Function write_encrypted_segment

nodedb/src/storage/segment.rs:147–184

Write a segment file with optional encryption. Layout (encrypted): `[preamble(16B plaintext)] [ciphertext(data_len + 16B auth_tag)] [footer(58B plaintext)]` Layout (unencrypted): `[data] [footer(58B plaintext)]` The preamble contains a freshly-generated random epoch. The nonce used for AES-256-GCM is `(preamble.epoch, min_lsn)`. The preamble bytes and a fixed AAD tag are included as Additional

(
    path: &Path,
    data: &[u8],
    footer: &SegmentFooter,
    key: Option<&nodedb_wal::crypto::WalEncryptionKey>,
)

/// Two segments that share `min_lsn` (e.g. LSN=0 at bootstrap) will have
/// different epochs and therefore non-colliding nonces.
pub fn write_encrypted_segment(
    path: &Path,
    data: &[u8],
    footer: &SegmentFooter,
    key: Option<&nodedb_wal::crypto::WalEncryptionKey>,
) -> crate::Result<()> {
    let mut file = std::fs::File::create(path)?;

    if let Some(key) = key {
        // Create a fresh-epoch key for this segment. Each segment write gets
        // its own epoch so even two segments with the same min_lsn (e.g.
        // both starting at LSN 0 at bootstrap) have non-colliding nonces.
        // `with_fresh_epoch` re-uses the same AES key bytes but generates a
        // new random epoch — the nonce for encryption will use this epoch,
        // and we record the same epoch in the preamble.
        let fresh_key = key.with_fresh_epoch().map_err(crate::Error::Wal)?;
        let epoch = *fresh_key.epoch();
        let preamble = SegmentPreamble::new_seg(epoch);
        let preamble_bytes = preamble.to_bytes();

        // AAD = preamble_bytes (binds ciphertext to this segment's preamble).
        let ciphertext = fresh_key
            .encrypt_aad(footer.min_lsn.as_u64(), &preamble_bytes, data)
            .map_err(|e| crate::Error::Storage {
                engine: "segment".into(),
                detail: format!("segment encryption failed: {e}"),
            })?;

        file.write_all(&preamble_bytes)?;
        file.write_all(&ciphertext)?;
    } else {
        file.write_all(data)?;
    }

    file.write_all(&footer.to_bytes())?;
    file.flush()?;
    Ok(())
}

/// Encrypt segment data into a self-describing byte envelope (no file I/O).
///