Imagine a branchless implementation of the AES-128 cipher. Are there any benefits over the implementation that contains branches, other than possible prevention of timing attacks?

Is it even true, that a branchless code can mitigate a timing attack? Are there any other side-channel attacks that are prevented in this way?


1 Answer 1


What makes crypto code vulnerable to timing attacks is data dependent timing variations. Branching according to a round counter, or to the key size, does not create a vulnerability. Most implementations of AES make no branch according to key or data value, and supressing other branches won't help.

The main source of data-dependent timing variations in AES is in the table lookups for the S-boxes: according to if the entry is in cache, or not, the table fetch will take different time. See for example Daniel J. Bernstein, Cache-timing attacks on AES (2004) [direct link to pdf] for more info.

Among ways to make AES code free of any timing dependencies are:

  • $\begingroup$ Is it possible to replace the lookup table (S-box) with a branchless code to eliminate the weakness? $\endgroup$ Nov 18, 2015 at 17:49
  • $\begingroup$ The link you provided mentions a constant-time AES implementation, I will read the whole PDF later, to understand things more thoroughly. Thanks. $\endgroup$ Nov 18, 2015 at 17:56

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