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 → Node 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 three new trees that GRIM doesn’t have (src/nostr/, src/nym/, and src/gui/views/goblin/), plus 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, removes the old clearnet/Tor transport in favor of Nym, 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 nym+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/, nym/, gui/views/goblin/
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.