Sign release checksums with Ed25519 (#63)

Publish SHA256SUMS for every release archive and sign it with a
detached Ed25519 signature (SHA256SUMS.sig), since paid code signing
(macOS Developer ID, Windows Authenticode) and Sigstore keyless (needs
a Fulcio-trusted OIDC issuer; self-hosted Gitea isn't one) are both
out of budget/scope.

- scripts/sign_checksums.py: dependency-light (cryptography only)
  helper to hash a directory of files into a sha256sum(1)-compatible
  SHA256SUMS manifest, sign it (domain-separated: b"bcc-release-v1|"
  + raw manifest bytes), and verify a signature. CLI has generate/
  sign/verify subcommands; verify doubles as the check path.
- tests/test_checksums.py: 15 unit + CLI-subprocess tests covering
  hashing, manifest formatting, sign/verify roundtrip, tamper
  detection, wrong-key rejection, domain-separation, and the
  no-key-provided failure path (must error, never write an empty/
  bogus .sig).
- .github/workflows/release.yml: Publish Release job now checks out
  the repo, flattens build artifacts, generates SHA256SUMS, and signs
  it from the RELEASE_SIGNING_KEY secret (base64 raw Ed25519 seed) if
  present. If the secret is absent, the release still publishes with
  a loud ::warning:: and no .sig — it never fails the release or
  publishes a bogus signature.
- README.md: new 'Verifying your download' section with the (still
  placeholder) public key, sha256sum -c / Get-FileHash commands, and
  an explicit statement that this does not remove Gatekeeper/
  SmartScreen warnings.
- requirements-dev.txt / ci.yml: add cryptography as a dev/test
  dependency for the new script and its tests.

Touches no files from bcc_core.py / tests/test_core.py /
pyproject.toml / bcc.spec to avoid colliding with concurrent work on
those files.

.github/workflows/ci.yml

@@ -62,8 +62,9 @@ jobs:
 
       # bcc_core has no GUI imports, so the test suite needs no PySide6 —
       # keeps CI fast and avoids Qt system-library headaches on the runner.
+      # cryptography is for tests/test_checksums.py (release signing helper).
       - name: Install test dependencies
-        run: pip install pytest
+        run: pip install pytest cryptography
 
       - name: Run tests
         run: python -m pytest -v

.github/workflows/release.yml

@@ -107,11 +107,72 @@ jobs:
       contents: write
 
     steps:
+      # Needed for scripts/sign_checksums.py — the release job otherwise
+      # only downloads build artifacts, it doesn't check out the repo.
+      - name: Checkout
+        uses: actions/checkout@v4
+
       - name: Download all artifacts
         uses: actions/download-artifact@v3
         with:
           path: artifacts
 
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      # download-artifact@v3 nests each artifact under a directory named
+      # after it (artifacts/<name>/<name>). Flatten into one directory so
+      # SHA256SUMS lists plain filenames, matching what `sha256sum -c`
+      # expects when run from inside an extracted release download.
+      - name: Collect release files
+        run: |
+          mkdir -p release-files
+          find artifacts -type f -exec cp {} release-files/ \;
+          ls -la release-files
+
+      - name: Generate SHA256SUMS
+        run: python3 scripts/sign_checksums.py generate release-files --out release-files/SHA256SUMS
+
+      # ── Sign the checksum manifest (best-effort) ──────────────────────
+      #
+      # BCC binaries are not code-signed (no budget for a paid cert). This
+      # is the free half: a checksum manifest, detached-signed with
+      # Ed25519, so a tampered download is detectable by anyone who
+      # checks. It does NOT remove Gatekeeper/SmartScreen warnings.
+      #
+      # The private key is a repo secret (RELEASE_SIGNING_KEY, base64 raw
+      # Ed25519 seed) generated via the Catalog Console (#62). If it's not
+      # set, we still publish the release — just without a .sig — rather
+      # than fail the release outright.
+      - name: Check for signing key
+        id: signing
+        run: |
+          if [ -n "${{ secrets.RELEASE_SIGNING_KEY }}" ]; then
+            echo "has_key=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "has_key=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Install signing dependencies
+        if: steps.signing.outputs.has_key == 'true'
+        run: pip install cryptography
+
+      - name: Sign SHA256SUMS
+        if: steps.signing.outputs.has_key == 'true'
+        env:
+          RELEASE_SIGNING_KEY: ${{ secrets.RELEASE_SIGNING_KEY }}
+        run: |
+          python3 scripts/sign_checksums.py sign \
+            --sums release-files/SHA256SUMS \
+            --out release-files/SHA256SUMS.sig
+
+      - name: Warn — release will be unsigned
+        if: steps.signing.outputs.has_key != 'true'
+        run: |
+          echo "::warning::RELEASE_SIGNING_KEY secret is not set — this release is being published WITHOUT a signed SHA256SUMS.sig. Add the secret (base64 raw Ed25519 seed, generated via the Catalog Console, #62) before the next tag."
+
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
         with:
@@ -119,7 +180,9 @@ jobs:
           draft: false
           prerelease: false
           generate_release_notes: false
-          files: artifacts/**/*
+          files: |
+            artifacts/**/*
+            release-files/SHA256SUMS*
           body: |
             ## Better Claude Config ${{ github.ref_name }}
 
