Which of the following is not a key size used by AES?

AES, or Advanced Encryption Standard, is a widely adopted symmetric encryption standard that uses specific key sizes to encrypt and decrypt data. The officially recognized key sizes for AES are 128 bits, 192 bits, and 256 bits. These key sizes allow for varying levels of security: 128 bits is generally considered secure for most applications, while 256 bits offers a higher level of security for sensitive data.

On the other hand, a key size of 512 bits is not utilized by AES. The standard was designed to function optimally with the aforementioned key sizes, and using a key size of 512 bits is not supported within the framework of AES. This limitation is rooted in AES's design and the mathematical principles behind its encryption algorithm.

Therefore, the answer identifies 512 bits as an invalid key size for AES, reinforcing the importance of understanding the specific parameters and standards that govern cryptographic systems.