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
104/// Why a request's credential is refused. The reply does not say whether the
105/// session id or the token was wrong.
106#[derive(Debug, Clone, Copy, PartialEq, Eq, thiserror::Error)]
107#[non_exhaustive]
108pub enum AuthError {
109    /// No `Proxy-Authorization` header.
110    #[error("this proxy serves sessions only: the request carries no session credential")]
111    Missing,
112    /// More than one header, a scheme other than `Basic`, or bad base64.
113    #[error("the session credential is malformed")]
114    Malformed,
115    /// No session has this id and token.
116    #[error("the session credential is not valid")]
117    Invalid,
118}
119
120/// Session mode: the credentials the proxy accepts and where their taint is read.
121pub struct Sessions<'a> {
122    credentials: Vec<Credential>,
123    taint: &'a dyn TaintLookup,
124    /// The last taint looked up per session, and when: what the open
125    /// connections are checked against again ([`Sessions::taint_within`]).
126    recent: Mutex<HashMap<String, (Instant, Taint)>>,
127}
128
129impl fmt::Debug for Sessions<'_> {
130    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
131        f.debug_struct("Sessions")
132            .field("credentials", &self.credentials)
133            .finish_non_exhaustive()
134    }
135}
136
137impl<'a> Sessions<'a> {
138    /// Accept `credentials`; look each session's taint up in `taint`.
139    #[must_use]
140    pub fn new(credentials: Vec<Credential>, taint: &'a dyn TaintLookup) -> Self {
141        Self {
142            credentials,
143            taint,
144            recent: Mutex::new(HashMap::new()),
145        }
146    }
147
148    /// The session that `headers` (every `Proxy-Authorization` value of the
149    /// request) authenticate. Every credential is compared in full, in time
150    /// that does not depend on where a guess first differs.
151    pub fn verify<T: AsRef<[u8]>>(&self, headers: &[T]) -> Result<&str, AuthError> {
152        let value = match headers {
153            [] => return Err(AuthError::Missing),
154            [one] => one.as_ref(),
155            _ => return Err(AuthError::Malformed),
156        };
157        let decoded = basic(value).ok_or(AuthError::Malformed)?;
158        let colon = decoded
159            .iter()
160            .position(|b| *b == b':')
161            .ok_or(AuthError::Malformed)?;
162        let (session, token) = (&decoded[..colon], &decoded[colon + 1..]);
163        let mut found = None;
164        for credential in &self.credentials {
165            let id = constant_time_eq(credential.session.as_bytes(), session);
166            let secret = constant_time_eq(&credential.token, token);
167            if id & secret {
168                found = Some(credential.session.as_str());
169            }
170        }
171        found.ok_or(AuthError::Invalid)
172    }
173
174    /// The taint of `session`, from the lookup.
175    pub fn taint(&self, session: &str) -> Result<Taint, String> {
176        let taint = self.taint.taint(session)?;
177        let mut recent = self.recent.lock().unwrap_or_else(PoisonError::into_inner);
178        recent.insert(session.to_owned(), (Instant::now(), taint.clone()));
179        Ok(taint)
180    }
181
182    /// The taint of `session` as looked up at most `max_age` ago, looked up
183    /// again when it is older. Each open connection of a session is checked
184    /// against its taint every so often; this keeps that to one lookup per
185    /// `max_age` for the whole session. A new connection uses [`Self::taint`].
186    pub fn taint_within(&self, session: &str, max_age: Duration) -> Result<Taint, String> {
187        {
188            let recent = self.recent.lock().unwrap_or_else(PoisonError::into_inner);
189            if let Some((at, taint)) = recent.get(session)
190                && at.elapsed() < max_age
191            {
192                return Ok(taint.clone());
193            }
194        }
195        self.taint(session)
196    }
197}
198
199/// The decoded `user:password` of a `Basic` credential (RFC 7617); the scheme
200/// is matched without regard to case.
201fn basic(value: &[u8]) -> Option<Zeroizing<Vec<u8>>> {
202    let value = value.trim_ascii();
203    let space = value.iter().position(u8::is_ascii_whitespace)?;
204    let (scheme, rest) = value.split_at(space);
205    if !scheme.eq_ignore_ascii_case(b"basic") {
206        return None;
207    }
208    base64(rest.trim_ascii())
209}
210
211/// Whether `a` and `b` are equal, in time that depends only on their lengths.
212#[must_use]
213pub fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
214    if a.len() != b.len() {
215        return false;
216    }
217    let diff = a
218        .iter()
219        .zip(b)
220        .fold(0u8, |acc, (x, y)| acc | std::hint::black_box(x ^ y));
221    std::hint::black_box(diff) == 0
222}
223
224/// Standard base64 (RFC 4648 §4), padding optional; `None` on any other byte.
225fn base64(input: &[u8]) -> Option<Zeroizing<Vec<u8>>> {
226    let data = input
227        .strip_suffix(b"==")
228        .or_else(|| input.strip_suffix(b"="))
229        .unwrap_or(input);
230    let padded = data.len() != input.len();
231    if data.is_empty() || data.len() % 4 == 1 || (padded && !input.len().is_multiple_of(4)) {
232        return None;
233    }
234    let mut out = Zeroizing::new(Vec::with_capacity(data.len() / 4 * 3 + 2));
235    let mut acc: u32 = 0;
236    let mut bits = 0u32;
237    for &c in data {
238        let v = match c {
239            b'A'..=b'Z' => c - b'A',
240            b'a'..=b'z' => c - b'a' + 26,
241            b'0'..=b'9' => c - b'0' + 52,
242            b'+' => 62,
243            b'/' => 63,
244            _ => return None,
245        };
246        acc = (acc << 6) | u32::from(v);
247        bits += 6;
248        if bits >= 8 {
249            bits -= 8;
250            out.push(u8::try_from((acc >> bits) & 0xff).ok()?);
251            acc &= (1 << bits) - 1;
252        }
253    }
254    Some(out)
255}
256
257#[cfg(test)]
258pub(crate) mod tests;
openmoat_proxy/session.rs

openmoat_proxy/
session.rs
