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//! Constant-time comparison utilities to prevent timing-based side-channel attacks.
//!
//! Timing attacks exploit measurable differences in how long comparisons take
//! depending on where they diverge, allowing an attacker to iteratively discover
//! secret values (e.g., HMAC tokens, API keys). All comparisons of secret material
//! must use the functions in this module instead of `==`.
use subtle::ConstantTimeEq;
/// Constant-time comparison utilities for security tokens
/// Uses subtle crate to ensure comparisons take the same time regardless of where differences occur
pub struct ConstantTimeOps;
impl ConstantTimeOps {
/// Compare two byte slices in constant time
///
/// Returns true if equal, false otherwise.
/// Time is independent of where the difference occurs, preventing timing attacks.
///
/// # Arguments
/// * `expected` - The expected (correct/known) value
/// * `actual` - The actual (untrusted) value from the user/attacker
///
/// # Examples
/// ```rust
/// use fraiseql_auth::constant_time::ConstantTimeOps;
/// let stored_token = b"secret_token_value";
/// let user_token = b"user_provided_token";
/// assert!(!ConstantTimeOps::compare(stored_token, user_token));
/// ```
#[must_use]
pub fn compare(expected: &[u8], actual: &[u8]) -> bool {
expected.ct_eq(actual).into()
}
/// Compare two strings in constant time
///
/// Converts strings to bytes and performs constant-time comparison.
/// Useful for comparing JWT tokens, session tokens, or other string-based secrets.
///
/// # Arguments
/// * `expected` - The expected (correct/known) string value
/// * `actual` - The actual (untrusted) string value from the user/attacker
#[must_use]
pub fn compare_str(expected: &str, actual: &str) -> bool {
Self::compare(expected.as_bytes(), actual.as_bytes())
}
/// Compare two slices of possibly different lengths in constant time.
///
/// Compares the common prefix before checking the lengths, so the duration
/// does not depend on *where* the values diverge. It does not hide the
/// lengths themselves — an attacker who can time the call learns roughly how
/// long the shorter input was.
///
/// # SECURITY
/// There is no length-hiding comparison in this module. One used to exist
/// (`compare_padded`) and it was not a comparison: it truncated both inputs
/// to a fixed length first, so two JWTs sharing a 512-byte prefix and
/// differing only in their signatures compared **equal** (#725). If you need
/// length hiding, compare digests of the two values rather than the values.
#[must_use]
pub fn compare_len_safe(expected: &[u8], actual: &[u8]) -> bool {
// If lengths differ, still compare constant-time
// First compare what we can, then check length
let min_len = expected.len().min(actual.len());
let prefix_equal = expected[..min_len].ct_eq(&actual[..min_len]);
let length_equal = u8::from(expected.len() == actual.len());
(prefix_equal.unwrap_u8() & length_equal) != 0
}
}