Tim Taubert @ttaubert Formal Verification for Fun and Profit Finding bugs with Cryptol and SAW December 2016, Hawaii

What is Formal Verification? “[…] Formal verification is the act of proving or disproving the correctness of intended algorithms […] using formal methods of mathematics.”

Example: f(x) = (x == 0) [Cryptol] is_zero : [64] -> [1] is_zero x = if x == 0 then 1 else 0

Example: f(x) = (x == 0) [C99] bool is_zero(uint64_t x) { return x == 0U; }

Example: f(x) = (x == 0) [C99, maybe const-time] bool is_zero(uint64_t x) { bool nz = false; for (size_t i = 0; i < sizeof(x) * 8; ++i) { nz |= x >> i & 1; } return !nz; }

Example: f(x) = (x == 0) [Rust] fn is_zero(x: u64) -> bool { return x == 0; } (Compile to LLVM IR) $ rustc --emit=llvm-ir -o is_zero.bc is_zero.rs

Case Study: UTF-16 to UTF-8 Encoder 1) Compute the length of the target buffer. 2) Fill the target buffer with UTF-8.

Thanks! Cryptol: http://cryptol.net/ SAW: http://saw.galois.com/