@startuml
  alt with pin

    Service -> privacyIDEA: POST /validate/check
    Service <-- privacyIDEA: transaction_id

  else without pin

    Service -> privacyIDEA: POST /validate/triggerchallenge
    Service <-- privacyIDEA: transaction_id

  end

  privacyIDEA -> "SMS Gateway": OTP

  ...User enters OTP from SMS...

  Service -> privacyIDEA: POST /validate/check
  Service <-- privacyIDEA
@enduml
