session key highlights its critical role in ensuring secure communication in symmetric encryption models.