@@ -139,5 +202,8 @@ jobs:
             xattr -cr /Applications/BetterClaudeConfig.app
             ```
 
+            ### Verifying your download
+            Every release includes `SHA256SUMS` (and, when the signing key is configured, a detached `SHA256SUMS.sig`). See [Verifying your download](https://git.avezzano.io/the_og/better-claude-config#verifying-your-download) in the README for commands. This proves you got the file we published — it does not remove Gatekeeper/SmartScreen warnings.
+
             ### Requirements
             No Python installation needed — the app is self-contained.

README.md

@@ -19,6 +19,65 @@ Pre-built self-contained binaries are attached to every [GitHub Release](../../r
 
 > **macOS Gatekeeper note:** the app is not notarized. On first launch, right-click → **Open**, or run `xattr -cr /Applications/BetterClaudeConfig.app` in a terminal.
 
+## Verifying your download
+
+BCC isn't code-signed — there's no budget for a paid certificate (macOS
+Developer ID, Windows Authenticode). Instead, every release publishes a
+`SHA256SUMS` file listing the checksum of each archive, detached-signed with
+Ed25519 as `SHA256SUMS.sig`. Both are attached to the release alongside the
+binaries.
+
+**What this proves:** the file you downloaded is byte-for-byte what we
+published, and the manifest itself was signed by our release key.
+
+**What this does NOT do:** it does not make the binary "safe," and it does
+**not** remove the macOS Gatekeeper or Windows SmartScreen warning — those
+are only suppressed by a paid OS-vendor certificate, which this project
+doesn't have. Verifying checksums is about detecting tampering in transit or
+on a mirror, not about vouching for the software.
+
+**Release signing public key** (Ed25519, base64, raw 32 bytes):
+
+```
+<PLACEHOLDER — AJ: paste the public key from the Catalog Console (#62) here>
+```
+
+### macOS / Linux
+
+```bash
+# From inside the folder you downloaded the release files into:
+sha256sum -c SHA256SUMS
+```
+
+If your `sha256sum` complains about missing files, download `SHA256SUMS`
+into the same directory as the archive you downloaded — it lists every
+platform's archive, and only the one(s) present will be checked.
+
+To also verify the manifest's signature (optional, requires Python +
+`pip install cryptography` and a checkout of this repo):
+
+```bash
+python3 scripts/sign_checksums.py verify \
+  --sums SHA256SUMS --sig SHA256SUMS.sig \
+  --pubkey-b64 "<the public key above>"
+```
+
+### Windows (PowerShell)
+
+```powershell
+Get-FileHash .\BetterClaudeConfig-Windows.zip -Algorithm SHA256
+```
+
+Compare the printed hash (case-insensitively) against the matching line in
+`SHA256SUMS`.
+
+### If a release has no `SHA256SUMS.sig`
+
+The signing key is a repo secret that has to be configured manually; if a
+release is missing the `.sig` file, the checksums themselves are still
+valid and safe to check against — the release workflow only skips signing,
+never checksum generation.
+
 ## Run from source
 
 ```bash
@@ -92,6 +151,7 @@ file is also listed, marked *legacy*, so you can copy them over.
 - `test_core.py` — unit suite for the core (`python test_core.py`).
 - `bcc.spec` — PyInstaller build spec (cross-platform).
 - `scripts/build_icons.py` — regenerates `icons/app.icns` and `icons/app.ico` from source PNGs.
+- `scripts/sign_checksums.py` — generates and Ed25519-signs the release `SHA256SUMS` manifest (see [Verifying your download](#verifying-your-download)).
 
 ## Building from source
 

requirements-dev.txt

