feat(git-sign-nostr): implement NIP-GS git object signing with Nostr keys (#459)

Author: tlongwell-block

Cargo.lock

@@ -19,6 +19,7 @@ members = [
     "crates/sprout-sdk",
     "crates/sprout-persona",
     "crates/git-credential-nostr",
+    "crates/git-sign-nostr",
 ]
 exclude = ["desktop/src-tauri"]
 resolver = "2"

Cargo.toml

@@ -0,0 +1,41 @@
+[package]
+name = "git-sign-nostr"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "NIP-GS git commit/tag signing program using Nostr secp256k1 keys"
+readme = "README.md"
+publish = false  # internal workspace tool, not published to crates.io
+
+[[bin]]
+name = "git-sign-nostr"
+path = "src/main.rs"
+
+[dependencies]
+# Base64 armor encoding/decoding for NIP-GS signature envelopes.
+# Not in workspace deps — each crate pins independently (same pattern as
+# sprout-relay, sprout-mcp, sprout-cli, git-credential-nostr).
+base64 = "0.22"
+
+# Hex encoding for BIP-340 signatures and public keys.
+hex = { workspace = true }
+
+# Secret key zeroization on drop.
+zeroize = { workspace = true, features = ["derive"] }
+
+# Nostr key parsing (nsec/npub bech32), secp256k1 Schnorr signing, SHA-256.
+# Uses the full default feature set because we need: Keys, PublicKey,
+# FromBech32, and the re-exported bitcoin::secp256k1 + bitcoin::hashes.
+nostr = { workspace = true }
+
+# JSON parsing for NIP-OA auth tag and envelope verification.
+serde_json = { workspace = true }
+
+# Timestamp formatting for GnuPG VALIDSIG status lines.
+chrono = { workspace = true }
+
+# Unix-specific: O_NOFOLLOW for keyfile open, fcntl for fd validation.
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"

crates/git-sign-nostr/Cargo.toml

@@ -0,0 +1,46 @@
+# git-sign-nostr
+
+NIP-GS signing program — signs git commits and tags with Nostr secp256k1 keys
+using BIP-340 Schnorr signatures.
+
+## Usage
+
+```bash
+# Configure git to use nostr signing
+git config gpg.format x509
+git config gpg.x509.program /path/to/git-sign-nostr
+git config commit.gpgsign true
+git config tag.gpgsign true
+git config user.signingkey <hex-pubkey>
+
+# Set the private key (env var)
+export NOSTR_PRIVATE_KEY=<hex-or-nsec>
+
+# Optional: NIP-OA owner attestation
+export SPROUT_AUTH_TAG='["auth","<owner-pk>","<conditions>","<owner-sig>"]'
+
+# Commits are now automatically signed
+git commit -m "signed with nostr"
+
+# Verify
+git verify-commit HEAD
+```
+
+## Key Loading Priority
+
+1. `NOSTR_PRIVATE_KEY` environment variable
+2. `SPROUT_PRIVATE_KEY` environment variable
+3. Keyfile at path from `git config nostr.keyfile`
+
+Keys may be hex (64 chars) or NIP-19 bech32 (`nsec1...`).
+
+## How It Works
+
+Git invokes this program as a signing/verification backend:
+
+- **Sign:** `git-sign-nostr --status-fd=2 -bsau <keyid>` — reads payload from
+  stdin, writes armored signature to stdout, status lines to fd 2 (stderr)
+- **Verify:** `git-sign-nostr --status-fd=1 --verify <sigfile> -` — reads
+  payload from stdin, verifies signature from file, status lines to fd 1 (stdout)
+
+See [NIP-GS](../../docs/nips/NIP-GS.md) for the full specification.