Refactored NotaryChangeProtocol and tests: moved the proposal verification step into the protocol.

Added another proposal verification step in the NotaryChangeProtocol.
Added the cause exception message to the 'notary change refused' error.

contracts/src/main/kotlin/com/r3corda/contracts/cash/Cash.kt

@@ -164,7 +164,7 @@ class Cash : FungibleAsset<Currency>() {
             throw InsufficientBalanceException(amount - gatheredAmount)
 
         val change = if (takeChangeFrom != null && gatheredAmount > amount) {
-            Amount<Issued<Currency>>(gatheredAmount.quantity - amount.quantity, takeChangeFrom.state.state.issuanceDef)
+            Amount<Issued<Currency>>(gatheredAmount.quantity - amount.quantity, takeChangeFrom.state.data.issuanceDef)
         } else {
             null
         }

core/src/main/kotlin/com/r3corda/core/contracts/Structures.kt

@@ -27,7 +27,17 @@ interface ContractState {
     /** Contract by which the state belongs */
     val contract: Contract
 
-    /** List of public keys for each party that can consume this state in a valid transaction. */
+    /**
+     * A _participant_ is any party that is able to consume this state in a valid transaction.
+     *
+     * The list of participants is required for certain types of transactions. For example, when changing the notary
+     * for this state ([TransactionType.NotaryChange]), every participants has to be involved and approve the transaction
+     * so that they receive the updated state, and don't end up in a situation where they can no longer use a state
+     * they possess, since someone consumed that state during the notary change process.
+     *
+     * The participants list should normally be derived from the contents of the state. E.g. for [Cash] the participants
+     * list should just contain the owner.
+     */
     val participants: List<PublicKey>
 }
 
@@ -172,6 +182,7 @@ data class Command(val value: CommandData, val signers: List<PublicKey>) {
     init {
         require(signers.isNotEmpty())
     }
+
     constructor(data: CommandData, key: PublicKey) : this(data, listOf(key))
 
     private fun commandDataToString() = value.toString().let { if (it.contains("@")) it.replace('$', '.').split("@")[0] else it }

core/src/main/kotlin/com/r3corda/core/contracts/TransactionBuilder.kt

@@ -10,9 +10,11 @@ import java.util.*
 
 /**
  * A TransactionBuilder is a transaction class that's mutable (unlike the others which are all immutable). It is
- * intended to be passed around contracts that may edit it by adding new states/commands or modifying the existing set.
+ * intended to be passed around contracts that may edit it by adding new states/commands. Then once the states
- * Then once the states and commands are right, this class can be used as a holding bucket to gather signatures from
+ * and commands are right, this class can be used as a holding bucket to gather signatures from multiple parties.
- * multiple parties.
+ *
+ * The builder can be customised for specific transaction types, e.g. where additional processing is needed
+ * before adding a state/command.
  */
 abstract class TransactionBuilder(protected val type: TransactionType = TransactionType.General(),
                                   protected val notary: Party? = null) {

core/src/main/kotlin/com/r3corda/core/contracts/TransactionTypes.kt

@@ -4,7 +4,7 @@ import com.r3corda.core.crypto.Party
 import com.r3corda.core.noneOrSingle
 import java.security.PublicKey
 
-/** Defines transaction validation rules for a specific transaction type */
+/** Defines transaction build & validation logic for a specific transaction type */
 sealed class TransactionType {
     override fun equals(other: Any?) = other?.javaClass == javaClass
     override fun hashCode() = javaClass.name.hashCode()
@@ -48,6 +48,7 @@ sealed class TransactionType {
 
     /** A general transaction type where transaction validity is determined by custom contract code */
     class General : TransactionType() {
+        /** Just uses the default [TransactionBuilder] with no special logic */
         class Builder(notary: Party? = null) : TransactionBuilder(General(), notary) {}
 
         /**

core/src/main/kotlin/protocols/NotaryChangeProtocol.kt

@@ -30,10 +30,9 @@ object NotaryChangeProtocol {
 
     data class Proposal(val stateRef: StateRef,
                         val newNotary: Party,
-                        val sessionIdForSend: Long,
+                        val stx: SignedTransaction)
-                        val sessionIdForReceive: Long)
 
