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I'm trying to validate the code on a cloud function but I always get same result "invalid_client". Bellow you have my NodeJs function with an example key:import * as functions from 'firebase-functions'; import * as rp from "request-promise-native"; import * as buildURL from "build-url"; import * as jwt from "jsonwebtoken"; const privateKey = '-----BEGIN PRIVATE KEY-----\n' + 'MIGTAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBHkwdwIBAQQgP8TqTrmSMJeaBZv+\n' + 'eOHFL8Y4b1jTIrb8d7FVIFR7vVOgCgYIKoZIzj0DAQehRANCAAQSqhtbJHJ0J24T\n' + 'vAxnANEY7m1yZJ/J7VTGXHGXWic/8xgvOIkhGnmdwgD/oCmIvjo/6yL1hlEx51hr\n' + 'EGErDIg1\n' + '-----END PRIVATE KEY-----'; const home = 'eu.long1.signinwithappleexample://eu.long1.signinwithappleexample/auth'; const clientId = 'eu.long1.signinwithappleexample.android'; const keyId = 'TB943KQS2Y'; const teamId = 'DKY2FBVP6L'; const audience = 'https://appleid.apple.com'; const tokenEndpoint = 'https://appleid.apple.com/auth/token'; export const callback = functions.https.onRequest(async (request, response) => { const data = <AuthResponse>request.body; let params: { [key: string]: string } = {state: data.state}; if (data.error) { response.redirect(buildURL(home, {queryParams: {...params, error: data.error}})); } else if (!data.code) { response.redirect(buildURL(home, {queryParams: {...params, error: 'no_code'}})); } else { try { const jwtOptions = { keyid: keyId, algorithm: 'ES256', issuer: teamId, audience: audience, subject: clientId, expiresIn: 60 }; const clientSecret = jwt.sign({}, privateKey, jwtOptions); const result = await rp.post(tokenEndpoint, { headers: {'Content-Type': 'application/x-www-form-urlencoded'}, formData: { client_id: clientId, client_secret: clientSecret, code: data.code, grant_type: 'authorization_code', redirect_uri: 'https://us-central1-flutter-sdk.cloudfunctions.net/callback' } }); response.redirect(buildURL(home, {queryParams: {...params, result: result.body}})); } catch (e) { response.redirect(buildURL(home, {queryParams: {...params, error: e.error}})); } } }); interface AuthResponse { readonly code: string | undefined readonly id_token: string | undefined readonly state: string readonly user: AuthResponseUser | undefined readonly error: string | undefined } interface AuthResponseUser { readonly name: AuthResponseUserName | undefined readonly email: string | undefined } interface AuthResponseUserName { readonly firstName: string | undefined readonly lastName: string | undefined }My idea is that the signing of the client_secret is wrong for some reason. Any help?