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: i32 = unsafe { CRYPTO_memcmp(a:a.as_ptr(), b: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}
120