-    class Handshake(val payload: Proposal,
+    class Handshake(val sessionIdForSend: Long,
                     replyTo: SingleMessageRecipient,
                     replySessionId: Long) : AbstractRequestMessage(replyTo, replySessionId)
 
@@ -102,18 +101,21 @@ object NotaryChangeProtocol {
         @Suspendable
         private fun getParticipantSignature(node: NodeInfo, stx: SignedTransaction, sessionIdForSend: Long): DigitalSignature.WithKey {
             val sessionIdForReceive = random63BitValue()
-            val proposal = Proposal(originalState.ref, newNotary, sessionIdForSend, sessionIdForReceive)
+            val proposal = Proposal(originalState.ref, newNotary, stx)
 
-            val handshake = Handshake(proposal, serviceHub.networkService.myAddress, sessionIdForReceive)
+            val handshake = Handshake(sessionIdForSend, serviceHub.networkService.myAddress, sessionIdForReceive)
-            val protocolInitiated = sendAndReceive<Boolean>(TOPIC_INITIATE, node.address, 0, sessionIdForReceive, handshake).validate { it }
+            sendAndReceive<Unit>(TOPIC_INITIATE, node.address, 0, sessionIdForReceive, handshake)
-            if (!protocolInitiated) throw Refused(node.identity, originalState)
 
-            val response = sendAndReceive<DigitalSignature.WithKey>(TOPIC_CHANGE, node.address, sessionIdForSend, sessionIdForReceive, stx)
+            val response = sendAndReceive<Result>(TOPIC_CHANGE, node.address, sessionIdForSend, sessionIdForReceive, proposal)
             val participantSignature = response.validate {
-                check(it.by == node.identity.owningKey) { "Not signed by the required participant" }
+                if (it.sig == null) throw NotaryChangeException(it.error!!)
-                it.verifyWithECDSA(stx.txBits)
+                else {
-                it
+                    check(it.sig.by == node.identity.owningKey) { "Not signed by the required participant" }
+                    it.sig.verifyWithECDSA(stx.txBits)
+                    it.sig
                 }
+            }
+
             return participantSignature
         }
 
@@ -132,38 +134,88 @@ object NotaryChangeProtocol {
         companion object {
             object VERIFYING : ProgressTracker.Step("Verifying Notary change proposal")
 
-            object SIGNING : ProgressTracker.Step("Signing Notary change transaction")
+            object APPROVING : ProgressTracker.Step("Notary change approved")
 
-            fun tracker() = ProgressTracker(VERIFYING, SIGNING)
+            object REJECTING : ProgressTracker.Step("Notary change rejected")
+
+            fun tracker() = ProgressTracker(VERIFYING, APPROVING, REJECTING)
         }
 
         @Suspendable
         override fun call() {
             progressTracker.currentStep = VERIFYING
+            val proposal = receive<Proposal>(TOPIC_CHANGE, sessionIdForReceive).validate { it }
 
-            val proposedTx = receive<SignedTransaction>(TOPIC_CHANGE, sessionIdForReceive).validate { validateTx(it) }
+            try {
+                verifyProposal(proposal)
+                verifyTx(proposal.stx)
+            } catch(e: Exception) {
+                // TODO: catch only specific exceptions. However, there are numerous validation exceptions
+                //       that might occur (tx validation/resolution, invalid proposal). Need to rethink how
+                //       we manage exceptions and maybe introduce some platform exception hierarchy
+                val myIdentity = serviceHub.storageService.myLegalIdentity
+                val state = proposal.stateRef
+                val reason = NotaryChangeRefused(myIdentity, state, e.message)
 
-            progressTracker.currentStep = SIGNING
+                reject(reason)
+                return
+            }
 
