//! Envelope encryption: ChaCha20-Poly1305 under a master key. Air-gapped
//! installs use a generated key file (backed up by the operator); a KMS
//! key source slots in behind the same seal/open surface for cloud.

use std::path::Path;

use chacha20poly1305::aead::{Aead, AeadCore, KeyInit, OsRng};
use chacha20poly1305::{ChaCha20Poly1305, Key, Nonce};

use crate::BrokerError;

/// A sealed (encrypted + authenticated) value.
#[derive(Debug, Clone)]
pub struct Sealed {
    pub ciphertext: Vec<u8>,
    pub nonce: Vec<u8>,
}

/// Master key loaded from a hex-encoded 32-byte file, mode 0600.
pub struct FileKey {
    cipher: ChaCha20Poly1305,
}

impl FileKey {
    /// Generates a fresh key file (done once by `clawmates-admin init`).
    pub fn generate(path: &Path) -> Result<(), BrokerError> {
        let key = ChaCha20Poly1305::generate_key(&mut OsRng);
        std::fs::write(path, hex::encode(key)).map_err(|e| BrokerError::Io(e.to_string()))?;
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            std::fs::set_permissions(path, std::fs::Permissions::from_mode(0o600))
                .map_err(|e| BrokerError::Io(e.to_string()))?;
        }
        Ok(())
    }

    pub fn load(path: &Path) -> Result<FileKey, BrokerError> {
        let encoded = std::fs::read_to_string(path).map_err(|e| BrokerError::Io(e.to_string()))?;
        let bytes = hex::decode(encoded.trim())
            .map_err(|e| BrokerError::Crypto(format!("key file is not hex: {e}")))?;
        if bytes.len() != 32 {
            return Err(BrokerError::Crypto(format!(
                "key must be 32 bytes, found {}",
                bytes.len()
            )));
        }
        Ok(FileKey {
            cipher: ChaCha20Poly1305::new(Key::from_slice(&bytes)),
        })
    }

    pub fn seal(&self, plaintext: &[u8]) -> Result<Sealed, BrokerError> {
        let nonce = ChaCha20Poly1305::generate_nonce(&mut OsRng);
        let ciphertext = self
            .cipher
            .encrypt(&nonce, plaintext)
            .map_err(|e| BrokerError::Crypto(e.to_string()))?;
        Ok(Sealed {
            ciphertext,
            nonce: nonce.to_vec(),
        })
    }

    pub fn open(&self, sealed: &Sealed) -> Result<Vec<u8>, BrokerError> {
        if sealed.nonce.len() != 12 {
            return Err(BrokerError::Crypto("nonce must be 12 bytes".into()));
        }
        self.cipher
            .decrypt(Nonce::from_slice(&sealed.nonce), sealed.ciphertext.as_ref())
            .map_err(|_| BrokerError::Crypto("decryption failed (tampered or wrong key)".into()))
    }
}