94 lines
2.4 KiB
Rust
94 lines
2.4 KiB
Rust
//! Utilities to safely compare cryptographic values.
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//!
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//! Extra care must be taken when comparing values in
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//! cryptographic code. If done incorrectly, it can lead
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//! to a [timing attack](https://en.wikipedia.org/wiki/Timing_attack).
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//! By analyzing the time taken to execute parts of a cryptographic
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//! algorithm, and attacker can attempt to compromise the
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//! cryptosystem.
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//!
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//! The utilities in this module are designed to be resistant
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//! to this type of attack.
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//!
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//! # Examples
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//!
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//! To perform a constant-time comparison of two arrays of the same length but different
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//! values:
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//!
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//! ```
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//! use boring2::memcmp::eq;
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//!
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//! // We want to compare `a` to `b` and `c`, without giving
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//! // away through timing analysis that `c` is more similar to `a`
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//! // than `b`.
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//! let a = [0, 0, 0];
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//! let b = [1, 1, 1];
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//! let c = [0, 0, 1];
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//!
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//! // These statements will execute in the same amount of time.
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//! assert!(!eq(&a, &b));
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//! assert!(!eq(&a, &c));
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//! ```
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use crate::ffi;
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use libc::size_t;
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/// Returns `true` iff `a` and `b` contain the same bytes.
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///
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/// This operation takes an amount of time dependent on the length of the two
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/// arrays given, but is independent of the contents of a and b.
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///
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/// # Panics
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///
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/// This function will panic the current task if `a` and `b` do not have the same
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/// length.
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///
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/// # Examples
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///
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/// To perform a constant-time comparison of two arrays of the same length but different
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/// values:
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///
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/// ```
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/// use boring2::memcmp::eq;
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///
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/// // We want to compare `a` to `b` and `c`, without giving
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/// // away through timing analysis that `c` is more similar to `a`
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/// // than `b`.
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/// let a = [0, 0, 0];
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/// let b = [1, 1, 1];
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/// let c = [0, 0, 1];
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///
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/// // These statements will execute in the same amount of time.
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/// assert!(!eq(&a, &b));
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/// assert!(!eq(&a, &c));
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/// ```
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#[must_use]
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pub fn eq(a: &[u8], b: &[u8]) -> bool {
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assert!(a.len() == b.len());
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let ret = unsafe {
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ffi::CRYPTO_memcmp(
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a.as_ptr() as *const _,
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b.as_ptr() as *const _,
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a.len() as size_t,
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)
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};
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ret == 0
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}
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#[cfg(test)]
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mod tests {
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use super::eq;
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#[test]
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fn test_eq() {
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assert!(eq(&[], &[]));
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assert!(eq(&[1], &[1]));
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assert!(!eq(&[1, 2, 3], &[1, 2, 4]));
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}
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#[test]
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#[should_panic]
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fn test_diff_lens() {
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let _ = eq(&[], &[1]);
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}
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}
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