openmoat_proxy/
session.rs

1//! Per-session proxy credentials (ADR-027).
2//!
3//! In session mode every request carries a `Basic` proxy credential whose user
4//! name is the audit session id and whose password is a random token only that
5//! session's processes were given. Clients send it when their proxy URL has
6//! userinfo (`http://<session>:<token>@127.0.0.1:<port>`). A request without a
7//! valid credential is refused (`proxy-session`) before its host is decided.
8//!
9//! The session id then selects the session's taint ([`TaintLookup`]) and its
10//! share of the connections (`slots.rs`).
11
12use std::collections::HashMap;
13use std::fmt;
14use std::net::SocketAddr;
15use std::sync::{Mutex, PoisonError};
16use std::time::{Duration, Instant};
17
18use openmoat_core::Taint;
19use zeroize::Zeroizing;
20
21/// Rule id for a request refused for its session: no valid credential, the
22/// session's connection share used up, or its history unreadable.
23pub const RULE_SESSION: &str = "proxy-session";
24
25/// Shortest token a [`Credential`] accepts, in characters.
26pub const MIN_TOKEN_LEN: usize = 32;
27
28/// Longest session id a [`Credential`] accepts.
29const MAX_SESSION_LEN: usize = 128;
30
31/// One session's credential: its audit session id and its token.
32pub struct Credential {
33    session: String,
34    token: Zeroizing<Vec<u8>>,
35}
36
37impl fmt::Debug for Credential {
38    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
39        // The token never appears in a log line or panic message.
40        f.debug_struct("Credential")
41            .field("session", &self.session)
42            .finish_non_exhaustive()
43    }
44}
45
46/// Why a [`Credential`] cannot be made.
47#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
48#[non_exhaustive]
49pub enum CredentialError {
50    /// The session id is empty, too long, or holds a character other than
51    /// ASCII letters, digits, `.`, `_` and `-`.
52    #[error("a session id is 1 to 128 ASCII letters, digits, `.`, `_` or `-`")]
53    Session,
54    /// The token is shorter than [`MIN_TOKEN_LEN`] or holds such a character.
55    #[error("a session token is at least 32 ASCII letters, digits, `.`, `_` or `-`")]
56    Token,
57}
58
59/// Characters that need no escaping in a URL's userinfo or a Basic credential.
60fn plain(bytes: &[u8]) -> bool {
61    bytes
62        .iter()
63        .all(|b| b.is_ascii_alphanumeric() || matches!(b, b'.' | b'_' | b'-'))
64}
65
66impl Credential {
67    /// The credential of `session`, which presents `token`.
68    pub fn new(session: &str, token: &[u8]) -> Result<Self, CredentialError> {
69        if session.is_empty() || session.len() > MAX_SESSION_LEN || !plain(session.as_bytes()) {
70            return Err(CredentialError::Session);
71        }
72        if token.len() < MIN_TOKEN_LEN || !plain(token) {
73            return Err(CredentialError::Token);
74        }
75        Ok(Self {
76            session: session.to_owned(),
77            token: Zeroizing::new(token.to_vec()),
78        })
79    }
80
81    /// The audit session id.
82    #[must_use]
83    pub fn session(&self) -> &str {
84        &self.session
85    }
86
87    /// `http://<session>:<token>@<addr>`: what the session's `HTTP_PROXY` and
88    /// `HTTPS_PROXY` hold. Zeroed when dropped.
89    #[must_use]
90    pub fn proxy_url(&self, addr: SocketAddr) -> Zeroizing<String> {
91        let token = String::from_utf8_lossy(&self.token);
92        Zeroizing::new(format!("http://{}:{token}@{addr}", self.session))
93    }
94}
95
96/// What a session was exposed to, read when one of its requests is decided.
97/// The CLI reads it from the audit log; tests pass a fixed map.
98pub trait TaintLookup: Send + Sync {
99    /// The taint of `session`. An error refuses the request: without the
100    /// session's history the proxy cannot tell whether it read a secret.
101    fn taint(&self, session: &str) -> Result<Taint, String>;
102
103    /// A mark that changes whenever a taint may have (the audit log's newest
104    /// event id), so a taint looked up before it changed is not used again.
105    /// `None` when there is none: a recent taint is then used for its age.
106    fn version(&self) -> Option<i64> {
107        None
108    }
109}
110
111/// Why a request's credential is refused. The reply does not say whether the
112/// session id or the token was wrong.
113#[derive(Debug, Clone, Copy, PartialEq, Eq, thiserror::Error)]
114#[non_exhaustive]
115pub enum AuthError {
116    /// No `Proxy-Authorization` header.
117    #[error("this proxy serves sessions only: the request carries no session credential")]
118    Missing,
119    /// More than one header, a scheme other than `Basic`, or bad base64.
120    #[error("the session credential is malformed")]
121    Malformed,
122    /// No session has this id and token.
123    #[error("the session credential is not valid")]
124    Invalid,
125}
126
127/// A taint as looked up: when, at which [`TaintLookup::version`], and what.
128type Looked = (Instant, Option<i64>, Taint);
129
130/// Session mode: the credentials the proxy accepts and where their taint is read.
131pub struct Sessions<'a> {
132    credentials: Vec<Credential>,
133    taint: &'a dyn TaintLookup,
134    /// The last taint looked up per session, when, and at which
135    /// [`TaintLookup::version`]: what the open connections are checked against
136    /// again ([`Sessions::taint_within`]).
