sgx_trts/src/memeq.rs

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//! Comparing buffer contents in constant time.
//!
//! This crate provides constant time memory comparison functions. These functions
//! are useful in cyptographic functions, defending against timing based side
//! channel attacks

use sgx_types::marker::{BytewiseEquality};
use core::mem;
use alloc::slice;

pub trait ConsttimeMemEq<T: BytewiseEquality + ?Sized = Self> {
    fn consttime_memeq(&self, other: &T) -> bool;

    fn consttime_memne(&self, other: &T) -> bool { !self.consttime_memeq(other) }
}

impl<T> ConsttimeMemEq<[T]> for [T]
    where T: Eq + BytewiseEquality
{
    fn consttime_memeq(&self, other: &[T]) -> bool {

        if self.len() != other.len() {
            return false;
        }
        if self.as_ptr() == other.as_ptr() {
            return true;
        }
        let size = mem::size_of_val(self);
        consttime_memequal(self.as_ptr() as * const u8,
                           other.as_ptr() as * const u8,
                           size) != 0
    }
}

impl<T> ConsttimeMemEq<T> for T
    where T: Eq + BytewiseEquality
{
    fn consttime_memeq(&self, other: &T) -> bool {

        let size = mem::size_of_val(self);
        if size == 0 {
            return true;
        }
        consttime_memequal(self as * const T as * const u8,
                           other as * const T as * const u8,
                           size) != 0
    }
}

fn consttime_memequal(b1: * const u8, b2: * const u8, l: usize) -> i32
{
    let mut res: i32 = 0;
    let mut len = l;
    let p1 = unsafe { slice::from_raw_parts(b1, l) };
    let p2 = unsafe { slice::from_raw_parts(b2, l) };

    while len > 0 {
        len -= 1;
        res |= i32::from(p1[len] ^ p2[len]);
    }
    /*
     * Map 0 to 1 and [1, 256) to 0 using only constant-time
     * arithmetic.
     *
     * This is not simply `!res' because although many CPUs support
     * branchless conditional moves and many compilers will take
     * advantage of them, certain compilers generate branches on
     * certain CPUs for `!res'.
     */
    (1 & ((res - 1) >> 8))
}