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53 lines
1.7 KiB
Rust
53 lines
1.7 KiB
Rust
use volatile::Volatile;
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/// Compare 2 byte slices in a way that ensures that the timing of the operation can't be used to
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/// glean any information about the data.
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#[inline(never)]
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#[cold]
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pub fn timing_safe_cmp(a: &[u8], b: &[u8]) -> bool {
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// we use raw pointers here to keep faithful to the C implementation and
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// to try to avoid any optimizations rustc might do with slices
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let len_a = a.len();
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let a = a.as_ptr();
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let len_b = b.len();
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let b = b.as_ptr();
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/* The volatile type declarations make sure that the compiler has no
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* chance to optimize and fold the code in any way that may change
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* the timing.
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*/
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let length: Volatile<usize>;
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let mut left: Volatile<*const u8>;
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let mut right: Volatile<*const u8>;
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let mut result: u8 = 0;
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/* loop count depends on length of b */
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length = Volatile::new(len_b);
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left = Volatile::new(std::ptr::null());
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right = Volatile::new(b);
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/* don't use else here to keep the amount of CPU instructions constant,
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* volatile forces re-evaluation
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* */
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if len_a == length.read() {
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left.write(Volatile::new(a).read());
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result = 0;
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}
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if len_a != length.read() {
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left.write(b);
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result = 1;
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}
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for _ in 0..length.read() {
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let l = left.read();
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left.write(l.wrapping_add(1));
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let r = right.read();
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right.write(r.wrapping_add(1));
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// safety: the 0..length range will always be either:
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// * as long as the length of both a and b, if len_a and len_b are equal
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// * as long as b, and both `left` and `right` are b
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result |= unsafe { l.read_volatile() ^ r.read_volatile() };
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}
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result == 0
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}
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