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Secret-key encryption uses one key, the secret key, to both encrypt and decrypt messages. This is also called symmetric encryption. The term "private key" is often used inappropriately to refer to the secret key. A private key is one of the keys in the public/private key pair for asymmetric cryptography. In this scheme, a user has two keys, one that is made public and one that is held privately. Refer to "Public-Key Cryptography."
In secret-key cryptography schemes, a single key is used to encrypt data. A secret key may be held by one person or exchanged between the sender and the receiver of a message. For example, if you encrypt data for storage on a hard drive, you remember the key and usually don't give it to someone else. But if you want to send secure messages to a business partner using symmetric cryptography, you need to make sure your partner knows the key that will decrypt the messages.
If secret-key cryptography is used to send secret messages between two parties, both the sender and receiver must have a copy of the secret key. However, the key may be compromised during transit. If you know the party you are exchanging messages with, you can give them the key in advance. However, if you need to send an encrypted message to someone you have never met, you'll need to figure out a way to exchange keys in a secure way. One method is to send it via another secure channel or even via overnight express, but this may be risky in some cases. See "Key Distribution and Management" for more information.
As mentioned, secret-key cryptography is often used to encrypt data on hard drives. The person encrypting the data holds the key privately and there is no problem with key distribution. Secret-key cryptography is also used for communication devices like bridges that encrypt all data that cross the link. A network administrator programs two devices with the same key, and then personally transports them to their physical locations.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.