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The public key plays a crucial role in the function of a digital envelope, where its primary purpose is to encrypt a symmetric key that is used to encrypt the actual message. This allows the encrypted symmetric key to be securely transmitted along with the encrypted message without exposing the symmetric key itself.

When the recipient receives the digital envelope, they can use their corresponding private key to decrypt the symmetric key. This ensures that only the intended recipient, who possesses the private key, can access the symmetric key and subsequently decrypt the message, thereby maintaining confidentiality.

In this context, the public key's role is essential for secure communication, making it vital to the concept of a digital envelope. The other options, such as decoding messages, validating the sender's identity, or enhancing performance metrics, do not align with the primary functions of a public key in this specific cryptographic process.