How strong is AES encryption? Here’s an excerpt from the fact sheet at NIST:
Because of its greater strength and efficiency, AES eventually will replace NIST’s earlier Data Encryption Standard (DES), in use since 1977, and Triple DES, approved in 1999. Assuming that one could build a machine that could recover a DES key in a second, then it would take that machine approximately 149 trillion (thousand-billion) years to crack a 128-bit AES key; this is longer than our universe has existed. In 1997, NIST invited the world’s best cryptographers to submit and help evaluate algorithms for the new encryption standard. This four-year effort resulted in the new AES.