fake_u2f.rb

# frozen_string_literal: true

module U2F
  # This class is for mocking a U2F device for testing purposes.
  class FakeU2F
    CURVE_NAME = 'prime256v1'.freeze

    attr_accessor :app_id, :counter, :key_handle_raw, :cert_subject

    # Initialize a new FakeU2F device for use in tests.
    #
    # app_id  - The appId/origin this is being tested against.
    # options - A Hash of optional parameters (optional).
    #           :counter      - The initial counter for this device.
    #           :key_handle   - The raw key-handle this device should use.
    #           :cert_subject - The subject field for the certificate generated
    #                           for this device.
    #
    # Returns nothing.
    def initialize(app_id, options = {})
      @app_id = app_id
      @counter = options.fetch(:counter, 0)
      @key_handle_raw = options.fetch(:key_handle, SecureRandom.random_bytes(32))
      @cert_subject = options.fetch(:cert_subject, '/CN=U2FTest')
    end

    # A registerResponse hash as returned by the u2f.register JavaScript API.
    #
    # challenge - The challenge to sign.
    # error     - Boolean. Whether to return an error response (optional).
    #
    # Returns a JSON encoded Hash String.
    def register_response(challenge, error = false)
      if error
        JSON.dump(errorCode: 4)
      else
        client_data_json = client_data(::U2F::ClientData::REGISTRATION_TYP, challenge)
        JSON.dump(
          registrationData: reg_registration_data(client_data_json),
          clientData: ::U2F.urlsafe_encode64(client_data_json)
        )
      end
    end

    # A SignResponse hash as returned by the u2f.sign JavaScript API.
    #
    # challenge - The challenge to sign.
    #
    # Returns a JSON encoded Hash String.
    def sign_response(challenge)
      client_data_json = client_data(::U2F::ClientData::AUTHENTICATION_TYP, challenge)
      JSON.dump(
        clientData: ::U2F.urlsafe_encode64(client_data_json),
        keyHandle: ::U2F.urlsafe_encode64(key_handle_raw),
        signatureData: auth_signature_data(client_data_json)
      )
    end

    # The appId specific public key as returned in the registrationData field of
    # a RegisterResponse Hash.
    #
    # Returns a binary formatted EC public key String.
    def origin_public_key_raw
      [origin_key.public_key.to_bn.to_s(16)].pack('H*')
    end

    # The raw device attestation certificate as returned in the registrationData
    # field of a RegisterResponse Hash.
    #
    # Returns a DER formatted certificate String.
    def cert_raw
      cert.to_der
    end

    private

    # The registrationData field returns in a RegisterResponse Hash.
    #
    # client_data_json - The JSON encoded clientData String.
    #
    # Returns a url-safe base64 encoded binary String.
    def reg_registration_data(client_data_json)
      ::U2F.urlsafe_encode64(
        [
          5,
          origin_public_key_raw,
          key_handle_raw.bytesize,
          key_handle_raw,
          cert_raw,
          reg_signature(client_data_json)
        ].pack("CA65CA#{key_handle_raw.bytesize}A#{cert_raw.bytesize}A*")
      )
    end

    # The signature field of a registrationData field of a RegisterResponse.
    #
    # client_data_json - The JSON encoded clientData String.
    #
    # Returns an ECDSA signature String.
    def reg_signature(client_data_json)
      payload = [
        "\x00",
        ::U2F::DIGEST.digest(app_id),
        ::U2F::DIGEST.digest(client_data_json),
        key_handle_raw,
        origin_public_key_raw
      ].join
      cert_key.sign(::U2F::DIGEST.new, payload)
    end

    # The signatureData field of a SignResponse Hash.
    #
    # client_data_json - The JSON encoded clientData String.
    #
    # Returns a url-safe base64 encoded binary String.
    def auth_signature_data(client_data_json)
      ::U2F.urlsafe_encode64(
        [
          1, # User present
          self.counter += 1,
          auth_signature(client_data_json)
        ].pack('CNA*')
      )
    end

    # The signature field of a signatureData field of a SignResponse Hash.
    #
    # client_data_json - The JSON encoded clientData String.
    #
    # Returns an ECDSA signature String.
    def auth_signature(client_data_json)
      data = [
        ::U2F::DIGEST.digest(app_id),
        1, # User present
        counter,
        ::U2F::DIGEST.digest(client_data_json)
      ].pack('A32CNA32')

      origin_key.sign(::U2F::DIGEST.new, data)
    end

    # The clientData hash as returned by registration and authentication
    # responses.
    #
    # typ       - The String value for the 'typ' field.
    # challenge - The String url-safe base64 encoded challenge parameter.
    #
    # Returns a JSON encoded Hash String.
    def client_data(typ, challenge)
      JSON.dump(
        challenge: challenge,
        origin: app_id,
        typ: typ
      )
    end

    # The appId-specific public/private key.
    #
    # Returns a OpenSSL::PKey::EC instance.
    def origin_key
      @origin_key ||= generate_ec_key
    end

    # The self-signed device attestation certificate.
    #
    # Returns a OpenSSL::X509::Certificate instance.
    def cert
      @cert ||= OpenSSL::X509::Certificate.new.tap do |c|
        c.subject = c.issuer = OpenSSL::X509::Name.parse(cert_subject)
        c.not_before = Time.now
        c.not_after = Time.now + 365 * 24 * 60 * 60
        c.public_key = cert_key
        c.serial = 0x1
        c.version = 0x0
        c.sign cert_key, ::U2F::DIGEST.new
      end
    end

    # The public key used for signing the device certificate.
    #
    # Returns a OpenSSL::PKey::EC instance.
    def cert_key
      @cert_key ||= generate_ec_key
    end

    # Generate an eliptic curve public/private key.
    #
    # Returns a OpenSSL::PKey::EC instance.
    def generate_ec_key
      OpenSSL::PKey::EC.new.tap do |ec|
        ec.group = OpenSSL::PKey::EC::Group.new(CURVE_NAME)
        ec.generate_key
        # https://bugs.ruby-lang.org/issues/8177
        ec.define_singleton_method(:private?) { private_key? }
        ec.define_singleton_method(:public?) { public_key? }
      end
    end
  end
end