release: sign SHA256SUMS with a pinned key, verify against it

verify-release.sh embeds the Bitflash release signing key and now requires that
the checksum signature validate against that exact key, not merely against some
key in the caller's keyring (#138). It also fetches releases from
releases.bitflash.network instead of the retired GitHub host, resolving 'latest'
from latest.txt.

Release signing key fingerprint:
  910A 2B4C CA87 9E81 FB4B  2AEA 1D4A 53D3 B78A A4B8

2 files changed, 108 insertions, 78 deletions

diff --git a/docs/release-verification.md b/docs/release-verification.md
index ecacd5f..32de74f 100644
--- a/docs/release-verification.md
+++ b/docs/release-verification.md
@@ -3,60 +3,82 @@
 Bitflash releases publish binaries and a `SHA256SUMS` file. The checksum file
 proves that the file you downloaded matches the file the release page names.
 A detached signature, `SHA256SUMS.asc`, proves that the checksum file itself was
-signed by a trusted release key.
+signed by the Bitflash release key.
 
 That gives users two separate checks:
 
-1. `gpg --verify SHA256SUMS.asc SHA256SUMS` checks who signed the checksums.
-2. `sha256sum -c SHA256SUMS` checks the binaries against those checksums.
+1. `gpg --verify SHA256SUMS.asc SHA256SUMS` — who signed the checksums.
+2. `sha256sum -c SHA256SUMS` — the binaries against those checksums.
 
-## User Check
+But the first check is only worth something if you require a **specific** key.
+A good signature by *some* key proves nothing; anyone can make one.
 
-From a shell with `curl` or `wget` and `sha256sum`:
+## The release signing key
 
-```bash
-scripts/verify-release.sh latest
+```text
+fingerprint  910A 2B4C CA87 9E81 FB4B  2AEA 1D4A 53D3 B78A A4B8
+uid          Bitflash Releases (bitflash.network release signing)
+             <releases@bitflash.network>
 ```
 
-For a specific release:
+The public key is published at
+`https://releases.bitflash.network/bitflash-release-key.asc` and embedded in
+`scripts/verify-release.sh`, so the tool's trust anchor lives in this
+repository rather than on the server it verifies.
+
+## User check
+
+From a shell with `curl` or `wget`, `sha256sum` and `gpg`:
 
 ```bash
-scripts/verify-release.sh v1.2.13
+scripts/verify-release.sh latest
 ```
 
-For release audits, require the signature:
+For a specific release:
 
 ```bash
-scripts/verify-release.sh v1.2.13 --require-signature
+scripts/verify-release.sh v1.2.15
 ```
 
-Older releases may not have `SHA256SUMS.asc`. In that case the script warns and
-still checks file integrity. New release audits should use `--require-signature`.
-
-## Maintainer Flow
+The script downloads the release, imports the pinned key into a throwaway
+keyring, and **refuses anything that is not a valid signature by the key above**
+before checking the hashes. Releases from before signing was introduced ship
+only `SHA256SUMS`; without `--require-signature` those are checked for integrity
+and reported as unsigned.
 
-After building release assets in the repository root:
+By hand, if you would rather not run the script:
 
 ```bash
-scripts/make-release-checksums.sh --sign --local-user RELEASE_KEY_ID
+curl -O https://releases.bitflash.network/bitflash-release-key.asc
+gpg --import bitflash-release-key.asc
+gpg --verify SHA256SUMS.asc SHA256SUMS   # must name the fingerprint above
+sha256sum -c SHA256SUMS
 ```
 
-Upload all built assets plus:
+## Maintainer flow
 
-```text
-SHA256SUMS
-SHA256SUMS.asc
+The signing key lives on a dedicated location, not in the build tree. After
+building the release assets in the repository root, write and sign the
+checksums:
+
+```bash
+sha256sum Bitflash-*.zip Bitflash-*.AppImage bitflash-node-* > SHA256SUMS
+gpg --armor --detach-sign --output SHA256SUMS.asc SHA256SUMS
 ```
 
-Keep the private signing key offline or on a dedicated release machine. Publish
-the public key fingerprint in the release notes and keep using the same key for
-future releases unless there is a clearly announced rotation.
+Publish, alongside the assets, `SHA256SUMS` and `SHA256SUMS.asc`, and update
+`latest.txt` on the releases host to the new tag. Keep using the same key; if it
+is ever rotated, announce the new fingerprint clearly and update the embedded
+key in `verify-release.sh`.
 
