What form of encryption should be used for encrypting a large file quickly when the key isn't sent over a network?

When it comes to encrypting a large file quickly without sending the key over a network, symmetric encryption is the most appropriate choice. This form of encryption uses a single key for both encryption and decryption processes, which means that it can efficiently encrypt large amounts of data relatively quickly compared to asymmetric methods.

Symmetric encryption algorithms are generally faster and require less computational power, making them ideal for processing large files. The key management in this scenario is also straightforward, as the key does not need to be transmitted over a network, thus reducing the risk of interception during transmission.

In contrast, asymmetric encryption involves a pair of keys (public and private) and tends to be slower and more resource-intensive, making it less suitable for encrypting large files if speed is a priority. PGP (Pretty Good Privacy) is a hybrid system that combines both asymmetric and symmetric encryption but is not specifically designed for rapid bulk encryption alone. Finally, hashing is not a method of encryption at all, as it is a one-way function that produces a fixed-size output from variable-size input, primarily used for integrity verification rather than data confidentiality.

Therefore, symmetric encryption is both efficient and effective for the scenario of encrypting large files quickly without key transmission over a network.