authenticateOrRejectWithChallenge

val challenge = HttpChallenge("MyAuth", Some("MyRealm"))

// your custom authentication logic:
def auth(creds: HttpCredentials): Boolean = true

def myUserPassAuthenticator(credentials: Option[HttpCredentials]): Future[AuthenticationResult[String]] =
  Future {
    credentials match {
      case Some(creds) if auth(creds) => Right("some-user-name-from-creds")
      case _                          => Left(challenge)
    }
  }

val route =
  Route.seal {
    path("secured") {
      authenticateOrRejectWithChallenge(myUserPassAuthenticator _) { userName =>
        complete("Authenticated!")
      }
    }
  }

// tests:
Get("/secured") ~> route ~> check {
  status shouldEqual StatusCodes.Unauthorized
  responseAs[String] shouldEqual "The resource requires authentication, which was not supplied with the request"
  header[`WWW-Authenticate`].get.challenges.head shouldEqual HttpChallenge("MyAuth", Some("MyRealm"))
}

val validCredentials = BasicHttpCredentials("John", "p4ssw0rd")
Get("/secured") ~> addCredentials(validCredentials) ~> // adds Authorization header
  route ~> check {
    status shouldEqual StatusCodes.OK
    responseAs[String] shouldEqual "Authenticated!"
  }