How do I generate an auth token using JWT for Google firebase?

Question

So I'm trying to authenticate the Firebase REST API. I'm using the Vapor framework for server side swift and I installed the JWT package.

I'm trying to use the data in the serviceAccountKey.json file and JWT to generate an auth token.

Here is the code I've tried:

let payload = try JSON(node: [
        "iat": Date().timeIntervalSince1970,
        "exp": Date().timeIntervalSince1970 + 3600,
        "iss": "client_email from serviceAccountKey.json",
        "aud": "https://accounts.google.com/o/oauth2/token",
        "scope": [
            "https://www.googleapis.com/auth/firebase.database",
            "https://www.googleapis.com/auth/userinfo.email"
        ]
    ])
    let privateKey = "copied from serviceAccountKey.json"

    let signer = try HS256(bytes: privateKey.bytes)

    let jwt = try JWT(payload: payload, signer: signer)
    let token = try jwt.createToken()
    print(token)

serviceAccountKey.json

{
  "type": "service_account",
  "project_id": "",
  "private_key_id": "",
  "private_key": "",
  "client_email": "",
  "client_id": "",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://accounts.google.com/o/oauth2/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": ""
}

Answer 1

At this time I am using Xcode 8.3.3. Package.swift contains:

let package = Package(
name: "StripePayment",
dependencies: [
    .Package(url: "https://github.com/vapor/vapor.git", majorVersion: 1, minor: 5),
    .Package(url:"https://github.com/vapor/jwt.git", majorVersion: 0,minor: 8),
     .Package(url: "https://github.com/SwiftyJSON/SwiftyJSON.git", versions: Version(1, 0, 0)..<Version(3, .max, .max))

],
exclude: [
    "Config",
    "Database",
    "Localization",
    "Public",
    "Resources",
    "Tests",
]
)

If you generate a service account credential you need to have in mind the following, taken from https://cloud.google.com/storage/docs/authentication : You can create a private key in the Cloud Platform Console by creating an OAuth Client ID for a service account. You can get your private key in JSON and PKCS12 format:

JSON keys are required if you are using Application Default Credentials in a production environment outside of Google Cloud Platform. JSON keys cannot be converted to other formats. PKCS12 (.p12) is supported by many different programming languages and libraries. If needed, you can convert the key into other formats using OpenSSL (see Converting the private key to other formats). However, PKCS12 keys cannot be converted to JSON format.

Note: You do NOT need to generate a service account at console.cloud.google.com . Just follow the steps 1...6 listed below.

1. Go to https://console.firebase.google.com , click on your project, next to Overview click on the wheel Settings, click on Service Accounts, scroll to the bottom of the page and click on Generate New Private Key.

2. Convert the p.12 (a.k.a pkcs12) file to .pem (a.k.a pkcs1) using OpenSSL

   cat /path/to/xxxx-privatekey.p12 | openssl pkcs12 -nodes -nocerts -passin pass:notasecret | openssl rsa > /path/to/secret.pem

3. Go to github and search VaporJWT and import it in Xcode. It will help you create a signed JSON Web Token.

4. On this github page you will learn how to extract the private key for RSA use.

5. Convert .pem to der
   openssl rsa -in /path/to/secret.pem -outform der -out /path/to/private.der

6. Convert .der to .base64
   openssl base64 -in /path/to/private.der -out /path/to/Desktop/private.txt
   In private.txt you have the private key encoded in base64 which you can finally use to sign your JWT. Then you can make calls to Google API with the signed JWT.

``

 import Vapor
 import VaporJWT

 let drop = Droplet()
 var tokenID:String!

 //set current date
 let dateNow = Date()

 // assign to expDate the validity period of the token returned by OAuth server (3600 seconds)
 var expDate = String(Int(dateNow.timeIntervalSince1970 + (60 * 60)))

// assign to iatDate the time when the call was made to request an access token
 var iatDate = String(Int(dateNow.timeIntervalSince1970))

// the header of the JSON Web Token (first part of the JWT)
 let headerJWT = ["alg":"RS256","typ":"JWT"]

 // the claim set of the JSON Web Token
 let jwtClaimSet =
   ["iss":"firebase-adminsdk-c7i38@fir-30c9e.iam.gserviceaccount.com",
     "scope":"https://www.googleapis.com/auth/firebase.database",
     "aud":"https://www.googleapis.com/oauth2/v4/token",
     "exp": expDate,
     "iat": iatDate]


 //Using VaporJWT construct a JSON Web Token and sign it with RS256 algorithm
 //The only signing algorithm supported by the Google OAuth 2.0 Authorization     
 //Server is RSA using SHA-256 hashing algorithm.

  let jwt = try JWT(headers: Node(node: headerJWT), payload: Node(node:jwtClaimSet), encoding: Base64URLEncoding(), signer: RS256(encodedKey: "copy paste here what you have in private.txt as explained at point 7 above "))

 // create the JSON Web Token
  let JWTtoken = try jwt.createToken()
 let grant_type = "urn:ietf:params:oauth:grant-type:jwt-bearer" // this value must not be changed
   let unreserved = "*-._"
   let allowed = NSMutableCharacterSet.alphanumeric()
    allowed.addCharacters(in: unreserved)

// percent or URL encode grant_type
 let grant_URLEncoded = grant_type.addingPercentEncoding(withAllowedCharacters: allowed as CharacterSet)

 // create a string made of grant_type and assertion. NOTE!!! only grant_type's value is URL encoded.
 //JSON Web Token value does not need to be URL encoded
   var fullString = "grant_type=\(grant_URLEncoded!)&assertion=\(JWTtoken)"


  //pass fullString in the body parameter
   drop.get("call") { request in


    let response =  try drop.client.post("https://www.googleapis.com/oauth2/v4/token", headers: ["Content-Type": "application/x-www-form-urlencoded"], query: [:],body: fullString)

   let serverResp = response.headers
   let serverBody = response.body.bytes
      let serverJson = try JSON(bytes: serverBody!)
        print(serverJson)

     return "Success"

Answer 2

If you just want to get things working, better off using version 1.5.0

.Package(url: "https://github.com/gtchance/FirebaseSwift.git", Version(1,5,0)),

And using the legacy secret. Project Settings>Service Accounts>Database Secrets