@@ -8,3 +8,4 @@ pillow>=10.0        # generates icons/app.ico during CI (Windows build)
 # Test / lint
 pytest>=8.0
 ruff>=0.6
+cryptography>=42.0   # release checksum signing (scripts/sign_checksums.py)

scripts/sign_checksums.py

@@ -0,0 +1,235 @@
+#!/usr/bin/env python3
+"""
+Generate a SHA256SUMS file for release artifacts and sign it with Ed25519.
+
+BCC ships PyInstaller binaries that are not code-signed (no budget for a
+macOS Developer ID / Windows Authenticode certificate). This script provides
+the free half of supply-chain integrity: a checksum manifest, detached-signed
+so downloaders can verify the file they got is the file we published.
+
+This does NOT remove Gatekeeper/SmartScreen warnings and does NOT prove the
+binary is safe to run -- only that it matches what the release signing key
+attested to.
+
+Usage:
+    # Hash every file in a directory into a SHA256SUMS-format manifest.
+    python scripts/sign_checksums.py generate <dir> --out SHA256SUMS
+
+    # Sign a manifest, producing a detached signature.
+    # Private key comes from $RELEASE_SIGNING_KEY (base64 raw Ed25519 seed)
+    # unless --key-b64 is given explicitly (mostly for tests).
+    python scripts/sign_checksums.py sign --sums SHA256SUMS --out SHA256SUMS.sig
+
+    # Verify a manifest against a detached signature and a public key.
+    python scripts/sign_checksums.py verify --sums SHA256SUMS --sig SHA256SUMS.sig \
+        --pubkey-b64 <base64 raw Ed25519 public key>
+
+The private key is generated and rotated via the Catalog Console (#62) --
+this script never generates or stores a key itself.
+"""
+
+from __future__ import annotations
+
+import argparse
+import base64
+import hashlib
+import os
+import sys
+from pathlib import Path
+
+# Domain separation prefix: ties every signature to "a BCC release checksum
+# manifest" so a signature can never be replayed against an unrelated
+# message signed by the same key.
+DOMAIN_PREFIX = b"bcc-release-v1|"
+
+CHUNK_SIZE = 1024 * 1024
+
+
+def sha256_file(path: Path) -> str:
+    """Return the lowercase hex SHA-256 digest of a file's contents."""
+    digest = hashlib.sha256()
+    with open(path, "rb") as fh:
+        while chunk := fh.read(CHUNK_SIZE):
+            digest.update(chunk)
+    return digest.hexdigest()
+
+
+def build_checksums_text(files: dict[str, str]) -> str:
+    """Build a sha256sum(1)-compatible manifest body.
+
+    `files` maps filename -> hex digest. Entries are sorted by filename for
+    a deterministic, diffable output. Format matches `sha256sum` exactly:
+    "<hash>  <filename>\n" (two spaces, no path components).
+    """
+    lines = [f"{digest}  {name}" for name, digest in sorted(files.items())]
+    body = "\n".join(lines)
+    return body + "\n" if body else ""
+
+
+def generate_checksums(directory: Path, *, exclude: set[str] | None = None) -> str:
+    """Hash every regular file directly inside `directory` (non-recursive)
+    and return the SHA256SUMS text. Filenames are recorded without any
+    directory prefix so the manifest can be verified from inside the
+    directory it describes.
+    """
+    exclude = exclude or set()
+    files: dict[str, str] = {}
+    for entry in sorted(directory.iterdir()):
+        if not entry.is_file():
+            continue
+        if entry.name in exclude:
+            continue
+        files[entry.name] = sha256_file(entry)
+    return build_checksums_text(files)
+
+
+def _signing_message(sums_text: str) -> bytes:
+    """The exact bytes that get signed: the domain prefix followed by the
+    raw bytes of the SHA256SUMS file content."""
