snapcast_server/
crypto.rs

1//! ChaCha20-Poly1305 encryption for audio chunks.
2//!
3//! Derives a 256-bit key from a pre-shared key via HKDF-SHA256.
4//! Each chunk is encrypted with a unique nonce (counter-based).
5
6use chacha20poly1305::aead::{Aead, KeyInit};
7use chacha20poly1305::{ChaCha20Poly1305, Nonce};
8use hkdf::Hkdf;
9use sha2::Sha256;
10
11/// Nonce size (12 bytes).
12const NONCE_SIZE: usize = 12;
13
14/// Derives a 256-bit encryption key from a PSK and salt via HKDF-SHA256.
15fn derive_key(psk: &[u8], salt: &[u8]) -> [u8; 32] {
16    let hk = Hkdf::<Sha256>::new(Some(salt), psk);
17    let mut key = [0u8; 32];
18    hk.expand(b"snapcast-f32lz4e", &mut key)
19        .expect("32 bytes is a valid HKDF-SHA256 output length");
20    key
21}
22
23/// Audio chunk encryptor.
24pub struct ChunkEncryptor {
25    cipher: ChaCha20Poly1305,
26    counter: u64,
27}
28
29impl ChunkEncryptor {
30    /// Create from PSK and session salt.
31    pub fn new(psk: &str, salt: &[u8]) -> Self {
32        let key = derive_key(psk.as_bytes(), salt);
33        Self {
34            cipher: ChaCha20Poly1305::new(&key.into()),
35            counter: 0,
36        }
37    }
38
39    /// Encrypt a chunk. Returns `[12-byte nonce][ciphertext + 16-byte tag]`.
40    pub fn encrypt(&mut self, plaintext: &[u8]) -> Result<Vec<u8>, chacha20poly1305::Error> {
41        let mut nonce_bytes = [0u8; NONCE_SIZE];
42        nonce_bytes[..8].copy_from_slice(&self.counter.to_le_bytes());
43        self.counter += 1;
44
45        let nonce = Nonce::from(nonce_bytes);
46        let ciphertext = self.cipher.encrypt(&nonce, plaintext)?;
47
48        let mut out = Vec::with_capacity(NONCE_SIZE + ciphertext.len());
49        out.extend_from_slice(&nonce_bytes);
50        out.extend_from_slice(&ciphertext);
51        Ok(out)
52    }
53}
54
55/// Audio chunk decryptor — see `snapcast-client` crate for the client-side implementation.
56/// Kept here only for roundtrip tests.
57#[cfg(test)]
58pub struct ChunkDecryptor {
59    cipher: ChaCha20Poly1305,
60}
61
62#[cfg(test)]
63impl ChunkDecryptor {
64    /// Create from PSK and session salt.
65    pub fn new(psk: &str, salt: &[u8]) -> Self {
66        let key = derive_key(psk.as_bytes(), salt);
67        Self {
68            cipher: ChaCha20Poly1305::new(&key.into()),
69        }
70    }
71
72    /// Decrypt a chunk. Input: `[12-byte nonce][ciphertext + 16-byte tag]`.
73    pub fn decrypt(&self, data: &[u8]) -> Result<Vec<u8>, chacha20poly1305::Error> {
74        if data.len() < NONCE_SIZE + 16 {
75            return Err(chacha20poly1305::Error);
76        }
77        let nonce = Nonce::from_slice(&data[..NONCE_SIZE]);
78        self.cipher.decrypt(nonce, &data[NONCE_SIZE..])
79    }
80}
81
82#[cfg(test)]
83mod tests {
84    use super::*;
85
86    #[test]
87    fn encrypt_decrypt_roundtrip() {
88        let salt = b"test-session-salt";
89        let mut enc = ChunkEncryptor::new("my-secret", salt);
90        let dec = ChunkDecryptor::new("my-secret", salt);
91
92        let plaintext = b"hello audio data";
93        let encrypted = enc.encrypt(plaintext).unwrap();
94
95        // 12 nonce + 16 plaintext + 16 tag = 44
96        assert_eq!(encrypted.len(), NONCE_SIZE + plaintext.len() + 16);
97
98        let decrypted = dec.decrypt(&encrypted).unwrap();
99        assert_eq!(decrypted, plaintext);
100    }
101
102    #[test]
103    fn wrong_key_fails() {
104        let salt = b"test-salt";
105        let mut enc = ChunkEncryptor::new("correct-key", salt);
106        let dec = ChunkDecryptor::new("wrong-key", salt);
107
108        let encrypted = enc.encrypt(b"secret audio").unwrap();
109        assert!(dec.decrypt(&encrypted).is_err());
110    }
111
112    #[test]
113    fn nonce_increments() {
114        let salt = b"nonce-test";
115        let mut enc = ChunkEncryptor::new("key", salt);
116
117        let a = enc.encrypt(b"chunk1").unwrap();
118        let b = enc.encrypt(b"chunk2").unwrap();
119
120        // Nonces should differ (first 12 bytes)
121        assert_ne!(&a[..NONCE_SIZE], &b[..NONCE_SIZE]);
122    }
123}
snapcast_server/crypto.rs

snapcast_server/
crypto.rs
