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// Copyright 2015-2016 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//! Constant-time operations.
use crate::{c, error};
/// Returns `Ok(())` if `a == b` and `Err(error::Unspecified)` otherwise.
/// The comparison of `a` and `b` is done in constant time with respect to the
/// contents of each, but NOT in constant time with respect to the lengths of
/// `a` and `b`.
pub fn verify_slices_are_equal(a: &[u8], b: &[u8]) -> Result<(), error::Unspecified> {
if a.len() != b.len() {
return Err(error::Unspecified);
}
let result = unsafe { CRYPTO_memcmp(a.as_ptr(), b.as_ptr(), a.len()) };
match result {
0 => Ok(()),
_ => Err(error::Unspecified),
}
}
prefixed_extern! {
fn CRYPTO_memcmp(a: *const u8, b: *const u8, len: c::size_t) -> c::int;
}
#[cfg(test)]
mod tests {
use crate::limb::LimbMask;
use crate::{bssl, error, rand};
#[test]
fn test_constant_time() -> Result<(), error::Unspecified> {
prefixed_extern! {
fn bssl_constant_time_test_main() -> bssl::Result;
}
Result::from(unsafe { bssl_constant_time_test_main() })
}
#[test]
fn constant_time_conditional_memcpy() -> Result<(), error::Unspecified> {
let rng = rand::SystemRandom::new();
for _ in 0..100 {
let mut out = rand::generate::<[u8; 256]>(&rng)?.expose();
let input = rand::generate::<[u8; 256]>(&rng)?.expose();
// Mask to 16 bits to make zero more likely than it would otherwise be.
let b = (rand::generate::<[u8; 1]>(&rng)?.expose()[0] & 0x0f) == 0;
let ref_in = input;
let ref_out = if b { input } else { out };
prefixed_extern! {
fn bssl_constant_time_test_conditional_memcpy(dst: &mut [u8; 256], src: &[u8; 256], b: LimbMask);
}
unsafe {
bssl_constant_time_test_conditional_memcpy(
&mut out,
&input,
if b { LimbMask::True } else { LimbMask::False },
)
}
assert_eq!(ref_in, input);
assert_eq!(ref_out, out);
}
Ok(())
}
#[test]
fn constant_time_conditional_memxor() -> Result<(), error::Unspecified> {
let rng = rand::SystemRandom::new();
for _ in 0..256 {
let mut out = rand::generate::<[u8; 256]>(&rng)?.expose();
let input = rand::generate::<[u8; 256]>(&rng)?.expose();
// Mask to 16 bits to make zero more likely than it would otherwise be.
let b = (rand::generate::<[u8; 1]>(&rng)?.expose()[0] & 0x0f) != 0;
let ref_in = input;
let mut ref_out = out;
if b {
ref_out
.iter_mut()
.zip(ref_in.iter())
.for_each(|(out, input)| {
*out ^= input;
});
}
prefixed_extern! {
fn bssl_constant_time_test_conditional_memxor(dst: &mut [u8; 256], src: &[u8; 256], b: LimbMask);
}
unsafe {
bssl_constant_time_test_conditional_memxor(
&mut out,
&input,
if b { LimbMask::True } else { LimbMask::False },
);
}
assert_eq!(ref_in, input);
assert_eq!(ref_out, out);
}
Ok(())
}
}