Preventing invalid spending of broadcasted states - Specific validation

MessageContract.kt

override fun verify(tx: LedgerTransaction) {
  val commandWithParties: CommandWithParties<Commands> = tx.commands.requireSingleCommand()
  val command = commandWithParties.value
  when (command) {
    is Commands.Send -> // validation for sending
    is Commands.Reply -> requireThat {
      // general requirements from previous snippet
      // precise requirements for `MessageState`
      "One input should be consumed when replying to a message." using (tx.inputs.size == 1)
      "Only one output state should be created when replying to a message." using (tx.outputs.size == 1)
      val output = tx.outputsOfType<MessageState>().single()
      val input = tx.inputsOfType<MessageState>().single()
      "Only the original message's recipient can reply to the message" using (output.sender == input.recipient)
      "The reply must be sent to the original sender" using (output.recipient == input.sender)
      // covered by the general requirements
      "The original sender must be included in the required signers" using commandWithParties.signers.contains(input.sender.owningKey)
      "The original recipient must be included in the required signers" using commandWithParties.signers.contains(input.recipient.owningKey)
    }
  }
}