-## Why This Matters
+## Why this matters
 
 `SHA256SUMS` alone protects against a broken download, but not against someone
-replacing both a binary and the checksum file. Signing `SHA256SUMS` means an
-attacker must also have the release signing key to make the replacement verify.
+replacing both a binary and the checksum file. Signing `SHA256SUMS`, and
+requiring a pinned key, means an attacker must also hold the release signing key
+to make the replacement verify. Serving from bitflash.network makes this the
+guarantee that matters: without a pinned key, "trust the site" is the only
+thing standing behind a download.
 
 This is not reproducible builds yet. It is the smaller, immediate step that
-makes every release asset auditable by users before they run it.
+makes every release asset auditable before anyone runs it.
diff --git a/scripts/verify-release.sh b/scripts/verify-release.sh
index 23e71d1..4fd4558 100755
--- a/scripts/verify-release.sh
+++ b/scripts/verify-release.sh
@@ -1,23 +1,50 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-repo="Bitflash-sh/bitflash"
-tag="${1:-latest}"
+# Verifies a Bitflash release: the checksum file's signature against a pinned
+# key, then the assets against the checksums.
+#
+# The signing key is embedded below, not fetched. That is the whole point of a
+# pinned key: the trust anchor ships with this script (in git), so it does not
+# depend on the same server that serves the release. A server that is
+# compromised can swap the binaries and the checksums, but it cannot forge a
+# signature by this key.
+
+host="https://releases.bitflash.network"
 require_signature=0
 
+# The Bitflash release signing key. Verification requires this exact key --
+# gpg's own "good signature" is not enough, because a good signature by *some*
+# key proves nothing. See issue #138.
+RELEASE_KEY_FP="910A2B4CCA879E81FB4B2AEA1D4A53D3B78AA4B8"
+read -r -d '' RELEASE_KEY <<'KEY' || true
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+
+mDMEanSOmRYJKwYBBAHaRw8BAQdAXj24i2BYUcAmOpoXwjE4ZTynHsifCZ+niwQK
+HqO0nMa0UEJpdGZsYXNoIFJlbGVhc2VzIChiaXRmbGFzaC5uZXR3b3JrIHJlbGVh
+c2Ugc2lnbmluZykgPHJlbGVhc2VzQGJpdGZsYXNoLm5ldHdvcms+iJAEExYKADgW
+IQSRCitMyoeegftLKuodSlPTt4qkuAUCanSOmQIbAwULCQgHAgYVCgkICwIEFgID
+AQIeAQIXgAAKCRAdSlPTt4qkuIi9AP96X3OcstiCTTgKOr16Vhds3LyuZ19q3Apq
+4KAcNuOUMAD+NqgJ1CqkLGPXfZLCtYBCRHXNYWxMMEk+ViLcRtdLMg4=
+=mxic
+-----END PGP PUBLIC KEY BLOCK-----
+KEY
+
 usage() {
   cat <<'EOF'
 Usage: scripts/verify-release.sh [tag|latest] [--require-signature]
 
 Downloads the release checksum file and every asset named in it, then verifies:
-  1. SHA256SUMS.asc, if present, against SHA256SUMS with gpg.
+  1. SHA256SUMS.asc against SHA256SUMS, requiring the pinned Bitflash release
+     key (fingerprint 910A 2B4C CA87 9E81 FB4B 2AEA 1D4A 53D3 B78A A4B8).
   2. SHA256SUMS against the downloaded assets.
 
-Use --require-signature for release audits. Without it, older releases that
-ship only SHA256SUMS are checked for integrity and reported as unsigned.
+Releases from before signing was introduced ship only SHA256SUMS; without
+--require-signature those are checked for integrity and reported as unsigned.
 EOF
 }
 
+tag="${1:-latest}"
 for arg in "${@:2}"; do
   case "$arg" in
     --require-signature) require_signature=1 ;;
@@ -25,19 +52,9 @@ for arg in "${@:2}"; do
     *) echo "unknown option: $arg" >&2; usage >&2; exit 2 ;;
   esac
 done
+if [ "$tag" = "-h" ] || [ "$tag" = "--help" ]; then usage; exit 0; fi
 
-if [ "$tag" = "-h" ] || [ "$tag" = "--help" ]; then
-  usage
-  exit 0
-fi
-
-need() {
-  if ! command -v "$1" >/dev/null 2>&1; then
-    echo "missing required command: $1" >&2
-    exit 1
-  fi
-}
-
+need() { command -v "$1" >/dev/null 2>&1 || { echo "missing required command: $1" >&2; exit 1; }; }
 need sed
 need awk
 