+    return DOMAIN_PREFIX + sums_text.encode("utf-8")
+
+
+def sign_checksums(seed_b64: str, sums_text: str) -> bytes:
+    """Sign `sums_text` with the Ed25519 private key encoded (base64, raw
+    32-byte seed) in `seed_b64`. Returns the raw 64-byte signature."""
+    # Imported lazily so `generate` mode (used on every CI run) never
+    # requires the `cryptography` package to be installed.
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+
+    seed = base64.b64decode(seed_b64)
+    if len(seed) != 32:
+        raise ValueError(f"expected a 32-byte raw Ed25519 seed, got {len(seed)} bytes")
+    private_key = Ed25519PrivateKey.from_private_bytes(seed)
+    return private_key.sign(_signing_message(sums_text))
+
+
+def verify_checksums(pubkey_b64: str, sums_text: str, signature: bytes) -> bool:
+    """Verify `signature` over `sums_text` against the base64-encoded raw
+    32-byte Ed25519 public key. Returns True/False; never raises for a bad
+    signature (only for malformed inputs)."""
+    from cryptography.exceptions import InvalidSignature
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
+
+    pubkey_bytes = base64.b64decode(pubkey_b64)
+    if len(pubkey_bytes) != 32:
+        raise ValueError(
+            f"expected a 32-byte raw Ed25519 public key, got {len(pubkey_bytes)} bytes"
+        )
+    public_key = Ed25519PublicKey.from_public_bytes(pubkey_bytes)
+    try:
+        public_key.verify(signature, _signing_message(sums_text))
+        return True
+    except InvalidSignature:
+        return False
+
+
+def public_key_b64_from_seed(seed_b64: str) -> str:
+    """Derive the base64 raw public key from a base64 raw seed. Handy for
+    local key-pair sanity checks; not used by the release workflow."""
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+    from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat
+
+    seed = base64.b64decode(seed_b64)
+    private_key = Ed25519PrivateKey.from_private_bytes(seed)
+    raw = private_key.public_key().public_bytes(Encoding.Raw, PublicFormat.Raw)
+    return base64.b64encode(raw).decode("ascii")
+
+
+# --------------------------------------------------------------------------- #
+# CLI
+# --------------------------------------------------------------------------- #
+def _cmd_generate(args: argparse.Namespace) -> int:
+    directory = Path(args.directory)
+    exclude = {"SHA256SUMS", "SHA256SUMS.sig"}
+    text = generate_checksums(directory, exclude=exclude)
+    out_path = Path(args.out)
+    out_path.write_text(text, encoding="utf-8")
+    print(f"Wrote {out_path} ({len(text.splitlines())} entries)")
+    return 0
+
+
+def _cmd_sign(args: argparse.Namespace) -> int:
+    seed_b64 = args.key_b64 or os.environ.get(args.key_env, "")
+    if not seed_b64:
+        print(
+            f"error: no signing key provided (checked --key-b64 and ${args.key_env})",
+            file=sys.stderr,
+        )
+        return 1
+    sums_text = Path(args.sums).read_text(encoding="utf-8")
+    signature = sign_checksums(seed_b64, sums_text)
+    Path(args.out).write_bytes(signature)
+    print(f"Wrote {args.out} ({len(signature)} bytes)")
+    return 0
+
+
+def _cmd_verify(args: argparse.Namespace) -> int:
+    pubkey_b64 = args.pubkey_b64 or os.environ.get(args.pubkey_env, "")
+    if not pubkey_b64:
+        print(
+            f"error: no public key provided (checked --pubkey-b64 and ${args.pubkey_env})",
+            file=sys.stderr,
+        )
+        return 1
+    sums_text = Path(args.sums).read_text(encoding="utf-8")
+    signature = Path(args.sig).read_bytes()
