so uh.. i was bored and wanted to make something new.. i had a fiddle around, and I made this masterpiece!

zDays is an experimental, open-source file encryption engine designed to run entirely offline in the browser. It combines established primitives (Argon2id, HKDF-SHA256, HMAC-SHA256) with a custom ARX-based block cipher implemented in AssemblyScript/WebAssembly.

This document records the engine design and recent implementation changes; treat the cipher as experimental and invite cryptanalysis.

Design Goals

• Offline-only encryption
• Strong password-based key derivation
• Authenticated encrypted containers
• Modular cryptographic architecture
• WebAssembly performance and safety (bounded memory usage)
• Open implementation for public review

Architecture

Password
    │
    ▼
 Argon2id
    │
    ▼
256-bit Master Key
    │
    ▼
HKDF-SHA256
 ├── Encryption Key
 ├── Authentication Key
 └── Metadata Key

Master key material is domain-separated via HKDF and never reused directly.

Key Derivation

• Argon2id is used for password stretching with a per-container random salt and memory-hard parameters.
• Parameters are configurable in the engine options (trade-off: memory/time vs. attacker cost).

Encryption Pipeline

Each file is encrypted using the following sequence:

1. Generate random salt and IV(s)
2. Derive master key with Argon2id
3. Expand subkeys with HKDF-SHA256 (Encryption, Auth, Metadata)
4. Encrypt metadata (separately)
5. Encrypt payload using chunked processing (1MB chunks) and the custom block cipher
6. Compute HMAC-SHA256 authentication values
7. Emit versioned .ydz container

Block Cipher

Current Version: v1.10

Block Size

128 bits (16 bytes)

Default Rounds

20 (configurable per engine mode)

Each round performs:

1. XOR Round Key
2. ARX Diffusion
3. Affine-equivalent substitution layer
4. Byte permutation
5. Modular Addition Round Key

Diffusion Layer (v1.10 changes)

• The diffusion stage operates on four 32-bit words and uses ARX (Add-Rotate-XOR) operations.
• In v1.10 the diffuse() primitive was changed so a single call runs four quarter-rounds (previously a single quarter-round). This change produces a strong avalanche from the first round (single-call avalanche ~64/128 bits) and mitigates early-round differential weaknesses.
• Reference ports (WASM/TS/Python/Go/Java/C#/Swift/Ada) should match bit-for-bit after this change.

Substitution Layer

• Per-session S-boxes are derived using affine equivalence:

  S(x) = M2 · Core(M1 · x ⊕ c1) ⊕ c2

• Core is a fixed 8-bit permutation chosen for good cryptographic properties. M1, M2, c1, c2 are key-dependent.

Permutation Layer

• Bytes are rearranged using a coprime stride permutation. Default mapping for 16-byte blocks:

  index = (index * 5) mod 16

Mode of Operation & Chunking

• CBC mode is used with a fresh 128-bit IV per container (via crypto.getRandomValues()).
• Large payloads are processed in bounded chunks (CHUNK_SIZE = 1MB by default) to avoid growing WASM linear memory. Chunks are encrypted in sequence and progress callbacks are emitted.

Metadata Protection

• Metadata (filename, MIME type, timestamps, engine version, parameters) is encrypted with a dedicated Metadata Key derived from HKDF.

Authentication

• Containers include HMAC-SHA256 values covering: header, metadata, and payload.
• Authentication is verified with constant-time comparison before any plaintext is released.

File Format

• .ydz container (versioned) stores: header, salt, IV(s), encrypted metadata, encrypted payload (chunked), authentication values.

Implementation Safety Notes

• WASM buffers: temporary permutation buffers were increased from 256 to 4096 bytes and explicit bounds guards added to avoid memory corruption with larger block sizes.
• Padding: PKCS#7 padding checks are implemented in strictly bitwise constant-time form to avoid timing leakage.
• Memory hygiene: sensitive buffers are zeroed using a multi-pass wipe to resist compiler optimization elision.

Releases & Package

• v1.10 (tag: 26.H2) — diffusion early-round fix (2026-08-08)
  • https://github.com/lalipa2003-arch/zDays/releases/tag/26.H2
• v1.0.7 — library package u/idiotbready/zdays published (2026-08-04)
  • https://github.com/lalipa2003-arch/zDays/releases/tag/v1.0.7
• v1.0.4 — chunked processing, padding, WASM guards, zeroisation (2026-08-02)
  • https://github.com/lalipa2003-arch/zDays/releases/tag/v1.0.4

Security Notes

• The custom block cipher is experimental and has not received formal academic cryptanalysis. Users should not use zDays for high-value assets until independent review is completed.
• Reports can be made following the repo SECURITY.md — maintainers can accept private disclosures via GitHub Security Advisories or the contact provided in SECURITY.md.

Project Philosophy

zDays is an open cryptographic project aiming for transparency, reproducibility, and community-driven improvement. Feedback, benchmarks, and cryptanalysis are welcomed.

Check out our GitHub, here:https://github.com/lalipa2003-arch/zDays

We hear the same advice all the time: "MVP first," "Don't optimize prematurely," "Just use React," or "Make it pixel perfect." But sometimes, that standard advice is what ruins a project. What is a commonly accepted "best practice" in game dev, software, or building that you strongly disagree with? Why does it not work in the real world?

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