@@ -46,39 +63,25 @@ if command -v sha256sum >/dev/null 2>&1; then
 elif command -v shasum >/dev/null 2>&1; then
   sha_check=(shasum -a 256 -c SHA256SUMS)
 else
-  echo "missing required command: sha256sum or shasum" >&2
-  exit 1
+  echo "missing required command: sha256sum or shasum" >&2; exit 1
 fi
 
 download() {
-  local url="$1"
-  local out="$2"
-  if command -v curl >/dev/null 2>&1; then
-    curl -fsSL "$url" -o "$out"
-  elif command -v wget >/dev/null 2>&1; then
-    wget -q "$url" -O "$out"
-  else
-    echo "missing required command: curl or wget" >&2
-    exit 1
-  fi
+  if command -v curl >/dev/null 2>&1; then curl -fsSL "$1" -o "$2"
+  elif command -v wget >/dev/null 2>&1; then wget -q "$1" -O "$2"
+  else echo "missing required command: curl or wget" >&2; exit 1; fi
 }
 
 if [ "$tag" = "latest" ]; then
-  tag="$(download "https://api.github.com/repos/$repo/releases/latest" - |
-    sed -n 's/.*"tag_name"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' |
-    head -1)"
-  if [ -z "$tag" ]; then
-    echo "could not resolve latest release tag" >&2
-    exit 1
-  fi
+  tag="$(download "$host/latest.txt" - 2>/dev/null | tr -d '[:space:]' || true)"
+  [ -n "$tag" ] || { echo "could not resolve the latest release tag from $host/latest.txt" >&2; exit 1; }
 fi
 
 workdir="$(mktemp -d "${TMPDIR:-/tmp}/bitflash-release-verify.XXXXXX")"
 cleanup() { rm -rf "$workdir"; }
 trap cleanup EXIT
 
-base_url="https://github.com/$repo/releases/download/$tag"
-
+base_url="$host/$tag"
 echo "Verifying Bitflash $tag"
 echo "Working directory: $workdir"
 cd "$workdir"
@@ -86,27 +89,32 @@ cd "$workdir"
 download "$base_url/SHA256SUMS" SHA256SUMS
 
 if download "$base_url/SHA256SUMS.asc" SHA256SUMS.asc 2>/dev/null; then
-  if ! command -v gpg >/dev/null 2>&1; then
-    echo "SHA256SUMS.asc exists, but gpg is not installed" >&2
+  need gpg
+  # Verify in a throwaway keyring holding ONLY the pinned key, and require that
+  # the signature validates against it with the pinned fingerprint. This is the
+  # difference between "signed" and "signed by Bitflash".
+  keyring="$workdir/keyring"
+  mkdir -p "$keyring"; chmod 700 "$keyring"
+  printf '%s\n' "$RELEASE_KEY" | GNUPGHOME="$keyring" gpg --batch --quiet --import 2>/dev/null
+  status="$(GNUPGHOME="$keyring" gpg --batch --status-fd=1 --verify SHA256SUMS.asc SHA256SUMS 2>/dev/null || true)"
+  if printf '%s' "$status" | grep -q "VALIDSIG.*$RELEASE_KEY_FP"; then
+    echo "Signature OK -- signed by the pinned Bitflash release key"
+  else
+    echo "SIGNATURE REJECTED: not a valid signature by the pinned release key" >&2
+    echo "  expected key: $RELEASE_KEY_FP" >&2
     exit 1
   fi
-  gpg --verify SHA256SUMS.asc SHA256SUMS
-  echo "Signature OK"
 else
   if [ "$require_signature" -eq 1 ]; then
-    echo "release does not publish SHA256SUMS.asc" >&2
-    exit 1
+    echo "release does not publish SHA256SUMS.asc" >&2; exit 1
   fi
-  echo "WARNING: no SHA256SUMS.asc found; checking hashes only"
+  echo "WARNING: no SHA256SUMS.asc found; checking hashes only (unsigned release)"
 fi
 
 awk '{print $2}' SHA256SUMS | while IFS= read -r file; do
-  file="${file#\*}"
-  file="${file#./}"
+  file="${file#\*}"; file="${file#./}"
   [ -z "$file" ] && continue
-  case "$file" in
-    */*|..*) echo "refusing unexpected checksum path: $file" >&2; exit 1 ;;
-  esac
+  case "$file" in */*|..*) echo "refusing unexpected checksum path: $file" >&2; exit 1 ;; esac
   echo "Downloading $file"
   download "$base_url/$file" "$file"
 done