snider 22e42d721a feat: SMSG v2 binary format with zstd compression + RFC-001 spec

- Add SMSG v2 format: binary attachments instead of base64 (~25% smaller)
   - Add zstd compression (klauspost/compress) - faster than gzip
   - Add RFC-001: Open Source DRM specification (status: Proposed)
   - Add live demo page at demo.dapp.fm with WASM decryption
   - Add mkdemo tool for generating encrypted demo files
   - Update README with proper documentation
   - Add format examples and failure case documentation

   Demo: https://demo.dapp.fm
   Master Password: PMVXogAJNVe_DDABfTmLYztaJAzsD0R7

2026-01-10 19:57:33 +00:00

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SMSG Format Specification

Overview

SMSG (Secure Message) is an encrypted container format using ChaCha20-Poly1305 authenticated encryption.

File Structure

┌─────────────────────────────────────────┐
│ Magic Number: "SMSG" (4 bytes)          │
├─────────────────────────────────────────┤
│ Version: uint16 (2 bytes)               │
├─────────────────────────────────────────┤
│ Header Length: uint32 (4 bytes)         │
├─────────────────────────────────────────┤
│ Header (JSON, plaintext)                │
│ - algorithm: "chacha20poly1305"         │
│ - manifest: {title, artist, license...} │
│ - nonce: base64                         │
├─────────────────────────────────────────┤
│ Encrypted Payload                       │
│ - Nonce (24 bytes for XChaCha20)        │
│ - Ciphertext + Auth Tag                 │
└─────────────────────────────────────────┘

Magic Number

Binary: 0x53 0x4D 0x53 0x47
ASCII: "SMSG"
Base64 (first 6 chars): "U01TRw"

Header (JSON, unencrypted)

{
  "algorithm": "chacha20poly1305",
  "manifest": {
    "title": "Track Title",
    "artist": "Artist Name",
    "license": "CC-BY-4.0",
    "expires": "2025-12-31T23:59:59Z",
    "tracks": [
      {"title": "Track 1", "start": 0, "trackNum": 1}
    ]
  }
}

The manifest is readable without decryption - this enables:

License validation before decryption
Metadata display in file browsers
Expiration enforcement

Encrypted Payload (JSON)

{
  "from": "artist@example.com",
  "to": "fan@example.com",
  "subject": "Album Title",
  "body": "Thank you for your purchase!",
  "attachments": [
    {
      "name": "track.mp3",
      "mime": "audio/mpeg",
      "content": "<base64-encoded-data>"
    }
  ]
}

Key Derivation

password → SHA-256 → 32-byte key

Simple but effective - the password IS the license key.

Storage Formats

Binary (.smsg)

Raw bytes. Canonical format for distribution.

53 4D 53 47 02 00 00 00 33 00 00 00 7B 22 61 6C ...
S  M  S  G  [ver]  [hdr len]  {"al...

Base64 Text (.smsg or .smsg.b64)

For embedding in JSON, URLs, or text-based transport.

U01TRwIAAAEzeyJhbGdvcml0aG0iOiJjaGFjaGEyMHBvbHkxMzA1Ii...

WASM API

// Initialize
const go = new Go();
await WebAssembly.instantiateStreaming(fetch('stmf.wasm'), go.importObject);
go.run(result.instance);

// Get metadata (no password needed)
const info = await BorgSMSG.getInfo(base64Content);
// info.manifest.title, info.manifest.expires, etc.

// Decrypt (requires password)
const msg = await BorgSMSG.decryptStream(base64Content, password);
// msg.attachments[0].data is Uint8Array (binary)
// msg.attachments[0].mime is MIME type

Security Properties

Authenticated Encryption: ChaCha20-Poly1305 provides both confidentiality and integrity
No Key Escrow: Password never transmitted, derived locally
Metadata Privacy: Only manifest is public; actual content encrypted
Browser-Safe: WASM runs in sandbox, keys never leave client

Use Cases

Use Case	Format	Notes
Direct download	Binary	Most efficient
Email attachment	Base64	Safe for text transport
IPFS/CDN	Binary	Content-addressed
Embedded in JSON	Base64	API responses
Browser demo	Either	Must detect format

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