Preventing invalid spending of broadcasted states - General contract validation

MessageContract.kt

override fun verify(tx: LedgerTransaction) {
  val commandWithParties: CommandWithParties<Commands> = tx.commands.requireSingleCommand()
  when (commandWithParties.value) {
    is Commands.Send -> // validation for sending
    is Commands.Reply -> requireThat {
      val inputPublicKeys = tx.inputs.flatMap { it.state.data.participants.map(AbstractParty::owningKey) }.toSet()
      "The input participant keys are a subset of the signing keys" using commandWithParties.signers.containsAll(inputPublicKeys)
      val outputPublicKeys = tx.outputStates.flatMap { it.participants.map(AbstractParty::owningKey) }.toSet()
      "The output participant keys are a subset of the signing keys" using commandWithParties.signers.containsAll(outputPublicKeys)
    }
  }
}