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
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 self-signed device attestation certificate.

 Returns a OpenSSL::X509::Certificate instance.