-            val mySignature = sign(proposedTx)
+            approve(proposal.stx)
-            val swapSignatures = sendAndReceive<List<DigitalSignature.WithKey>>(TOPIC_CHANGE, otherSide, sessionIdForSend, sessionIdForReceive, mySignature)
+        }
+
+        @Suspendable
+        private fun approve(stx: SignedTransaction) {
+            progressTracker.currentStep = APPROVING
+
+            val mySignature = sign(stx)
+            val response = Result.noError(mySignature)
+            val swapSignatures = sendAndReceive<List<DigitalSignature.WithKey>>(TOPIC_CHANGE, otherSide, sessionIdForSend, sessionIdForReceive, response)
 
             val allSignatures = swapSignatures.validate { signatures ->
-                signatures.forEach { it.verifyWithECDSA(proposedTx.txBits) }
+                signatures.forEach { it.verifyWithECDSA(stx.txBits) }
                 signatures
             }
 
-            val finalTx = proposedTx + allSignatures
+            val finalTx = stx + allSignatures
             finalTx.verify()
             serviceHub.recordTransactions(listOf(finalTx))
         }
 
         @Suspendable
-        private fun validateTx(stx: SignedTransaction): SignedTransaction {
+        private fun reject(e: NotaryChangeRefused) {
+            progressTracker.currentStep = REJECTING
+            val response = Result.withError(e)
+            send(TOPIC_CHANGE, otherSide, sessionIdForSend, response)
+        }
+
+        /**
+         * Check the notary change proposal.
+         *
+         * For example, if the proposed new notary has the same behaviour (e.g. both are non-validating)
+         * and is also in a geographically convenient location we can just automatically approve the change.
+         * TODO: In more difficult cases this should call for human attention to manually verify and approve the proposal
+         */
+        @Suspendable
+        private fun verifyProposal(proposal: NotaryChangeProtocol.Proposal) {
+            val newNotary = proposal.newNotary
+            val isNotary = serviceHub.networkMapCache.notaryNodes.any { it.identity == newNotary }
+            require(isNotary) { "The proposed node $newNotary does not run a Notary service " }
+
+            val state = proposal.stateRef
+            val proposedTx = proposal.stx.tx
+            require(proposedTx.inputs.contains(state)) { "The proposed state $state is not in the proposed transaction inputs" }
+
+            // An example requirement
+            val blacklist = listOf("Evil Notary")
+            require(!blacklist.contains(newNotary.name)) { "The proposed new notary $newNotary is not trusted by the party" }
+        }
+
+        @Suspendable
+        private fun verifyTx(stx: SignedTransaction) {
             checkMySignatureRequired(stx.tx)
             checkDependenciesValid(stx)
             checkValid(stx)
-            return stx
         }
 
         private fun checkMySignatureRequired(tx: WireTransaction) {
@@ -189,8 +241,20 @@ object NotaryChangeProtocol {
         }
     }
 
-    /** Thrown when a participant refuses to change the notary of the state */
+    // TODO: similar classes occur in other places (NotaryProtocol), need to consolidate
-    class Refused(val identity: Party, val originalState: StateAndRef<*>) : Exception() {
+    data class Result private constructor(val sig: DigitalSignature.WithKey?, val error: NotaryChangeRefused?) {
-        override fun toString() = "A participant $identity refused to change the notary of state $originalState"
+        companion object {
+            fun withError(error: NotaryChangeRefused) = Result(null, error)
+            fun noError(sig: DigitalSignature.WithKey) = Result(sig, null)
+        }
     }
 }
+
+/** Thrown when a participant refuses to change the notary of the state */
+class NotaryChangeRefused(val identity: Party, val state: StateRef, val cause: String?) {
+    override fun toString() = "A participant $identity refused to change the notary of state $state"
+}
+
+class NotaryChangeException(val error: NotaryChangeRefused) : Exception() {
+    override fun toString() = "${super.toString()}: Notary change failed - ${error.toString()}"
+}

core/src/test/kotlin/com/r3corda/core/contracts/TransactionGraphSearchTests.kt

@@ -26,13 +26,13 @@ class TransactionGraphSearchTests {
      * @param signer signer for the two transactions and their commands.
      */
     fun buildTransactions(command: CommandData, signer: KeyPair): GraphTransactionStorage {
-        val originTx = TransactionBuilder().apply {
+        val originTx = TransactionType.General.Builder().apply {
-            addOutputState(DummyContract.State(random31BitValue(), DUMMY_NOTARY))
+            addOutputState(DummyContract.State(random31BitValue()), DUMMY_NOTARY)
             addCommand(command, signer.public)
             signWith(signer)
         }.toSignedTransaction(false)
-        val inputTx = TransactionBuilder().apply {
+        val inputTx = TransactionType.General.Builder().apply {
-            addInputState(originTx.tx.outRef<DummyContract.State>(0).ref)
+            addInputState(originTx.tx.outRef<DummyContract.State>(0))
             signWith(signer)
         }.toSignedTransaction(false)
         return GraphTransactionStorage(originTx, inputTx)

