GRIM: the wallet engine Goblin forks

Summary. Goblin is a fork of GRIM, a cross-platform Grin wallet and integrated node written in Rust on egui. Goblin keeps GRIM’s entire money engine (seed/key management, node and chain sync, and the Mimblewimble slatepack transaction state machine) unmodified, and adds its payments experience in new modules alongside it.

Motivation

Writing a correct cryptocurrency wallet is hard and security-critical; writing a Grin wallet and a full Grin node is harder still. GRIM had already done that work: a mature, audited egui app with the complete Grin stack vendored in. Rather than reimplement any of it, Goblin forks GRIM and treats it as a stable foundation, so all of Goblin’s new code is about messaging and transport, not about money primitives. This keeps the risky surface (key handling, transaction building, consensus) on well-trodden upstream code.

How it works

GRIM bundles the Grin node and wallet libraries as path dependencies and drives them from an egui UI. Goblin inherits all of that:

• Seed & keys. BIP-39 mnemonic (12–24 words), the wallet master seed, and Grin’s output/rangeproof key derivation: all GRIM/Grin code. Your Grin funds are controlled by this seed. (Goblin’s nostr identity is deliberately separate; see Identity.)
• Integrated node + sync. GRIM can run a full Grin node or talk to an external one, track the chain tip, and scan for your outputs. Goblin exposes this under Settings → Advanced but does not change it.
• The slatepack transaction machine. Grin’s interactive flow (Standard (send) and Invoice (request), each a two-step slate exchange) and the slatepack armor encoding live in the Grin wallet library. Goblin’s entire job is to carry these slatepacks; it never alters how they’re built or validated.
• The egui shell & platform layer. Window, fonts, Android/desktop entry points, camera, and storage abstractions come from GRIM.

What Goblin adds lives in two new trees that GRIM doesn’t have (src/nostr/ and src/gui/views/goblin/), plus a src/tor/ transport copied from GRIM’s own proven Tor engine and pointed at the relay, and a handful of hooks into the wallet lifecycle (src/wallet/wallet.rs) to start/stop the Nostr service and dispatch WalletTask::Nostr* jobs.

What Goblin changes in upstream is intentionally minimal: it swaps the default presented surface to the Goblin UI, routes all of its Nostr and HTTP traffic through that embedded Tor transport, and rebrands. The original GRIM tree is kept side-by-side (at ../grim) precisely so the fork can be diffed and stay close to upstream.

Reference

• Crate identity. The package is still named grim (version = "0.3.6"), the binary is goblin; see goblin/Cargo.toml ([package], [[bin]]). The node/wallet libraries are path deps: grin_api = { path = "node/api" }, grin_chain, grin_wallet_*, etc.
• Versioning is build-number based, off the fork point. goblin/build.rs defines GOBLIN_FORK_BASE = "b51a46b" (the GRIM commit Goblin forked from) and computes Build N = number of commits since the fork via git rev-list. An explicit GOBLIN_BUILD env var overrides it (used by CI single-commit public builds). So Goblin ships “Build 97,” not a semver.
• Release profiles. [profile.release] strips symbols (the tor+nostr+grin tree is ~16 MB of symbols); [profile.release-apk] adds opt-level="z", lto, panic="abort" for Android size.
• Upstream. GRIM lives at https://code.gri.mw/GUI/grim (author Ardocrat). Goblin’s repository field still points there.

To see exactly what the fork changed, diff the two trees:

diff -ru ../grim/src ./src        # new: nostr/, gui/views/goblin/; tor/ from GRIM
diff -ru ../grim/Cargo.toml ./Cargo.toml

References

• goblin/Cargo.toml: package name, binary, path deps, release profiles.
• goblin/build.rs:5-6: GOBLIN_FORK_BASE = "b51a46b"; build-count logic at :11-34.
• goblin/README.md: “Goblin is a fork of the Grim egui GRIN wallet…”.
• Reference copy of upstream GRIM: ../grim (sibling of the goblin tree).
• egui: https://github.com/emilk/egui.