+    ok = verify_checksums(pubkey_b64, sums_text, signature)
+    if ok:
+        print("OK: signature is valid")
+        return 0
+    print("FAILED: signature is invalid", file=sys.stderr)
+    return 1
+
+
+def build_parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
+    )
+    sub = parser.add_subparsers(dest="mode", required=True)
+
+    p_gen = sub.add_parser(
+        "generate", help="hash every file in a directory into a SHA256SUMS manifest"
+    )
+    p_gen.add_argument("directory", help="directory whose files should be hashed (non-recursive)")
+    p_gen.add_argument("--out", required=True, help="path to write the SHA256SUMS manifest to")
+    p_gen.set_defaults(func=_cmd_generate)
+
+    p_sign = sub.add_parser("sign", help="detached-sign a SHA256SUMS manifest with Ed25519")
+    p_sign.add_argument("--sums", required=True, help="path to the SHA256SUMS manifest to sign")
+    p_sign.add_argument("--out", required=True, help="path to write the detached signature to")
+    p_sign.add_argument(
+        "--key-b64", default=None, help="base64 raw Ed25519 seed (overrides --key-env)"
+    )
+    p_sign.add_argument(
+        "--key-env",
+        default="RELEASE_SIGNING_KEY",
+        help="environment variable holding the base64 seed (default: RELEASE_SIGNING_KEY)",
+    )
+    p_sign.set_defaults(func=_cmd_sign)
+
+    p_verify = sub.add_parser(
+        "verify", help="verify a SHA256SUMS manifest against a detached signature"
+    )
+    p_verify.add_argument("--sums", required=True, help="path to the SHA256SUMS manifest")
+    p_verify.add_argument("--sig", required=True, help="path to the detached signature")
+    p_verify.add_argument(
+        "--pubkey-b64", default=None, help="base64 raw Ed25519 public key (overrides --pubkey-env)"
+    )
+    p_verify.add_argument(
+        "--pubkey-env",
+        default="RELEASE_SIGNING_PUBKEY",
+        help="environment variable holding the base64 public key (default: RELEASE_SIGNING_PUBKEY)",
+    )
+    p_verify.set_defaults(func=_cmd_verify)
+
+    return parser
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = build_parser()
+    args = parser.parse_args(argv)
+    return args.func(args)
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/test_checksums.py

@@ -0,0 +1,234 @@
+"""Tests for scripts/sign_checksums.py: SHA256SUMS generation and detached
+Ed25519 signing/verification for release artifacts."""
+
+from __future__ import annotations
+
+import base64
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "scripts"))
+
+import sign_checksums as sc
+
+cryptography = pytest.importorskip("cryptography")
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey  # noqa: E402
+from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat  # noqa: E402
+
+
+def _make_keypair() -> tuple[str, str]:
+    """Return (seed_b64, pubkey_b64) for a fresh Ed25519 keypair."""
+    private_key = Ed25519PrivateKey.generate()
+    seed = private_key.private_bytes_raw()
+    pubkey = private_key.public_key().public_bytes(Encoding.Raw, PublicFormat.Raw)
+    return base64.b64encode(seed).decode("ascii"), base64.b64encode(pubkey).decode("ascii")
+
+
+# --------------------------------------------------------------------------- #
+# sha256_file / build_checksums_text / generate_checksums
+# --------------------------------------------------------------------------- #
+def test_sha256_file_matches_hashlib(tmp_path):
+    f = tmp_path / "a.txt"
+    f.write_bytes(b"hello world")
+    import hashlib
+
+    assert sc.sha256_file(f) == hashlib.sha256(b"hello world").hexdigest()