core/src/test/kotlin/com/r3corda/core/node/WalletUpdateTests.kt

@@ -5,6 +5,7 @@ import com.r3corda.core.crypto.SecureHash
 import com.r3corda.core.node.services.Wallet
 import com.r3corda.core.testing.DUMMY_NOTARY
 import org.junit.Test
+import java.security.PublicKey
 import kotlin.test.assertEquals
 
 
@@ -12,14 +13,15 @@ class WalletUpdateTests {
 
     object DummyContract : Contract {
 
-        override fun verify(tx: TransactionForVerification) {
+        override fun verify(tx: TransactionForContract) {
         }
 
         override val legalContractReference: SecureHash = SecureHash.sha256("")
     }
 
     private class DummyState : ContractState {
-        override val notary = DUMMY_NOTARY
+        override val participants: List<PublicKey>
+            get() = emptyList()
         override val contract = WalletUpdateTests.DummyContract
     }
 
@@ -29,11 +31,11 @@ class WalletUpdateTests {
     private val stateRef3 = StateRef(SecureHash.randomSHA256(), 3)
     private val stateRef4 = StateRef(SecureHash.randomSHA256(), 4)
 
-    private val stateAndRef0 = StateAndRef<DummyState>(DummyState(), stateRef0)
+    private val stateAndRef0 = StateAndRef(TransactionState(DummyState(), DUMMY_NOTARY), stateRef0)
-    private val stateAndRef1 = StateAndRef<DummyState>(DummyState(), stateRef1)
+    private val stateAndRef1 = StateAndRef(TransactionState(DummyState(), DUMMY_NOTARY), stateRef1)
-    private val stateAndRef2 = StateAndRef<DummyState>(DummyState(), stateRef2)
+    private val stateAndRef2 = StateAndRef(TransactionState(DummyState(), DUMMY_NOTARY), stateRef2)
-    private val stateAndRef3 = StateAndRef<DummyState>(DummyState(), stateRef3)
+    private val stateAndRef3 = StateAndRef(TransactionState(DummyState(), DUMMY_NOTARY), stateRef3)
-    private val stateAndRef4 = StateAndRef<DummyState>(DummyState(), stateRef4)
+    private val stateAndRef4 = StateAndRef(TransactionState(DummyState(), DUMMY_NOTARY), stateRef4)
 
     @Test
     fun `nothing plus nothing is nothing`() {

node/src/main/kotlin/com/r3corda/node/internal/testing/TestUtils.kt

@@ -3,6 +3,8 @@ package com.r3corda.node.internal.testing
 import com.r3corda.core.contracts.StateAndRef
 import com.r3corda.core.contracts.DummyContract
 import com.r3corda.core.contracts.StateRef
+import com.r3corda.core.contracts.TransactionState
+import com.r3corda.core.contracts.TransactionType
 import com.r3corda.core.crypto.Party
 import com.r3corda.core.seconds
 import com.r3corda.core.testing.DUMMY_NOTARY
@@ -11,8 +13,8 @@ import com.r3corda.node.internal.AbstractNode
 import java.time.Instant
 import java.util.*
 
-fun issueState(node: AbstractNode, notary: Party = DUMMY_NOTARY): StateAndRef<*> {
+fun issueState(node: AbstractNode): StateAndRef<*> {
-    val tx = DummyContract().generateInitial(node.info.identity.ref(0), Random().nextInt(), notary)
+    val tx = DummyContract().generateInitial(node.info.identity.ref(0), Random().nextInt(), DUMMY_NOTARY)
     tx.signWith(node.storage.myLegalIdentityKey)
     tx.signWith(DUMMY_NOTARY_KEY)
     val stx = tx.toSignedTransaction()
@@ -20,6 +22,20 @@ fun issueState(node: AbstractNode, notary: Party = DUMMY_NOTARY): StateAndRef<*>
     return StateAndRef(tx.outputStates().first(), StateRef(stx.id, 0))
 }
 
