Which of the following is a popular symmetric encryption standard?

AES, or Advanced Encryption Standard, is a widely adopted symmetric encryption standard that plays a crucial role in securing data. It was established by the National Institute of Standards and Technology (NIST) in 2001 and is recognized for its efficiency and robust security features.

Symmetric encryption means that the same key is used for both encryption and decryption. AES operates on fixed block sizes of 128 bits and supports key lengths of 128, 192, and 256 bits, making it a flexible choice for various security needs. Its strength lies in both the complexity of the algorithms behind it and its ability to withstand attacks, thus making it the standard choice for many applications, whether in government, financial institutions, or in software solutions for data protection.

In contrast, RSA is an asymmetric encryption standard that relies on a pair of keys (public and private) for securing data; SHA-256 is a cryptographic hash function rather than an encryption standard; and PGP (Pretty Good Privacy) combines both symmetric and asymmetric encryption methods but is not classified as a symmetric encryption standard on its own. Therefore, AES stands out as the correct answer for a popular symmetric encryption standard.