Table of Contents

• What it demonstrates
• How it works
• Build and run
• Source layout
• Wire format
• Security notes
• Related
• License

This demo encrypts and decrypts in the browser with no server in the loop. Pick a cipher, supply a passphrase or keyfile, and produce armored text or a binary .lvthn file. The output is byte-compatible with the lvthn command-line tool, so a file encrypted here decrypts with lvthn decrypt and the reverse.

The code is written to be read. The Encrypt-then-MAC construction, the HMAC input layout, and the scrypt parameters are documented inline in src/app.js.

Ciphers. Three suites are selectable on encrypt: Serpent-256-CBC with HMAC-SHA256 (Encrypt-then-MAC), XChaCha20-Poly1305, and AES-256-GCM-SIV. On decrypt the cipher byte in the file header selects the suite automatically, so no UI flag is needed. Encryption and decryption run through leviathan-crypto's SealStreamPool, which splits the data into chunks across a worker pool.

Passphrase path. scrypt (N=32768, r=8, p=1, dkLen=32) with a fresh random 32-byte salt, producing a 32-byte master key. The parameters match the lvthn CLI exactly.

Keyfile path. The keyfile is read as 32 raw bytes and used directly as the master key. The keygen panel produces 32 random bytes (256-bit) from WebCrypto; download it as a binary keyfile or copy the hex. A key generated here or by lvthn keygen works in either tool.

Requires Bun.

bun install
bun bake
# → dist/index.html

Open dist/index.html in any modern browser. No server required.

bake runs build.ts, which pre-bundles the cipher pool workers, bundles leviathan.web-entry.ts with Bun, and inlines that bundle, src/style.css, src/app.js, and its helper modules into src/template.html. The result is a self-contained HTML file with no external runtime dependencies.

web/
├── src/
│   ├── template.html          # page structure and markup
│   ├── style.css              # all styles
│   ├── app.js                 # encrypt, decrypt, keygen UI logic
│   ├── crypto.js              # scrypt derivation and key handling
│   ├── format.js              # LVTHN header read/write
│   ├── cipher-suites.js       # pool cipher suite wrappers
│   └── workers/               # generated cipher pool workers
├── leviathan.web-entry.ts     # leviathan-crypto bundle entry
├── build.ts                   # build script
└── dist/
    └── index.html             # built output

Files use the LVTHN v3 format, byte-compatible with the lvthn CLI across all three cipher suites.

Offset  Size  Field    Description
------  ----  -----    -----------
0       5     magic    "LVTHN" (0x4c 0x56 0x54 0x48 0x4e)
5       1     version  0x03
6       1     cipher   0x01 = Serpent-256-CBC + HMAC-SHA256
                       0x02 = XChaCha20-Poly1305
                       0x03 = AES-256-GCM-SIV
7       1     kdf      0x01 = scrypt (N=32768, r=8, p=1, dkLen=32)
                       0x02 = keyfile (raw 32 bytes)
8       1     flags    0x00 (reserved)
9       32    salt     scrypt salt; all-zero for keyfile mode
41+     N     payload  SealStreamPool output (20-byte preamble + chunks)

The cipher byte at offset 6 selects the pool on decrypt. See cli/FORMAT.md for the full specification, including the pool preamble layout and the counter-nonce scheme.

• Encrypt-then-MAC. The Serpent suite authenticates the header (with the HMAC field zeroed) concatenated with the ciphertext, so any modification fails before decryption.
• Authenticated decryption. Every suite rejects a tampered file rather than returning partial plaintext.
• Client-side only. Key material never leaves the browser, and nothing is sent over the network after the page loads.

• leviathan-crypto: the underlying library
• tamper demo, kyber demo, jwt demo, cli tool
• covcom: a production secure messenger built on the same library
• LVTHN wire format: full binary specification
• Live demo: leviathan.3xi.club/web

Leviathan and its demos are written under the MIT license.

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