How to use the oauth-sign.rsasign function in oauth-sign

function getSignature (r) {
  var sign
  r.headers.Authorization.slice('OAuth '.length).replace(/, /g, ',').split(',').forEach(function (v) {
    if (v.slice(0, 'oauth_signature="'.length) === 'oauth_signature="') {
      sign = v.slice('oauth_signature="'.length, -1)
    }
  })
  return decodeURIComponent(sign)
}

// Tests from Twitter documentation https://dev.twitter.com/docs/auth/oauth

var hmacsign = oauth.hmacsign
var hmacsign256 = oauth.hmacsign256
var rsasign = oauth.rsasign
var rsaPrivatePEM = fs.readFileSync(path.join(__dirname, 'ssl', 'test.key'))
var reqsign
var reqsign256
var reqsignRSA
var accsign
var accsign256
var accsignRSA
var upsign
var upsign256
var upsignRSA

tape('reqsign', function (t) {
  reqsign = hmacsign('POST', 'https://api.twitter.com/oauth/request_token',
    { oauth_callback: 'http://localhost:3005/the_dance/process_callback?service_provider_id=11',
      oauth_consumer_key: 'GDdmIQH6jhtmLUypg82g',
      oauth_nonce: 'QP70eNmVz8jvdPevU3oJD2AfF7R7odC2XJcn4XlZJqk',