ADR-0001 — Rust core with UniFFI bindings

• Status: Accepted
• Date: 2026-07-11

Context

One protocol implementation must serve desktop, iOS, Android, headless relays, and eventually microcontroller-class mesh gateways. Duplicate implementations of security code across platforms multiply audit surface and guarantee divergence.

Decision

All crypto, protocol, transport, and storage logic lives in one Rust workspace (kult-* crates). Clients consume it through UniFFI-generated bindings (Kotlin, Swift) and Tauri (desktop). kult-crypto stays no_std+alloc compatible.

Alternatives considered

• Go: mature go-libp2p, but gomobile FFI is clumsy, GC pauses and binary size hurt on mobile, and there is no credible embedded story.
• TypeScript/Node: fastest UI iteration, unacceptable for constant-time crypto and off-grid/embedded targets.
• C/C++ core: maximum portability, memory-unsafety in exactly the code that must not have it.

Consequences

Single audited implementation; memory safety in the TCB; strongest ecosystem fit (RustCrypto, rust-libp2p, zeroize/subtle). Cost: UniFFI binding maintenance and slower UI iteration than a pure-JS stack — accepted.