feat: verify release checksum and cosign signature (#550)

* feat: verify release checksum and cosign signature

Download checksums.txt for the release and verify the SHA-256 of the
downloaded archive against it. When cosign is available in PATH, also
download checksums.txt.sigstore.json and verify the signature against
the goreleaser/goreleaser-pro release workflow identity. Both steps
degrade gracefully (with a warning) when the corresponding artifacts
or tooling are missing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* test: use install() for checksum e2e tests

Drop the http-client download helper from verifyChecksum integration
tests; call goreleaser.install() instead so the test exercises the
public API path and avoids duplicating download logic.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/goreleaser.ts

@@ -1,26 +1,26 @@
+import * as crypto from 'crypto';
 import * as fs from 'fs';
 import * as path from 'path';
-import * as util from 'util';
 import yaml from 'js-yaml';
 import * as context from './context';
 import * as github from './github';
 import * as core from '@actions/core';
+import * as exec from '@actions/exec';
+import * as io from '@actions/io';
 import * as tc from '@actions/tool-cache';
 
 export async function install(distribution: string, version: string): Promise<string> {
   const release: github.GitHubRelease = await github.getRelease(distribution, version);
   const filename = getFilename(distribution);
-  const downloadUrl = util.format(
-    'https://github.com/goreleaser/%s/releases/download/%s/%s',
-    distribution,
-    release.tag_name,
-    filename
-  );
+  const baseUrl = `https://github.com/goreleaser/${distribution}/releases/download/${release.tag_name}`;
+  const downloadUrl = `${baseUrl}/${filename}`;
 
   core.info(`Downloading ${downloadUrl}`);
   const downloadPath: string = await tc.downloadTool(downloadUrl);
   core.debug(`Downloaded to ${downloadPath}`);
 
+  await verifyChecksum(distribution, release.tag_name, downloadPath, filename);
+
   core.info('Extracting GoReleaser');
   let extPath: string;
   if (context.osPlat == 'win32') {
@@ -45,6 +45,92 @@ export async function install(distribution: string, version: string): Promise<st
   return exePath;
 }
 
+export async function verifyChecksum(
+  distribution: string,
+  tag: string,
+  archivePath: string,
+  filename: string
+): Promise<void> {
+  const baseUrl = `https://github.com/goreleaser/${distribution}/releases/download/${tag}`;
+  let checksumsPath: string;
+  try {
+    core.info(`Downloading ${baseUrl}/checksums.txt`);
+    checksumsPath = await tc.downloadTool(`${baseUrl}/checksums.txt`);
+  } catch (e) {
+    core.warning(`Skipping checksum verification: unable to download checksums.txt: ${e.message}`);
+    return;
+  }
+
+  const sha256 = crypto.createHash('sha256').update(fs.readFileSync(archivePath)).digest('hex');
+  const expected = findChecksum(fs.readFileSync(checksumsPath, 'utf8'), filename);
+  if (!expected) {
+    throw new Error(`Could not find ${filename} in checksums.txt`);
+  }
+  if (expected.toLowerCase() !== sha256.toLowerCase()) {
+    throw new Error(`Checksum mismatch for ${filename}: expected ${expected}, got ${sha256}`);
+  }
+  core.info(`Checksum verified for ${filename}`);
+
+  await verifyCosignSignature(distribution, tag, baseUrl, checksumsPath);
+}
+
+export const findChecksum = (checksumsContent: string, filename: string): string | undefined => {
+  const match = checksumsContent
+    .split('\n')
+    .map(line => line.trim().split(/\s+/))
+    .find(parts => parts.length >= 2 && parts[1].replace(/^[*]/, '') === filename);
+  return match ? match[0] : undefined;
+};
+
+async function verifyCosignSignature(
+  distribution: string,
+  tag: string,
+  baseUrl: string,
+  checksumsPath: string
+): Promise<void> {
+  const cosign = await io.which('cosign', false);
+  if (!cosign) {
+    core.info('cosign not found in PATH, skipping signature verification');
+    return;
+  }
+
+  let bundlePath: string;
+  try {
+    core.info(`Downloading ${baseUrl}/checksums.txt.sigstore.json`);
+    bundlePath = await tc.downloadTool(`${baseUrl}/checksums.txt.sigstore.json`);
+  } catch (e) {
+    core.warning(`Skipping cosign signature verification: unable to download sigstore bundle: ${e.message}`);
+    return;
+  }
+
+  const certificateIdentity = getCertificateIdentity(distribution, tag);
+  core.info(`Verifying checksums.txt signature with cosign (identity: ${certificateIdentity})`);
+  await exec.exec(cosign, [
+    'verify-blob',
+    '--certificate-identity',
+    certificateIdentity,
+    '--certificate-oidc-issuer',
+    'https://token.actions.githubusercontent.com',
+    '--bundle',
+    bundlePath,
+    checksumsPath
+  ]);
+  core.info('cosign signature verified');
+}
+
+export const getCertificateIdentity = (distribution: string, tag: string): string => {
+  const pro = isPro(distribution);
+  if (tag === 'nightly') {
+    const workflow = pro ? 'nightly-pro.yml' : 'nightly-oss.yml';
+    const repo = pro ? 'goreleaser-pro-internal' : 'goreleaser';
+    return `https://github.com/goreleaser/${repo}/.github/workflows/${workflow}@refs/heads/main`;
+  }
+  if (pro) {
+    return `https://github.com/goreleaser/goreleaser-pro-internal/.github/workflows/release-pro.yml@refs/tags/${tag}`;
+  }
+  return `https://github.com/goreleaser/goreleaser/.github/workflows/release.yml@refs/tags/${tag}`;
+};
+
 export const distribSuffix = (distribution: string): string => {
   return isPro(distribution) ? '-pro' : '';
 };
@@ -81,7 +167,7 @@ const getFilename = (distribution: string): string => {
   const platform: string = context.osPlat == 'win32' ? 'Windows' : context.osPlat == 'darwin' ? 'Darwin' : 'Linux';
   const ext: string = context.osPlat == 'win32' ? 'zip' : 'tar.gz';
   const suffix: string = distribSuffix(distribution);
-  return util.format('goreleaser%s_%s_%s.%s', suffix, platform, arch, ext);
+  return `goreleaser${suffix}_${platform}_${arch}.${ext}`;
 };
 
 export async function getDistPath(yamlfile: string): Promise<string> {