When it uses a secure algorithm, symmetric key encryption can be extemely secure. One of the most widely-used symmetric key encryption systems is the U.S. Government-designated Advanced Encryption Standard. When you use it with its most secure 256-bit key length, it would take about a billion years for a 10 petaflop computer to guess the key through a brute-force attack. Since, as of November 2012, the fastest computer in the world runs at 17 petaflops, 256-bit AES is essentially unbreakable.
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