Google OAuth 2.0 Service Account Authorization

service_account.rb

require 'multi_json'
require 'base64'
require 'openssl'
require 'faraday'

now = Time.now.utc.to_i

def encode(hash)
  Base64.urlsafe_encode64(MultiJson.dump(hash))
end

header = {
  alg: "RS256",
  typ: "JWT"
}

claim = {
  iss: ARGV[0],
  scope: "https://www.google.com/m8/feeds/",
  aud: "https://accounts.google.com/o/oauth2/token",
  exp: now + 3500,
  iat: now.to_i
}

assertion = [encode(header), encode(claim)].join('.')

cert = OpenSSL::PKCS12.new File.open(ARGV[1]){|f| f.read}, 'notasecret'

signature = Base64.urlsafe_encode64 cert.key.sign(OpenSSL::Digest::SHA256.new, assertion)

signed_assertion = [assertion,signature].join('.')

response = Faraday.post("https://accounts.google.com/o/oauth2/token", 
  grant_type: 'assertion',
  assertion_type: 'http://oauth.net/grant_type/jwt/1.0/bearer',
  assertion: signed_assertion
)