If you want to make calls from GCF to IAP protected app, you should indeed be using ID tokens. There are no examples in Nodejs so I made one using this as a reference (style may be wrong since that's the first time I touch nodejs). Unlike regular JWT claims set, it should not contain scope and have target_audience.
/**
* Make IAP request
*
*/
exports.CfToIAP = function CfToIAP (req, res) {
var crypto = require('crypto'),
request = require('request');
var token_URL = "https://www.googleapis.com/oauth2/v4/token";
// service account private key (copied from service_account.json)
var key = "-----BEGIN PRIVATE KEY-----\nMIIEvQexsQ1DBNe12345GRwAZM=\n-----END PRIVATE KEY-----\n";
// craft JWT
var JWT_header = new Buffer(JSON.stringify({ alg: "RS256", typ: "JWT" })).toString('base64');
// prepare claims set
var iss = "[email protected]"; // service account email address (copied from service_account.json)
var aud = "https://www.googleapis.com/oauth2/v4/token";
var iat = Math.floor(new Date().getTime() / 1000);
var exp = iat + 120; // no need for a long linved token since it's not cached
var target_audience = "12345.apps.googleusercontent.com"; // this is the IAP client ID that can be obtained by clicking 3 dots -> Edit OAuth Client in IAP configuration page
var claims = {
iss: iss,
aud: aud,
iat: iat,
exp: exp,
target_audience: target_audience
};
var JWT_claimset = new Buffer(JSON.stringify(claims)).toString('base64');
// concatenate header and claimset
var unsignedJWT = [JWT_header, JWT_claimset].join('.');
// sign JWT
var JWT_signature = crypto.createSign('RSA-SHA256').update(unsignedJWT).sign(key, 'base64');
var signedJWT = [unsignedJWT, JWT_signature].join('.');
// get id_token and make IAP request
request.post({url:token_URL, form: {grant_type:'urn:ietf:params:oauth:grant-type:jwt-bearer', assertion:signedJWT}}, function(err,res,body){
var data = JSON.parse(body);
var bearer = ['Bearer', data.id_token].join(' ');
var options = {
url: 'https://1234.appspot.com/', // IAP protected GAE app
headers: {
'User-Agent': 'cf2IAP',
'Authorization': bearer
}
};
request(options, function (err, res, body) {
console.log('error:', err);
});
});
res.send('done');
};
/**
* package.json
*
*/
{
"name": "IAP-test",
"version": "0.0.1",
"dependencies": {
"request": ">=2.83"
}
}
Update: Bundling service account key is not recommended, so a better option is to use the metadata server. For the below sample to work Google Identity and Access Management (IAM) API should be enabled and App Engine default service account should have Service Account Actor role (default Editor is not enough):
/**
* Make request from CF to a GAE app behind IAP:
* 1) get access token from the metadata server.
* 2) prepare JWT and use IAM APIs projects.serviceAccounts.signBlob method to avoid bundling service account key.
* 3) 'exchange' JWT for ID token.
* 4) make request with ID token.
*
*/
exports.CfToIAP = function CfToIAP (req, res) {
// imports and constants
const request = require('request');
const user_agent = '<user_agent_to_identify_your_CF_call>';
const token_URL = "https://www.googleapis.com/oauth2/v4/token";
const project_id = '<project_ID_where_CF_is_deployed>';
const service_account = [project_id,
'@appspot.gserviceaccount.com'].join(''); // app default service account for CF project
const target_audience = '<IAP_client_ID>';
const IAP_GAE_app = '<IAP_protected_GAE_app_URL>';
// prepare request options and make metadata server access token request
var meta_req_opts = {
url: ['http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/',
service_account,
'/token'].join(''),
headers: {
'User-Agent': user_agent,
'Metadata-Flavor': 'Google'
}
};
request(meta_req_opts, function (err, res, body) {
// get access token from response
var meta_resp_data = JSON.parse(body);
var access_token = meta_resp_data.access_token;
// prepare JWT that is {Base64url encoded header}.{Base64url encoded claim set}.{Base64url encoded signature}
// https://developers.google.com/identity/protocols/OAuth2ServiceAccount for more info
var JWT_header = new Buffer(JSON.stringify({ alg: "RS256", typ: "JWT" })).toString('base64');
var iat = Math.floor(new Date().getTime() / 1000);
// prepare claims set and base64 encode it
var claims = {
iss: service_account,
aud: token_URL,
iat: iat,
exp: iat + 60, // no need for a long lived token since it's not cached
target_audience: target_audience
};
var JWT_claimset = new Buffer(JSON.stringify(claims)).toString('base64');
// concatenate JWT header and claims set and get signature usign IAM APIs projects.serviceAccounts.signBlob method
var to_sign = [JWT_header, JWT_claimset].join('.');
// sign JWT using IAM APIs projects.serviceAccounts.signBlob method
var signature_req_opts = {
url: ['https://iam.googleapis.com/v1/projects/',
project_id,
'/serviceAccounts/',
service_account,
':signBlob'].join(''),
method: "POST",
json: {
"bytesToSign": new Buffer(to_sign).toString('base64')
},
headers: {
'User-Agent': user_agent,
'Authorization': ['Bearer', access_token].join(' ')
}
};
request(signature_req_opts, function (err, res, body) {
// get signature from response and form JWT
var JWT_signature = body.signature;
var JWT = [JWT_header, JWT_claimset, JWT_signature].join('.');
// obtain ID token
request.post({url:token_URL, form: {grant_type:'urn:ietf:params:oauth:grant-type:jwt-bearer', assertion:JWT}}, function(err, res, body){
// use ID token to make a request to the IAP protected GAE app
var ID_token_resp_data = JSON.parse(body);
var ID_token = ID_token_resp_data.id_token;
var IAP_req_opts = {
url: IAP_GAE_app,
headers: {
'User-Agent': user_agent,
'Authorization': ['Bearer', ID_token].join(' ')
}
};
request(IAP_req_opts, function (err, res, body) {
console.log('error:', err);
});
});
});
});
res.send('done');
};
