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constant-time: add secure memcmp_le function

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The compare function should do a little-endian comparision, therefore
copy the code from quinier/memsec and don't revert the loop, tada, le.

Signed-off-by: Paul Spooren <mail@aparcar.org>
This commit is contained in:
Paul Spooren
2024-03-11 12:25:58 +01:00
committed by Karolin Varner
parent c69fd889fb
commit b2de384fcf
1 changed files with 16 additions and 1 deletions
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17
constant-time/src/compare.rs
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@@ -1,3 +1,18 @@
use core::ptr;
/// Little endian memcmp version of quinier/memsec
/// https://github.com/quininer/memsec/blob/bbc647967ff6d20d6dccf1c85f5d9037fcadd3b0/src/lib.rs#L30
#[inline(never)]
pub unsafe fn memcmp_le(b1: *const u8, b2: *const u8, len: usize) -> i32 {
let mut res = 0;
for i in 0..len {
let diff =
i32::from(ptr::read_volatile(b1.add(i))) - i32::from(ptr::read_volatile(b2.add(i)));
res = (res & (((diff - 1) & !diff) >> 8)) | diff;
}
((res - 1) >> 8) + (res >> 8) + 1
}
/// compares two slices of memory content and returns an integer indicating the relationship between
/// the slices
///
@@ -20,5 +35,5 @@
#[inline]
pub fn compare(a: &[u8], b: &[u8]) -> i32 {
assert!(a.len() == b.len());
unsafe { memsec::memcmp(a.as_ptr(), b.as_ptr(), a.len()) }
unsafe { memcmp_le(a.as_ptr(), b.as_ptr(), a.len()) }
}