+
+
+def test_build_checksums_text_sorted_and_formatted():
+    files = {"zeta.zip": "aa" * 32, "alpha.zip": "bb" * 32}
+    text = sc.build_checksums_text(files)
+    lines = text.splitlines()
+    assert lines[0].endswith("alpha.zip")
+    assert lines[1].endswith("zeta.zip")
+    # Standard sha256sum format: hash, two spaces, filename.
+    assert lines[0] == f"{'bb' * 32}  alpha.zip"
+
+
+def test_build_checksums_text_empty():
+    assert sc.build_checksums_text({}) == ""
+
+
+def test_generate_checksums_from_directory(tmp_path):
+    (tmp_path / "b.bin").write_bytes(b"second")
+    (tmp_path / "a.bin").write_bytes(b"first")
+    (tmp_path / "subdir").mkdir()
+    (tmp_path / "subdir" / "ignored.bin").write_bytes(b"nested, not hashed")
+
+    text = sc.generate_checksums(tmp_path)
+    lines = text.splitlines()
+    assert len(lines) == 2
+    assert lines[0].endswith("a.bin")
+    assert lines[1].endswith("b.bin")
+    assert "subdir" not in text
+
+
+def test_generate_checksums_excludes_manifest_files(tmp_path):
+    (tmp_path / "archive.zip").write_bytes(b"payload")
+    (tmp_path / "SHA256SUMS").write_text("stale")
+    (tmp_path / "SHA256SUMS.sig").write_bytes(b"stale-sig")
+
+    text = sc.generate_checksums(tmp_path, exclude={"SHA256SUMS", "SHA256SUMS.sig"})
+    assert "archive.zip" in text
+    assert "SHA256SUMS" not in text.replace("archive.zip", "")
+
+
+# --------------------------------------------------------------------------- #
+# sign_checksums / verify_checksums
+# --------------------------------------------------------------------------- #
+def test_sign_then_verify_roundtrip():
+    seed_b64, pubkey_b64 = _make_keypair()
+    sums_text = "deadbeef" * 8 + "  BetterClaudeConfig-Linux.tar.gz\n"
+
+    signature = sc.sign_checksums(seed_b64, sums_text)
+    assert len(signature) == 64
+    assert sc.verify_checksums(pubkey_b64, sums_text, signature) is True
+
+
+def test_verify_rejects_tampered_checksums():
+    seed_b64, pubkey_b64 = _make_keypair()
+    sums_text = "aa" * 32 + "  file.zip\n"
+    signature = sc.sign_checksums(seed_b64, sums_text)
+
+    tampered = "bb" * 32 + "  file.zip\n"
+    assert sc.verify_checksums(pubkey_b64, tampered, signature) is False
+
+
+def test_verify_rejects_wrong_key():
+    seed_b64, _ = _make_keypair()
+    _, other_pubkey_b64 = _make_keypair()
+    sums_text = "cc" * 32 + "  file.zip\n"
+    signature = sc.sign_checksums(seed_b64, sums_text)
+
+    assert sc.verify_checksums(other_pubkey_b64, sums_text, signature) is False
+
+
+def test_domain_prefix_is_applied():
+    """The signed message must be prefixed, not the raw manifest bytes --
+    otherwise a signature over this manifest could be replayed as a
+    signature over an unrelated message with the same bytes elsewhere."""
+    seed_b64, pubkey_b64 = _make_keypair()
+    sums_text = "11" * 32 + "  file.zip\n"
+
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey as PK
+
+    seed = base64.b64decode(seed_b64)
+    raw_signature = PK.from_private_bytes(seed).sign(sums_text.encode("utf-8"))
+
+    # A signature over the raw (unprefixed) bytes must NOT verify via our
+    # domain-separated verify function.
+    assert sc.verify_checksums(pubkey_b64, sums_text, raw_signature) is False
+
+    # But our own sign_checksums() output does verify.
+    good_signature = sc.sign_checksums(seed_b64, sums_text)
+    assert sc.verify_checksums(pubkey_b64, sums_text, good_signature) is True
+
+
+def test_sign_checksums_rejects_bad_seed_length():
+    bad_seed_b64 = base64.b64encode(b"too-short").decode("ascii")
