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};
10
11/// PKCE code verifier length (43-128 characters per RFC 7636)
12const CODE_VERIFIER_LENGTH: usize = 64;
13
14/// Characters allowed in code verifier (unreserved URI characters)
15const CODE_VERIFIER_CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~";
16
17/// PKCE challenge pair containing verifier and challenge strings.
18#[derive(Debug, Clone)]
19pub struct PkceChallenge {
20    /// The code verifier (random string, kept secret by client)
21    pub code_verifier: String,
22    /// The code challenge (SHA-256 hash of verifier, sent to authorization server)
23    pub code_challenge: String,
24    /// The challenge method (always "S256" for SHA-256)
25    pub code_challenge_method: String,
26}
27
28impl PkceChallenge {
29    /// Create a new PKCE challenge from a code verifier.
30    pub fn from_verifier(code_verifier: String) -> Result<Self> {
31        let code_challenge = compute_s256_challenge(&code_verifier)?;
32        Ok(Self {
33            code_verifier,
34            code_challenge,
35            code_challenge_method: "S256".to_string(),
36        })
37    }
38}
39
40/// Generate a cryptographically secure PKCE challenge pair.
41///
42/// This function generates a random code verifier and computes
43/// the corresponding S256 code challenge.
44///
45/// # Example
46/// ```
47/// use vtcode_auth::generate_pkce_challenge;
48///
49/// let challenge = generate_pkce_challenge().unwrap();
50/// println!("Verifier: {}", challenge.code_verifier);
51/// println!("Challenge: {}", challenge.code_challenge);
52/// ```
53pub fn generate_pkce_challenge() -> Result<PkceChallenge> {
54    let code_verifier = generate_code_verifier()?;
55    PkceChallenge::from_verifier(code_verifier)
56}
57
58/// Generate a cryptographically random code verifier per RFC 7636 §4.1.
59///
60/// Uses `ring::rand::SystemRandom` (backed by the OS CSPRNG) instead of a
61/// user-space PRNG to ensure ≥128 bits of entropy as required by the spec.
62fn generate_code_verifier() -> Result<String> {
63    let rng = SystemRandom::new();
64    let charset_len = CODE_VERIFIER_CHARSET.len() as u8;
65    let max_valid = (256u16 - 256u16 % charset_len as u16) as u8;
66    let mut verifier = String::with_capacity(CODE_VERIFIER_LENGTH);
67    let mut buf = [0u8; 1];
68
69    while verifier.len() < CODE_VERIFIER_LENGTH {
70        rng.fill(&mut buf)
71            .map_err(|_| anyhow::anyhow!("failed to read from OS random source"))?;
72        // Rejection sampling to avoid modulo bias.
73        if buf[0] < max_valid {
74            let idx = (buf[0] % charset_len) as usize;
75            verifier.push(CODE_VERIFIER_CHARSET[idx] as char);
76        }
77    }
78
79    Ok(verifier)
80}
81
82/// Compute S256 code challenge from a code verifier.
83///
84/// S256 = BASE64URL(SHA256(code_verifier))
85fn compute_s256_challenge(code_verifier: &str) -> Result<String> {
86    let mut hasher = Sha256::new();
87    hasher.update(code_verifier.as_bytes());
88    let hash = hasher.finalize();
89
90    Ok(URL_SAFE_NO_PAD.encode(hash))
91}
92
93#[cfg(test)]
94mod tests {
95    use super::*;
96
97    #[test]
98    fn test_generate_pkce_challenge() {
99        let challenge = generate_pkce_challenge().unwrap();
100
101        // Verify code verifier length
102        assert_eq!(challenge.code_verifier.len(), CODE_VERIFIER_LENGTH);
103
104        // Verify all characters are in allowed charset
105        for c in challenge.code_verifier.chars() {
106            assert!(CODE_VERIFIER_CHARSET.contains(&(c as u8)), "Invalid character in verifier: {c}");
107        }
108
109        // Verify challenge method
110        assert_eq!(challenge.code_challenge_method, "S256");
111
112        // Verify challenge is valid base64url (43 chars for SHA-256)
113        assert_eq!(challenge.code_challenge.len(), 43);
114    }
115
116    #[test]
117    fn test_deterministic_challenge() {
118        // Same verifier should produce same challenge
119        let verifier = "test_verifier_string_for_deterministic_test";
120        let challenge1 = PkceChallenge::from_verifier(verifier.to_string()).unwrap();
121        let challenge2 = PkceChallenge::from_verifier(verifier.to_string()).unwrap();
122
123        assert_eq!(challenge1.code_challenge, challenge2.code_challenge);
124    }
125
126    #[test]
127    fn test_unique_verifiers() {
128        // Multiple calls should produce different verifiers
129        let c1 = generate_pkce_challenge().unwrap();
130        let c2 = generate_pkce_challenge().unwrap();
131
132        assert_ne!(c1.code_verifier, c2.code_verifier);
133    }
134}
vtcode_auth/pkce.rs

vtcode_auth/
pkce.rs
