cellos_server/
auth.rs

1//! Bearer token authentication.
2//!
3//! The server requires `Authorization: Bearer <token>` on every route.
4//! Tokens are loaded from `CELLOS_SERVER_API_TOKEN` at startup; the
5//! server refuses to start if the env var is unset. Constant-time compare
6//! avoids leaking length/prefix information via timing.
7
8use axum::http::{header, HeaderMap};
9
10use crate::error::{AppError, AppErrorKind};
11
12/// Verify the incoming `Authorization` header against the configured
13/// token. Returns `Ok(())` on match, RFC-9457 `Unauthorized` otherwise.
14pub fn require_bearer(headers: &HeaderMap, expected: &str) -> Result<(), AppError> {
15    let auth = headers
16        .get(header::AUTHORIZATION)
17        .ok_or_else(|| AppError::new(AppErrorKind::Unauthorized, "missing Authorization header"))?
18        .to_str()
19        .map_err(|_| AppError::new(AppErrorKind::Unauthorized, "non-ascii Authorization header"))?;
20
21    let token = auth
22        .strip_prefix("Bearer ")
23        .ok_or_else(|| AppError::new(AppErrorKind::Unauthorized, "expected Bearer scheme"))?;
24
25    if constant_time_eq(token.as_bytes(), expected.as_bytes()) {
26        Ok(())
27    } else {
28        Err(AppError::new(AppErrorKind::Unauthorized, "invalid token"))
29    }
30}
31
32/// Length-aware constant-time compare. Same length AND same bytes → true.
33/// We intentionally do NOT short-circuit on length to avoid leaking the
34/// expected token length to a timing attacker.
35fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
36    // Always iterate over the longer slice so the loop count does not
37    // depend on the secret length.
38    let len = a.len().max(b.len());
39    let mut diff: u8 = (a.len() != b.len()) as u8;
40    for i in 0..len {
41        let av = *a.get(i).unwrap_or(&0);
42        let bv = *b.get(i).unwrap_or(&0);
43        diff |= av ^ bv;
44    }
45    diff == 0
46}
47
48#[cfg(test)]
49mod tests {
50    use super::*;
51
52    #[test]
53    fn missing_header_unauthorized() {
54        let h = HeaderMap::new();
55        assert!(require_bearer(&h, "s3cr3t").is_err());
56    }
57
58    #[test]
59    fn wrong_scheme_unauthorized() {
60        let mut h = HeaderMap::new();
61        h.insert(header::AUTHORIZATION, "Basic abc".parse().unwrap());
62        assert!(require_bearer(&h, "s3cr3t").is_err());
63    }
64
65    #[test]
66    fn wrong_token_unauthorized() {
67        let mut h = HeaderMap::new();
68        h.insert(header::AUTHORIZATION, "Bearer wrong".parse().unwrap());
69        assert!(require_bearer(&h, "s3cr3t").is_err());
70    }
71
72    #[test]
73    fn correct_token_ok() {
74        let mut h = HeaderMap::new();
75        h.insert(header::AUTHORIZATION, "Bearer s3cr3t".parse().unwrap());
76        assert!(require_bearer(&h, "s3cr3t").is_ok());
77    }
78}
cellos_server/auth.rs

cellos_server/
auth.rs
