The UltraScale FPGA encryption system uses the AES-GCM authenticated encryption algorithm. AES is an official standard supported by the NIST and the U.S. Department of Commerce (see the Advanced Encryption Standard (AES) (FIPS PUB 197) and GCM specification The Galois/Counter Mode of Operation (GCM) (Specification) for more information).
An advantage of AES-GCM is that it also supports built-in authentication. The UltraScale FPGA AES encryption system uses a 256-bit encryption key (the alternate key lengths of 128 and 192 bits described by NIST are not implemented) to encrypt or decrypt blocks of 128 bits of data at a time. According to NIST, there are 1.1 x 1077 possible key combinations for a 256-bit key. For the most secure approach, it is recommended that you create this 256-bit key manually rather than use the pseudo-random key generator feature provided by Vivado.