Honzo supports content encryption via an EXTRA entry under the org.nisoku.drm namespace. The design separates content encryption from key management. Chapters are encrypted with a random content encryption key (CEK). The CEK is then wrapped for each authorized recipient.

Model

Key Envelope

The DRM envelope is a MessagePack map stored in the EXTRA entry:

{
  "version": 1,
  "content_encryption_alg": "AES-256-GCM",
  "key_wrapping_alg": "ECDH-X25519-HKDF-SHA256",
  "license_url": "https://example.com/license/abc123",
  "expiry": "2026-01-01T00:00:00Z",
  "recipients": [
    {
      "ephemeral_pub": "<32 bytes base64>",
      "encrypted_cek": "<48 bytes base64>",
      "recipient_id": "[email protected]"
    }
  ]
}

Each recipient gets their own envelope entry. The CEK is wrapped using ECDH with X25519:

Reading Encrypted Files

use honzo_io::HonzoReader;

let private_key: [u8; 32] = load_private_key();

let mut reader = HonzoReader::new(&data, 1)
    .with_drm_key(&private_key);
reader.parse().unwrap();

for entry in reader.toc_entries() {
    let chunk_data = reader.read_chunk(&entry).unwrap();
}

If a DRM key is not provided, encrypted chunks cannot be read:

let reader = HonzoReader::new(&data, 1);
reader.parse().unwrap();

match reader.read_chunk(&entry) {
    Err(e) if e.is_drm_error() => {
        println!("This chapter requires a DRM key");
    }
    _ => {}
}

Building Encrypted Files

use honzo_io::{HonzoBuilder, DrmConfig};

let drm = DrmConfig::new()
    .encrypt_chunks(&[0, 1, 2])
    .add_recipient(&alice_pub_key, "[email protected]")
    .expiry("2026-01-01T00:00:00Z")
    .license_url("https://example.com/license/abc123");

let hzo = HonzoBuilder::new()
    .with_drm(drm)
    .add_chapter("Chapter 1", ...)
    .add_chapter("Chapter 2", ...)
    .add_chapter("Chapter 3", ...)
    .finalize()
    .unwrap();

Supported Algorithms

Next Steps