What is AES and what key sizes does it support?

The Advanced Encryption Standard, or AES, is the current US government standard for symmetric encryption, adopted in 2001 to replace the Data Encryption Standard. AES is a block cipher that processes data in 128-bit blocks and supports three key sizes: 128, 192, and 256 bits. Longer keys provide stronger security but require more processing. AES-256 is required for top secret government data and is considered resistant to all known attacks including theoretical quantum computing threats.