ring/
constant_time.rs

1// Copyright 2015-2016 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15//! Constant-time operations.
16
17use crate::{c, error};
18
19/// Returns `Ok(())` if `a == b` and `Err(error::Unspecified)` otherwise.
20/// The comparison of `a` and `b` is done in constant time with respect to the
21/// contents of each, but NOT in constant time with respect to the lengths of
22/// `a` and `b`.
23pub fn verify_slices_are_equal(a: &[u8], b: &[u8]) -> Result<(), error::Unspecified> {
24    if a.len() != b.len() {
25        return Err(error::Unspecified);
26    }
27    let result = unsafe { CRYPTO_memcmp(a.as_ptr(), b.as_ptr(), a.len()) };
28    match result {
29        0 => Ok(()),
30        _ => Err(error::Unspecified),
31    }
32}
33
34prefixed_extern! {
35    fn CRYPTO_memcmp(a: *const u8, b: *const u8, len: c::size_t) -> c::int;
36}
37
38#[cfg(test)]
39mod tests {
40    use crate::limb::LimbMask;
41    use crate::{bssl, error, rand};
42
43    #[test]
44    fn test_constant_time() -> Result<(), error::Unspecified> {
45        prefixed_extern! {
46            fn bssl_constant_time_test_main() -> bssl::Result;
47        }
48        Result::from(unsafe { bssl_constant_time_test_main() })
49    }
50
51    #[test]
52    fn constant_time_conditional_memcpy() -> Result<(), error::Unspecified> {
53        let rng = rand::SystemRandom::new();
54        for _ in 0..100 {
55            let mut out = rand::generate::<[u8; 256]>(&rng)?.expose();
56            let input = rand::generate::<[u8; 256]>(&rng)?.expose();
57
58            // Mask to 16 bits to make zero more likely than it would otherwise be.
59            let b = (rand::generate::<[u8; 1]>(&rng)?.expose()[0] & 0x0f) == 0;
60
61            let ref_in = input;
62            let ref_out = if b { input } else { out };
63
64            prefixed_extern! {
65                fn bssl_constant_time_test_conditional_memcpy(dst: &mut [u8; 256], src: &[u8; 256], b: LimbMask);
66            }
67            unsafe {
68                bssl_constant_time_test_conditional_memcpy(
69                    &mut out,
70                    &input,
71                    if b { LimbMask::True } else { LimbMask::False },
72                )
73            }
74            assert_eq!(ref_in, input);
75            assert_eq!(ref_out, out);
76        }
77
78        Ok(())
79    }
80
81    #[test]
82    fn constant_time_conditional_memxor() -> Result<(), error::Unspecified> {
83        let rng = rand::SystemRandom::new();
84        for _ in 0..256 {
85            let mut out = rand::generate::<[u8; 256]>(&rng)?.expose();
86            let input = rand::generate::<[u8; 256]>(&rng)?.expose();
87
88            // Mask to 16 bits to make zero more likely than it would otherwise be.
89            let b = (rand::generate::<[u8; 1]>(&rng)?.expose()[0] & 0x0f) != 0;
90
91            let ref_in = input;
92            let mut ref_out = out;
93            if b {
94                ref_out
95                    .iter_mut()
96                    .zip(ref_in.iter())
97                    .for_each(|(out, input)| {
98                        *out ^= input;
99                    });
100            }
101
102            prefixed_extern! {
103                fn bssl_constant_time_test_conditional_memxor(dst: &mut [u8; 256], src: &[u8; 256], b: LimbMask);
104            }
105            unsafe {
106                bssl_constant_time_test_conditional_memxor(
107                    &mut out,
108                    &input,
109                    if b { LimbMask::True } else { LimbMask::False },
110                );
111            }
112
113            assert_eq!(ref_in, input);
114            assert_eq!(ref_out, out);
115        }
116
117        Ok(())
118    }
119}
ring/constant_time.rs

ring/
constant_time.rs
