generate-client-assertion

#!/usr/bin/env ruby
#
# USAGE
# -----
#
#   generate-client-assertion
#     --aud={ISSUER}
#     --key={PRIVATE_KEY_IN_JWK_FORMAT}
#     --sub={CLIENT_ID}
#     [--duration={DURATION_IN_SECONDS}]
#


require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'json-jwt'
  gem 'optparse'
end

require 'securerandom'
require 'time'


#------------------------------------------------------------
# main
#------------------------------------------------------------
def main(args)
  # Process the command line options.
  options = Options.process(args)

  # Prepare the payload of the client assertion.
  payload = build_payload(options)

  # Generate a JWS by signing the payload with the key.
  assertion = sign(payload, options.key)

  # Write the client assertion to the standard output.
  puts assertion
end


#------------------------------------------------------------
# Prepare the payload of the client assertion.
#------------------------------------------------------------
def build_payload(options)
  # The current time in seconds for time-related claims
  now = Time.now.to_i

  # Payload of a client assertion (RFC 7523)
  {
    iss: options.subject,
    sub: options.subject,
    aud: options.audience,
    iat: now,
    exp: now + options.duration,
    jti: SecureRandom.uuid
  }
end


#------------------------------------------------------------
# Generate a JWS by signing the payload with the key.
#------------------------------------------------------------
def sign(payload, jwk)
  # Prepare a JWT with the payload.
  jwt = JSON::JWT.new(payload)

  # Sign the JWT with the key and convert it to JWS.
  jwt.sign(jwk, jwk[:alg]).to_s
end


#------------------------------------------------------------
# Command line options
#------------------------------------------------------------
class Options < OptionParser
  DEFAULT_DURATION = 3600

  DESC_AUDIENCE = "The value of the 'aud' claim. For example, the URL of the token endpoint."
  DESC_DURATION = "The duration of the client assertion in seconds. The default value is #{DEFAULT_DURATION}."
  DESC_KEY      = "A file containing a private key in the JWK format."
  DESC_SUBJECT  = "The value of the 'sub' claim. The client ID."

  attr_reader :audience, :duration, :key, :subject

  def initialize
    super

    @audience = nil
    @duration = DEFAULT_DURATION
    @key      = nil
    @subject  = nil

    self.on('-a AUDIENCE', '--aud=AUDIENCE', DESC_AUDIENCE) do |audience|
      @audience = audience
    end

    self.on('-d DURATION', '--duration=DURATION', /^[1-9][0-9]+$/, DESC_DURATION) do |duration|
      @duration = duration.to_i
    end

    self.on('-k FILE', '--key=FILE', DESC_KEY) do |file|
      @key = read_jwk(file)
    end

    self.on('-s SUBJECT', '--sub=SUBJECT', DESC_SUBJECT) do |subject|
      @subject = subject
    end
  end

  private

  def read_jwk(file)
    json = File.read(file)
    hash = JSON.parse(json, {symbolize_names: true})
    JSON::JWK.new(hash)
  end

  def error_if_missing(value, option)
    if value.nil?
      raise OptionParser::ParseError.new "'#{option}' is missing."
    end
  end

  public

  def verify
    error_if_missing(@audience, '--aud=AUDIENCE')
    error_if_missing(@key,      '--key=FILE')
    error_if_missing(@subject,  '--sub=SUBJECT')
  end

  def self.process(args)
    options = Options.new
    options.parse(args)
    options.verify()

    return options
  end
end


#------------------------------------------------------------
# Entry Point
#------------------------------------------------------------
main(ARGV)