dynamic construction of oidc issuer (#195)

Signed-off-by: Brian DeHamer <[email protected]>

src/main.ts

@@ -1,14 +1,21 @@
 import { buildSLSAProvenancePredicate } from '@actions/attest'
 import * as core from '@actions/core'
 
+const VALID_SERVER_URLS = [
+  'https://github.com',
+  new RegExp('^https://[a-z0-9-]+\\.ghe\\.com$')
+] as const
+
 /**
  * The main function for the action.
  * @returns {Promise<void>} Resolves when the action is complete.
  */
 export async function run(): Promise<void> {
   try {
+    const issuer = getIssuer()
+
     // Calculate subject from inputs and generate provenance
-    const predicate = await buildSLSAProvenancePredicate()
+    const predicate = await buildSLSAProvenancePredicate(issuer)
 
     core.setOutput('predicate', predicate.params)
     core.setOutput('predicate-type', predicate.type)
@@ -18,3 +25,21 @@ export async function run(): Promise<void> {
     core.setFailed(error.message)
   }
 }
+
+// Derive the current OIDC issuer based on the server URL
+function getIssuer(): string {
+  const serverURL = process.env.GITHUB_SERVER_URL || 'https://github.com'
+
+  // Ensure the server URL is a valid GitHub server URL
+  if (!VALID_SERVER_URLS.some(valid_url => serverURL.match(valid_url))) {
+    throw new Error(`Invalid server URL: ${serverURL}`)
+  }
+
+  let host = new URL(serverURL).hostname
+
+  if (host === 'github.com') {
+    host = 'githubusercontent.com'
+  }
+
+  return `https://token.actions.${host}`
+}