137    recent: Mutex<HashMap<String, Looked>>,
138}
139
140impl fmt::Debug for Sessions<'_> {
141    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
142        f.debug_struct("Sessions")
143            .field("credentials", &self.credentials)
144            .finish_non_exhaustive()
145    }
146}
147
148impl<'a> Sessions<'a> {
149    /// Accept `credentials`; look each session's taint up in `taint`.
150    #[must_use]
151    pub fn new(credentials: Vec<Credential>, taint: &'a dyn TaintLookup) -> Self {
152        Self {
153            credentials,
154            taint,
155            recent: Mutex::new(HashMap::new()),
156        }
157    }
158
159    /// The session that `headers` (every `Proxy-Authorization` value of the
160    /// request) authenticate. Every credential is compared in full, in time
161    /// that does not depend on where a guess first differs.
162    pub fn verify<T: AsRef<[u8]>>(&self, headers: &[T]) -> Result<&str, AuthError> {
163        let value = match headers {
164            [] => return Err(AuthError::Missing),
165            [one] => one.as_ref(),
166            _ => return Err(AuthError::Malformed),
167        };
168        let decoded = basic(value).ok_or(AuthError::Malformed)?;
169        let colon = decoded
170            .iter()
171            .position(|b| *b == b':')
172            .ok_or(AuthError::Malformed)?;
173        let (session, token) = (&decoded[..colon], &decoded[colon + 1..]);
174        let mut found = None;
175        for credential in &self.credentials {
176            let id = constant_time_eq(credential.session.as_bytes(), session);
177            let secret = constant_time_eq(&credential.token, token);
178            if id & secret {
179                found = Some(credential.session.as_str());
180            }
181        }
182        found.ok_or(AuthError::Invalid)
183    }
184
185    /// The taint of `session`, from the lookup.
186    pub fn taint(&self, session: &str) -> Result<Taint, String> {
187        // the mark before the lookup: an event recorded during it changes it
188        let version = self.taint.version();
189        let taint = self.taint.taint(session)?;
190        let mut recent = self.recent.lock().unwrap_or_else(PoisonError::into_inner);
191        recent.insert(session.to_owned(), (Instant::now(), version, taint.clone()));
192        Ok(taint)
193    }
194
195    /// The taint of `session` as looked up at most `max_age` ago and with no
196    /// event recorded since ([`TaintLookup::version`]), else looked up again.
197    /// Each open connection of a session is checked against its taint every so
198    /// often; this keeps that to one lookup per new event, at most one per
199    /// `max_age`, for the whole session. A new connection uses [`Self::taint`].
200    pub fn taint_within(&self, session: &str, max_age: Duration) -> Result<Taint, String> {
201        let version = self.taint.version();
202        {
203            let recent = self.recent.lock().unwrap_or_else(PoisonError::into_inner);
204            if let Some((at, seen, taint)) = recent.get(session)
205                && at.elapsed() < max_age
206                && *seen == version
207            {
208                return Ok(taint.clone());
209            }
210        }
211        self.taint(session)
212    }
213}
214
215/// The decoded `user:password` of a `Basic` credential (RFC 7617); the scheme
216/// is matched without regard to case.
217fn basic(value: &[u8]) -> Option<Zeroizing<Vec<u8>>> {
218    let value = value.trim_ascii();
219    let space = value.iter().position(u8::is_ascii_whitespace)?;
220    let (scheme, rest) = value.split_at(space);
221    if !scheme.eq_ignore_ascii_case(b"basic") {
222        return None;
223    }
224    base64(rest.trim_ascii())
225}
226
227/// Whether `a` and `b` are equal, in time that depends only on their lengths.
228#[must_use]
229pub fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
230    if a.len() != b.len() {
231        return false;
232    }
233    let diff = a
234        .iter()
235        .zip(b)
236        .fold(0u8, |acc, (x, y)| acc | std::hint::black_box(x ^ y));
237    std::hint::black_box(diff) == 0
238}
239
240/// Standard base64 (RFC 4648 §4), padding optional; `None` on any other byte.
241fn base64(input: &[u8]) -> Option<Zeroizing<Vec<u8>>> {
242    let data = input
243        .strip_suffix(b"==")
244        .or_else(|| input.strip_suffix(b"="))
245        .unwrap_or(input);
246    let padded = data.len() != input.len();
247    if data.is_empty() || data.len() % 4 == 1 || (padded && !input.len().is_multiple_of(4)) {
248        return None;
249    }
250    let mut out = Zeroizing::new(Vec::with_capacity(data.len() / 4 * 3 + 2));
251    let mut acc: u32 = 0;
252    let mut bits = 0u32;
253    for &c in data {
254        let v = match c {
255            b'A'..=b'Z' => c - b'A',
256            b'a'..=b'z' => c - b'a' + 26,
257            b'0'..=b'9' => c - b'0' + 52,
258            b'+' => 62,
259            b'/' => 63,
260            _ => return None,
261        };
262        acc = (acc << 6) | u32::from(v);
263        bits += 6;
264        if bits >= 8 {
265            bits -= 8;
266            out.push(u8::try_from((acc >> bits) & 0xff).ok()?);
267            acc &= (1 << bits) - 1;
268        }
269    }
270    Some(out)
271}
272
273#[cfg(test)]
274pub(crate) mod tests;
openmoat_proxy/session.rs

openmoat_proxy/
session.rs
