Ais when the sender cryptographically marks a message with its private key, which is achieved by a cryptographic algorithm applied to the message or to a small block of data that is a function of the message.

Symmetric Key Encryption

Explanation:

Symmetric key encryption is a type of encryption in which only one key is used by sender to encrypt and receiver to decrypt a message(i.e both sender and receiver have the same key). This is different for asymmetric key encryption where a set of keys (private and public)  is used to encrypt and decrypt a message.

Note: The goal of encryption is to secure messages against third party intrusion.

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