+    with pytest.raises(ValueError):
+        sc.sign_checksums(bad_seed_b64, "irrelevant\n")
+
+
+def test_verify_checksums_rejects_bad_pubkey_length():
+    seed_b64, _ = _make_keypair()
+    sig = sc.sign_checksums(seed_b64, "irrelevant\n")
+    bad_pubkey_b64 = base64.b64encode(b"too-short").decode("ascii")
+    with pytest.raises(ValueError):
+        sc.verify_checksums(bad_pubkey_b64, "irrelevant\n", sig)
+
+
+def test_public_key_b64_from_seed_matches_generated_pubkey():
+    seed_b64, pubkey_b64 = _make_keypair()
+    assert sc.public_key_b64_from_seed(seed_b64) == pubkey_b64
+
+
+# --------------------------------------------------------------------------- #
+# CLI (end-to-end, via subprocess so argparse wiring is exercised too)
+# --------------------------------------------------------------------------- #
+SCRIPT = Path(__file__).resolve().parent.parent / "scripts" / "sign_checksums.py"
+
+
+def _run(*args, env=None):
+    return subprocess.run(
+        [sys.executable, str(SCRIPT), *args],
+        capture_output=True,
+        text=True,
+        env=env,
+    )
+
+
+def test_cli_generate_sign_verify_roundtrip(tmp_path, monkeypatch):
+    seed_b64, pubkey_b64 = _make_keypair()
+
+    release_dir = tmp_path / "release-files"
+    release_dir.mkdir()
+    (release_dir / "BetterClaudeConfig-Linux.tar.gz").write_bytes(b"fake archive contents")
+    (release_dir / "BetterClaudeConfig-macOS.zip").write_bytes(b"fake zip contents")
+
+    sums_path = release_dir / "SHA256SUMS"
+    sig_path = release_dir / "SHA256SUMS.sig"
+
+    gen = _run("generate", str(release_dir), "--out", str(sums_path))
+    assert gen.returncode == 0, gen.stderr
+    assert sums_path.exists()
+    body = sums_path.read_text()
+    assert "BetterClaudeConfig-Linux.tar.gz" in body
+    assert "BetterClaudeConfig-macOS.zip" in body
+
+    sign = _run("sign", "--sums", str(sums_path), "--out", str(sig_path), "--key-b64", seed_b64)
+    assert sign.returncode == 0, sign.stderr
+    assert sig_path.exists()
+    assert sig_path.stat().st_size == 64
+
+    verify = _run(
+        "verify",
+        "--sums",
+        str(sums_path),
+        "--sig",
+        str(sig_path),
+        "--pubkey-b64",
+        pubkey_b64,
+    )
+    assert verify.returncode == 0, verify.stderr
+    assert "OK" in verify.stdout
+
+
+def test_cli_sign_without_key_fails_loudly(tmp_path):
+    sums_path = tmp_path / "SHA256SUMS"
+    sums_path.write_text("aa" * 32 + "  file.zip\n")
+    sig_path = tmp_path / "SHA256SUMS.sig"
+
+    import os
+
+    env = {k: v for k, v in os.environ.items() if k != "RELEASE_SIGNING_KEY"}
+    result = _run("sign", "--sums", str(sums_path), "--out", str(sig_path), env=env)
+
+    assert result.returncode != 0
+    assert not sig_path.exists(), "must never write a bogus/empty signature file"
+    assert "no signing key" in result.stderr.lower()
+
+
+def test_cli_verify_detects_tampering(tmp_path):
+    seed_b64, pubkey_b64 = _make_keypair()
+    sums_path = tmp_path / "SHA256SUMS"
+    sums_path.write_text("aa" * 32 + "  file.zip\n")
+    sig_path = tmp_path / "SHA256SUMS.sig"
+
+    _run("sign", "--sums", str(sums_path), "--out", str(sig_path), "--key-b64", seed_b64)
+
+    sums_path.write_text("bb" * 32 + "  file.zip\n")  # tamper after signing
+    verify = _run(
+        "verify",
+        "--sums",
+        str(sums_path),
+        "--sig",
+        str(sig_path),
+        "--pubkey-b64",
+        pubkey_b64,
+    )
+    assert verify.returncode != 0
+    assert "FAILED" in verify.stdout + verify.stderr