A block cipher operated on blocks of individually encrypted data that are encrypted individually. In the case of AES a block is 128 bits which is equal to 16 bytes. Symmetric means that the same key is used to encrypt and decrypt data.
AES is available in variants with different key lengths: 128 bit, 192 bit, and 256 bit key. The longer the key is the higher is the level of security.
The Advanced Encryption Standard is a standard ratified by National Institute of Standards and Technology (NIST) in the year 2000 as a replacement for the Data Encryption Standard (DES) from 1977. AES was later approved as the FIPS standard, is included in ISO/IEC 18033-3, and is the only publicly available cipher approved by NSA for communication at top secret level.
Currently, no significant weaknesses have been found in AES. This means brute force is the only existing way of attacking it. Brute force can also be described as the method of trial and error: every possible key is tried until the correct one is found.
So how long would it take to find the key? From June 2017 the world’s fastest supercomputer has been TaihuLight. It performs ~ 100 petaFLOPs or 1.0E17 operations per second. The AES 256 key space is 2^256 ~= 1.1E77. For a full key space search, TaihuLight would need in the order of 1.1E77/1.0E17 = 1E60 seconds. But it would not be able to complete the task! In a much shorter time – in 3.7E17 second – Earth would have been swallowed by the Sun, which would have become a red dwarf.
Let us assume the unthinkable: that we were able to design a computer with the performance of TaihuLight from just one atom and that we transformed every atom of Earth into a such a computer: Earth is estimated to consist of 1E50 atoms. With that many supercomputers, the 256 bit key space could be traversed in 1E10 seconds or 316 years. So even with the help of a computer that we cannot imagine it would take more than a lifetime to make use of brute force against AES 256.
The emergence of Quantum computers will change the cryptography landscape. When sufficiently large quantum computers become available, Grover’s algorithm will allow the key place to search in O(sqrt(n)) time. When quantum technology becomes available AES 256 will have the same security level as AES 128 has today.
Basically, AES 256 is available as software or hardware implementation. Hardware implementation allows for increase security and performance compared to software. Hardware AES 256 can perform 10Gbps without significant latency.
Hardware encryption is typically much less complex than similar software encryption. And reduced complexity can be translated into less vulnerability to malware and errors.