# base64-ng Professional Secure Plan
Date: 2026-05-13
## Objective
Build `base64-ng` as a modern, secure, `no_std`-first Base64 implementation for Rust. The crate must improve on the established `base64` crate only where evidence supports the claim: stricter release process, stronger verification, zero-copy ergonomics, optional streaming, and hardware acceleration that remains subordinate to proven scalar behavior.
## Current Baseline
- Rust stable: `1.95.0`.
- License: `MIT OR Apache-2.0`.
- Project name: `base64-ng`.
- Runtime and dev dependency graph: zero external crates.
- Local testing system modeled after `fluxheim`: check script, release gate, dependency policy, audit config, Miri when installed, SBOM, reproducible build check, and CI.
## Dependency Policy
The default position is no external crates.
This policy is enforced by:
```sh
scripts/validate-dependencies.sh
```
Allowed without adding dependencies:
- Base64 scalar logic.
- In-place decode.
- `alloc` convenience APIs.
- `std::io` streaming wrappers.
- Architecture SIMD through `core::arch`.
- Runtime CPU dispatch through the Rust standard library where available.
- Deterministic tests, table tests, and handcrafted malformed-input cases.
External crates require written justification before inclusion:
- `tokio` may be accepted only behind the optional `tokio` feature for async stream wrappers.
- Fuzzing crates may live only in a future `fuzz/` workspace or tool-specific harness, not in normal runtime dependencies.
- Benchmark crates may live only in `dev-dependencies` or an isolated bench workspace.
- Kani support must stay feature-gated and must not affect normal users.
Rejected by default:
- Helper crates for simple bit manipulation, table generation, feature selection, error formatting, or CLI tooling.
- Git dependencies.
- Runtime dependencies for the default feature set.
- Dependencies with unclear licensing, advisories, yanked releases, or unnecessary transitive graphs.
Any dependency addition must answer:
- Why can this not be implemented clearly with `core`, `alloc`, or `std`?
- Is it runtime, dev-only, fuzz-only, or CI-only?
- What transitive dependencies does it add?
- Does `cargo deny check`, `cargo audit`, and `cargo license --json` remain clean?
- Can the feature remain optional?
## Architecture
### Layers
1. `core`
- `no_std` scalar implementation.
- Reference semantics for all future fast paths.
- No unsafe code.
2. `alloc`
- `Vec<u8>` decode/encode helpers.
- Encoded `String` helper.
- Optional feature.
3. `simd`
- Future AVX2, AVX-512, and ARM NEON.
- Runtime dispatch under `std`.
- Compile-time target-feature paths for embedded or specialized builds.
- Unsafe isolated and documented.
4. `stream`
- `std::io::{Read, Write}` wrappers.
- Chunk-boundary state machines.
5. `tokio`
- Future async wrappers.
- Explicit optional dependency.
### API Policy
- Stable Rust only for public APIs.
- No `generic_const_exprs` or unstable const-generic tricks in the public surface.
- Use zero-sized engines and trait-based alphabets.
- Strict decoding is default.
- Legacy behavior must be opt-in and named.
## Security Design
### Hard Rules
- Scalar code forbids unsafe.
- Unsafe SIMD must live under dedicated modules.
- Every unsafe block requires a local safety explanation.
- Every SIMD path must have deterministic and fuzzed differential tests against scalar.
- Padding behavior must be canonical by default.
- Whitespace and non-alphabet bytes are rejected by default.
- Public buffer-size calculations must have checked variants for untrusted metadata.
- No dependency without license and advisory review.
### Verification
Phase 1:
- Unit tests for RFC 4648 vectors.
- Integration tests for round trips.
- `cargo clippy` with warnings denied.
- `cargo audit`.
- `cargo deny check`.
- `cargo license --json`.
Phase 2:
- Miri for scalar/in-place APIs.
- cargo-fuzz for malformed inputs, round trips, and differential tests.
- Kani proofs for bounded in-place decoding.
Phase 3:
- SIMD equivalence fuzzing.
- Per-architecture benchmark evidence.
- Release evidence archived under `target/release-evidence`.
## Roadmap
### v0.1
- Scalar strict encoding and decoding.
- Standard and URL-safe alphabets.
- Padded and unpadded modes.
- Caller-owned output buffers.
- Stable compile-time encoding into caller-sized arrays.
- `alloc` `Vec<u8>` and encoded `String` helpers.
- `std::io` streaming wrappers.
- Miri integrated into CI and the local release gate when installed.
- In-place encoding.
- In-place decoding.
- Hardened test/release scripts.
### v0.2
- Document panic-free public API expectations for scalar encode/decode paths.
- Add focused panic-free tests for malformed and size-boundary inputs.
- Strengthen bounded-memory documentation around checked length helpers and
caller-owned output APIs.
- Explicit legacy decode mode.
- Migration guide for projects moving from the `base64` crate, including strict
defaults and compatibility differences.
- More exhaustive malformed-input tests.
- Design an explicit constant-time scalar decode API for sensitive payloads,
separate from the default fast strict decoder.
### v0.3
- Maintain isolated fuzz targets for arbitrary decode input, in-place decoding,
stream chunk boundaries, and differential checks against established Base64
implementations for canonical inputs.
- Provide dependency-free clear-tail encode/decode APIs for callers that want
unused caller-owned output bytes cleared on success and full caller-owned
output buffers cleared on error.
- Reconsider an optional `zeroize` feature only if users require a stronger
best-effort cleanup primitive with a justified dependency tradeoff.
- Add release evidence documentation for audit, license, SBOM, and
reproducibility artifacts.
- Document the unsafe SIMD admission bar before adding architecture-specific
code, keeping the `simd` feature reserved until that evidence exists.
- Isolated scalar comparison benchmark harness first; consider Criterion only
if its larger dependency graph is justified by better measurement quality.
- Prototype and verify the constant-time decode path with no secret-indexed
table lookups during Base64 symbol mapping. Generated-code review remains
required before making a formal cryptographic constant-time guarantee.
### v0.4
- AVX2 and NEON prototypes.
- Runtime feature dispatch.
- Scalar/SIMD differential testing.
- Cross-architecture CI evidence.
- Keep SIMD unsafe code isolated from the scalar core with documented invariants
for every unsafe block.
### v0.5
- AVX-512 implementation.
- CPU dispatch hardening.
- Tokio streaming wrappers.
- Async cancellation and partial-read tests.
- Streaming fuzz and regression tests for adjacent framed payloads.
### v1.0
- Kani proofs complete for scalar in-place decode.
- Formal or tool-backed evidence for panic-free scalar public APIs.
- Published security and migration documentation for strict-by-default adoption.
- Constant-time decode guarantee either formally documented with supporting
verification evidence or explicitly excluded from the stable API contract.
- Fuzz corpus stabilized.
- API freeze.
- Release gate mandatory.
## Release Gate
The release gate must pass before every release:
```sh
scripts/stable_release_gate.sh
```
Minimum required evidence:
- Formatting clean.
- Clippy clean.
- Tests pass under default, all-features, and no-default-features.
- Docs build.
- Dependency licenses accepted.
- RustSec advisories clean.
- Cargo license inventory generated.
- SBOM generated.
- Reproducible package/build check passes.
## Commit Policy
- Commit every completed, verified unit of work.
- Leave pushing to maintainers.
- Never hide failed checks. If a tool is missing locally, record that clearly.