build provenance stmt from OIDC claims

Signed-off-by: Brian DeHamer <bdehamer@github.com>

packages/attest/src/oidc.ts

@@ -0,0 +1,102 @@
+import {getIDToken} from '@actions/core'
+import {HttpClient} from '@actions/http-client'
+import * as jwt from 'jsonwebtoken'
+import jwks from 'jwks-rsa'
+
+const OIDC_AUDIENCE = 'nobody'
+
+const REQUIRED_CLAIMS = [
+  'iss',
+  'ref',
+  'sha',
+  'repository',
+  'event_name',
+  'workflow_ref',
+  'repository_id',
+  'repository_owner_id',
+  'runner_environment',
+  'run_id',
+  'run_attempt'
+] as const
+
+export type ClaimSet = {[K in (typeof REQUIRED_CLAIMS)[number]]: string}
+
+type OIDCConfig = {
+  jwks_uri: string
+}
+
+export const getIDTokenClaims = async (issuer: string): Promise<ClaimSet> => {
+  try {
+    const token = await getIDToken(OIDC_AUDIENCE)
+    const claims = await decodeOIDCToken(token, issuer)
+    assertClaimSet(claims)
+    return claims
+  } catch (error) {
+    throw new Error(`Failed to get ID token: ${error.message}`)
+  }
+}
+
+const decodeOIDCToken = async (
+  token: string,
+  issuer: string
+): Promise<jwt.JwtPayload> => {
+  // Verify and decode token
+  return new Promise((resolve, reject) => {
+    jwt.verify(
+      token,
+      getPublicKey(issuer),
+      {audience: OIDC_AUDIENCE, issuer},
+      (err, decoded) => {
+        if (err) {
+          reject(err)
+        } else if (!decoded || typeof decoded === 'string') {
+          reject(new Error('No decoded token'))
+        } else {
+          resolve(decoded)
+        }
+      }
+    )
+  })
+}
+
+// Returns a callback to locate the public key for the given JWT header. This
+// involves two calls:
+// 1. Fetch the OpenID configuration to get the JWKS URI.
+// 2. Fetch the public key from the JWKS URI.
+const getPublicKey =
+  (issuer: string): jwt.GetPublicKeyOrSecret =>
+  (header: jwt.JwtHeader, callback: jwt.SigningKeyCallback) => {
+    // Look up the JWKS URI from the issuer's OpenID configuration
+    new HttpClient('actions/attest')
+      .getJson<OIDCConfig>(`${issuer}/.well-known/openid-configuration`)
+      .then(data => {
+        if (!data.result) {
+          callback(new Error('No OpenID configuration found'))
+        } else {
+          // Fetch the public key from the JWKS URI
+          jwks({jwksUri: data.result.jwks_uri}).getSigningKey(
+            header.kid,
+            (err, key) => {
+              callback(err, key?.getPublicKey())
+            }
+          )
+        }
+      })
+      .catch(err => {
+        callback(err)
+      })
+  }
+
+function assertClaimSet(claims: jwt.JwtPayload): asserts claims is ClaimSet {
+  const missingClaims: string[] = []
+
+  for (const claim of REQUIRED_CLAIMS) {
+    if (!(claim in claims)) {
+      missingClaims.push(claim)
+    }
+  }
+
+  if (missingClaims.length > 0) {
+    throw new Error(`Missing claims: ${missingClaims.join(', ')}`)
+  }
+}