//! Sealed artifact format definition.
//!
//! Binary layout (big-endian):
//!
//! ```text
//! Offset  Size   Field
//! ──────  ─────  ─────────────────────────────────────────────────────────
//! 0       8      magic: b"ENGRAM01"
//! 8       2      version: u16 (currently 1)
//! 10      *      JSON-encoded SealedArtifact body (algorithm_id, nonce, …)
//! ```
//!
//! The body is JSON so the format is self-describing and forward-compatible.
//! Future versions can add new fields without breaking older parsers.

use serde::{Deserialize, Serialize};

/// Magic header bytes — identify an Engram sealed artifact.
pub const MAGIC: [u8; 8] = *b"ENGRAM01";

/// Current artifact format version.
pub const FORMAT_VERSION: u16 = 1;

// ── Configuration ─────────────────────────────────────────────────────────────

/// Which algorithm was used to seal the artifact.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum SealAlgorithm {
    /// AES-256-GCM — current default, quantum-resistant at 256-bit.
    Aes256Gcm,
    /// CRYSTALS-Kyber 768 — when ml-kem crate stabilizes.
    MlKem768,
    /// CRYSTALS-Kyber 1024 — when ml-kem crate stabilizes.
    MlKem1024,
}

impl SealAlgorithm {
    pub fn id(&self) -> &'static str {
        match self {
            SealAlgorithm::Aes256Gcm => "aes256gcm-v1",
            SealAlgorithm::MlKem768 => "mlkem768-v1",
            SealAlgorithm::MlKem1024 => "mlkem1024-v1",
        }
    }
}

/// How the deployment key is derived / bound.
#[derive(Debug, Clone)]
pub enum DeploymentBinding {
    /// Read the seal key from an environment variable (e.g. `ENGRAM_SEAL_KEY`).
    EnvironmentKey(String),
    /// Bind to this machine's hostname + OS + CPU model (BLAKE3 hash of all three).
    MachineFingerprint,
    /// No binding — key is the zero vector. For testing only; offers no security.
    None,
}

/// Configuration for the sealing operation.
#[derive(Debug, Clone)]
pub struct SealConfig {
    pub algorithm: SealAlgorithm,
    pub deployment_binding: DeploymentBinding,
}

impl Default for SealConfig {
    fn default() -> Self {
        Self {
            algorithm: SealAlgorithm::Aes256Gcm,
            deployment_binding: DeploymentBinding::EnvironmentKey("ENGRAM_SEAL_KEY".into()),
        }
    }
}

// ── Artifact ──────────────────────────────────────────────────────────────────

/// A quantum-sealed bytecode artifact.
///
/// This is the output of `el seal` / the `prod` compilation target.
/// It is serialized to disk as: `MAGIC || VERSION_u16_be || JSON(body)`.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SealedArtifact {
    /// Algorithm identifier — matches [`SealAlgorithm::id()`].
    pub algorithm_id: String,

    /// BLAKE3-keyed MAC over `(algorithm_id || nonce || ciphertext)`.
    /// Used to detect tampering before attempting decryption.
    /// In the PQ upgrade path, this becomes an ML-DSA signature.
    pub signature: Vec<u8>,

    /// The binding-protected symmetric key.
    ///
    /// In the current scheme: `symmetric_key XOR BLAKE3(binding_material)`.
    /// Without the deployment key, the binding material cannot be derived,
    /// so the symmetric key cannot be recovered.
    ///
    /// In the ML-KEM upgrade: this becomes the KEM-encapsulated key ciphertext.
    pub encapsulated_key: Vec<u8>,

    /// 96-bit AES-GCM nonce.
    pub nonce: Vec<u8>,

    /// Encrypted bytecode (AES-256-GCM ciphertext including the 128-bit auth tag).
    pub ciphertext: Vec<u8>,

    /// BLAKE3 hash of the binding material. Allows the unsealer to verify
    /// it is running in the correct deployment environment before decryption.
    /// `None` if [`DeploymentBinding::None`] was used.
    pub deployment_fingerprint: Option<Vec<u8>>,
}

impl SealedArtifact {
    /// Serialize to the on-disk wire format: `MAGIC || version_be16 || JSON`.
    pub fn to_bytes(&self) -> Result<Vec<u8>, serde_json::Error> {
        let mut out = Vec::with_capacity(256);
        out.extend_from_slice(&MAGIC);
        out.extend_from_slice(&FORMAT_VERSION.to_be_bytes());
        let json = serde_json::to_vec(self)?;
        out.extend_from_slice(&json);
        Ok(out)
    }

    /// Deserialize from the on-disk wire format.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, crate::SealError> {
        if bytes.len() < 10 {
            return Err(crate::SealError::Serialization("artifact too short".into()));
        }
        let magic: [u8; 8] = bytes[..8].try_into().unwrap();
        if magic != MAGIC {
            return Err(crate::SealError::InvalidMagic(magic));
        }
        // Version check (bytes 8..10) — currently we only support v1
        let version = u16::from_be_bytes([bytes[8], bytes[9]]);
        if version != FORMAT_VERSION {
            return Err(crate::SealError::UnsupportedAlgorithm(format!("format v{version}")));
        }
        let body = &bytes[10..];
        serde_json::from_slice(body).map_err(|e| crate::SealError::Serialization(e.to_string()))
    }
}