1 files changed, 84 insertions, 0 deletions
diff --git a/src/crypto.rs b/src/crypto.rs
new file mode 100644
index 0000000..1480040
--- /dev/null
+++ b/src/crypto.rs
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+//! XChaCha20-Poly1305 authenticated encryption.
+//!
+//! Uses OS-provided randomness (`arc4random_buf`) via
+//! `tesseras_dht::sys::random_bytes` for key and nonce
+//! generation.
+
+use chacha20poly1305::{
+    XChaCha20Poly1305, XNonce,
+    aead::{Aead, KeyInit},
+};
+
+/// XChaCha20 extended nonce size (24 bytes).
+const NONCE_SIZE: usize = 24;
+
+/// XChaCha20-Poly1305 key size (32 bytes).
+pub const KEY_SIZE: usize = 32;
+
+/// Generate a random 32-byte encryption key.
+pub fn generate_key() -> [u8; KEY_SIZE] {
+    let mut key = [0u8; KEY_SIZE];
+    tesseras_dht::sys::random_bytes(&mut key);
+    key
+}
+
+/// Encrypt plaintext with a random nonce. Returns `nonce || ciphertext`.
+pub fn encrypt(key: &[u8; KEY_SIZE], plaintext: &[u8]) -> Vec<u8> {
+    let cipher = XChaCha20Poly1305::new(key.into());
+    let mut nonce_bytes = [0u8; NONCE_SIZE];
+    tesseras_dht::sys::random_bytes(&mut nonce_bytes);
+    let nonce = XNonce::from(nonce_bytes);
+    let ciphertext = cipher
+        .encrypt(&nonce, plaintext)
+        .expect("encryption should not fail");
+    let mut out = Vec::with_capacity(NONCE_SIZE + ciphertext.len());
+    out.extend_from_slice(&nonce_bytes);
+    out.extend_from_slice(&ciphertext);
+    out
+}
+
+/// Decrypt `nonce || ciphertext`. Returns `None` if authentication fails.
+pub fn decrypt(key: &[u8; KEY_SIZE], data: &[u8]) -> Option<Vec<u8>> {
+    if data.len() < NONCE_SIZE {
+        return None;
+    }
+    let (nonce_bytes, ciphertext) = data.split_at(NONCE_SIZE);
+    let nonce = XNonce::from_slice(nonce_bytes);
+    let cipher = XChaCha20Poly1305::new(key.into());
+    cipher.decrypt(nonce, ciphertext).ok()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trip() {
+        let key = generate_key();
+        let sealed = encrypt(&key, b"hello");
+        let opened = decrypt(&key, &sealed).unwrap();
+        assert_eq!(opened, b"hello");
+    }
+
+    #[test]
+    fn wrong_key_fails() {
+        let key = generate_key();
+        let wrong = generate_key();
+        let sealed = encrypt(&key, b"secret");
+        assert!(decrypt(&wrong, &sealed).is_none());
+    }
+
+    #[test]
+    fn truncated_fails() {
+        let key = generate_key();
+        assert!(decrypt(&key, &[0u8; 10]).is_none());
+    }
+
+    #[test]
+    fn different_nonces() {
+        let key = generate_key();
+        let a = encrypt(&key, b"same");
+        let b = encrypt(&key, b"same");
+        assert_ne!(a, b);
+    }
+}