vtcode_auth/
pkce.rs

1//! PKCE (Proof Key for Code Exchange) utilities for OAuth 2.0.
2//!
3//! Implements RFC 7636 for secure OAuth flows without client secrets.
4//! Uses SHA-256 (S256) code challenge method as recommended by the spec.
5
6use anyhow::Result;
7use base64::{Engine, engine::general_purpose::URL_SAFE_NO_PAD};
8use ring::rand::{SecureRandom, SystemRandom};
9use sha2::{Digest, Sha256};
10use std::fmt;
11
12/// PKCE code verifier length (43-128 characters per RFC 7636)
13const CODE_VERIFIER_LENGTH: usize = 64;
14
15/// Characters allowed in code verifier (unreserved URI characters)
16const CODE_VERIFIER_CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~";
17
18/// PKCE challenge pair containing verifier and challenge strings.
19///
20/// Custom `Debug` redacts the `code_verifier` — it is a secret that must
21/// never leave the client. The `code_challenge` and method are safe to
22/// display (they are sent to the authorization server in the URL).
23#[derive(Clone)]
24pub struct PkceChallenge {
25    /// The code verifier (random string, kept secret by client)
26    pub(crate) code_verifier: String,
27    /// The code challenge (SHA-256 hash of verifier, sent to authorization server)
28    pub(crate) code_challenge: String,
29    /// The challenge method (always "S256" for SHA-256)
30    pub(crate) code_challenge_method: String,
31}
32
33impl fmt::Debug for PkceChallenge {
34    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
35        f.debug_struct("PkceChallenge")
36            .field("code_verifier", &"<redacted>")
37            .field("code_challenge", &self.code_challenge)
38            .field("code_challenge_method", &self.code_challenge_method)
39            .finish()
40    }
41}
42
43impl PkceChallenge {
44    /// Create a new PKCE challenge from a code verifier.
45    fn from_verifier(code_verifier: String) -> Result<Self> {
46        let code_challenge = compute_s256_challenge(&code_verifier)?;
47        Ok(Self {
48            code_verifier,
49            code_challenge,
50            code_challenge_method: "S256".to_string(),
51        })
52    }
53}
54
55/// Generate a cryptographically secure PKCE challenge pair.
56///
57/// This function generates a random code verifier and computes
58/// the corresponding S256 code challenge.
59///
60/// # Example
61/// ```
62/// use vtcode_auth::generate_pkce_challenge;
63///
64/// let challenge = generate_pkce_challenge().unwrap();
65/// // code_verifier is a secret — never print or log it.
66/// assert!(!challenge.code_challenge.is_empty());
67/// ```
68pub fn generate_pkce_challenge() -> Result<PkceChallenge> {
69    let code_verifier = generate_code_verifier()?;
70    PkceChallenge::from_verifier(code_verifier)
71}
72
73/// Generate a cryptographically random code verifier per RFC 7636 §4.1.
74///
75/// Uses `ring::rand::SystemRandom` (backed by the OS CSPRNG) instead of a
76/// user-space PRNG to ensure ≥128 bits of entropy as required by the spec.
77fn generate_code_verifier() -> Result<String> {
78    let rng = SystemRandom::new();
79    let charset_len = CODE_VERIFIER_CHARSET.len() as u8;
80    let max_valid = (256u16 - 256u16 % charset_len as u16) as u8;
81    let mut verifier = String::with_capacity(CODE_VERIFIER_LENGTH);
82    let mut buf = [0u8; 1];
83
84    while verifier.len() < CODE_VERIFIER_LENGTH {
85        rng.fill(&mut buf)
86            .map_err(|_| anyhow::anyhow!("failed to read from OS random source"))?;
87        // Rejection sampling to avoid modulo bias.
88        if buf[0] < max_valid {
89            let idx = (buf[0] % charset_len) as usize;
90            verifier.push(CODE_VERIFIER_CHARSET[idx] as char);
91        }
92    }
93
94    Ok(verifier)
95}
96
97/// Compute S256 code challenge from a code verifier.
98///
99/// S256 = BASE64URL(SHA256(code_verifier))
100fn compute_s256_challenge(code_verifier: &str) -> Result<String> {
101    let mut hasher = Sha256::new();
102    hasher.update(code_verifier.as_bytes());
103    let hash = hasher.finalize();
104
105    Ok(URL_SAFE_NO_PAD.encode(hash))
106}
107
108#[cfg(test)]
109mod tests {
110    use super::*;
111
112    #[test]
113    fn test_generate_pkce_challenge() {
114        let challenge = generate_pkce_challenge().unwrap();
115
116        // Verify code verifier length
117        assert_eq!(challenge.code_verifier.len(), CODE_VERIFIER_LENGTH);
118
119        // Verify all characters are in allowed charset
120        for c in challenge.code_verifier.chars() {
121            assert!(CODE_VERIFIER_CHARSET.contains(&(c as u8)), "Invalid character in verifier: {c}");
122        }
123
124        // Verify challenge method
125        assert_eq!(challenge.code_challenge_method, "S256");
126
127        // Verify challenge is valid base64url (43 chars for SHA-256)
128        assert_eq!(challenge.code_challenge.len(), 43);
129    }
130
131    #[test]
132    fn test_deterministic_challenge() {
133        // Same verifier should produce same challenge
134        let verifier = "test_verifier_string_for_deterministic_test";
135        let challenge1 = PkceChallenge::from_verifier(verifier.to_string()).unwrap();
136        let challenge2 = PkceChallenge::from_verifier(verifier.to_string()).unwrap();
137
138        assert_eq!(challenge1.code_challenge, challenge2.code_challenge);
139    }
140
141    #[test]
142    fn test_unique_verifiers() {
143        // Multiple calls should produce different verifiers
144        let c1 = generate_pkce_challenge().unwrap();
145        let c2 = generate_pkce_challenge().unwrap();
146
147        assert_ne!(c1.code_verifier, c2.code_verifier);
148    }
149}
vtcode_auth/pkce.rs

vtcode_auth/
pkce.rs
