Symmetric encryption is the standard method for securing data at rest, such as on a hard drive. This type of encryption uses a single secret key for both the encryption and decryption processes. Algorithms like the Advanced Encryption Standard (AES) are symmetric and are favored for this purpose because they are computationally efficient and fast, making them ideal for encrypting large volumes of data. Full-disk encryption (FDE) and file-level encryption systems almost universally rely on symmetric ciphers for the actual data encryption process.

A. File-based encryption: This describes the scope or method of applying encryption (i.e., encrypting individual files), not the fundamental cryptographic type used to perform the encryption.

B. Hash encryption: Hashing is a one-way cryptographic function that creates a fixed-size string of bits from data. It is used for integrity, not confidentiality, as it cannot be reversed to recover the original data.

D. Public Key Infrastructure (PKI): PKI is a framework that uses asymmetric encryption for managing digital certificates and secure key exchange. Asymmetric encryption is too slow for encrypting large volumes of data like a hard drive.

1. Cisco Press, CCNA Cyber Ops SECFND 210-250 Official Cert Guide, Omar Santos, Joseph Muniz, Stefano De Crescenzo, 2017.

Reference: Chapter 11, "Cryptography," Section "Symmetric Encryption Algorithms."

Content: The text explains that symmetric algorithms like AES are "used to encrypt bulk amounts of data" due to their speed and efficiency, contrasting them with slower asymmetric algorithms used for key exchange.

2. MIT OpenCourseWare, 6.857 Computer and Network Security, Spring 2017, Prof. Ronald Rivest.

Reference: Lecture 3 Notes, "Symmetric-Key Ciphers: DES, AES, Modes of Operation."

Content: The course material details that symmetric-key ciphers like AES are the standard for encrypting large quantities of data, such as files or entire disks, because they are designed for high performance. This is contrasted with public-key (asymmetric) cryptography, which is noted as being significantly slower.