+fun issueMultiPartyState(nodeA: AbstractNode, nodeB: AbstractNode): StateAndRef<DummyContract.MultiOwnerState> {
+    val state = TransactionState(DummyContract.MultiOwnerState(0,
+            listOf(nodeA.info.identity.owningKey, nodeB.info.identity.owningKey)), DUMMY_NOTARY)
+    val tx = TransactionType.NotaryChange.Builder().withItems(state)
+    tx.signWith(nodeA.storage.myLegalIdentityKey)
+    tx.signWith(nodeB.storage.myLegalIdentityKey)
+    tx.signWith(DUMMY_NOTARY_KEY)
+    val stx = tx.toSignedTransaction()
+    nodeA.services.recordTransactions(listOf(stx))
+    nodeB.services.recordTransactions(listOf(stx))
+    val stateAndRef = StateAndRef(state, StateRef(stx.id, 0))
+    return stateAndRef
+}
+
 fun issueInvalidState(node: AbstractNode, notary: Party = DUMMY_NOTARY): StateAndRef<*> {
     val tx = DummyContract().generateInitial(node.info.identity.ref(0), Random().nextInt(), notary)
     tx.setTime(Instant.now(), notary, 30.seconds)

node/src/main/kotlin/com/r3corda/node/internal/testing/WalletFiller.kt

@@ -3,7 +3,7 @@ package com.r3corda.node.internal.testing
 import com.r3corda.contracts.cash.Cash
 import com.r3corda.core.contracts.Amount
 import com.r3corda.core.contracts.Issued
-import com.r3corda.core.contracts.TransactionBuilder
+import com.r3corda.core.contracts.TransactionType
 import com.r3corda.core.crypto.Party
 import com.r3corda.core.node.ServiceHub
 import com.r3corda.core.serialization.OpaqueBytes
@@ -34,7 +34,7 @@ object WalletFiller {
             // this field as there's no other database or source of truth we need to sync with.
             val depositRef = myIdentity.ref(ref)
 
-            val issuance = TransactionBuilder()
+            val issuance = TransactionType.General.Builder()
             val freshKey = services.keyManagementService.freshKey()
             cash.generateIssue(issuance, Amount(pennies, Issued(depositRef, howMuch.token)), freshKey.public, notary)
             issuance.signWith(myKey)

node/src/main/kotlin/com/r3corda/node/services/NotaryChangeService.kt

@@ -17,37 +17,11 @@ class NotaryChangeService(net: MessagingService, val smm: StateMachineManager) :
         )
     }
 
-    private fun handleChangeNotaryRequest(req: NotaryChangeProtocol.Handshake): Boolean {
+    private fun handleChangeNotaryRequest(req: NotaryChangeProtocol.Handshake) {
-        val proposal = req.payload
-        val autoAccept = checkProposal(proposal)
-
-        if (autoAccept) {
         val protocol = NotaryChangeProtocol.Acceptor(
                 req.replyTo as SingleMessageRecipient,
-                    proposal.sessionIdForReceive,
+                req.sessionID!!,
-                    proposal.sessionIdForSend)
+                req.sessionIdForSend)
         smm.add(NotaryChangeProtocol.TOPIC_CHANGE, protocol)
     }
-        return autoAccept
-    }
-
-    /**
-     * Check the notary change for a state proposal and decide whether to allow the change and initiate the protocol
-     * or deny the change.
-     *
-     * For example, if the proposed new notary has the same behaviour (e.g. both are non-validating)
-     * and is also in a geographically convenient location we can just automatically approve the change.
-     * TODO: In more difficult cases this should call for human attention to manually verify and approve the proposal
-     */
-    private fun checkProposal(proposal: NotaryChangeProtocol.Proposal): Boolean {
-        val newNotary = proposal.newNotary
-        val isNotary = smm.serviceHub.networkMapCache.notaryNodes.any { it.identity == newNotary }
-        require(isNotary) { "The proposed node $newNotary does not run a Notary service " }
-
-        // An example requirement
-        val blacklist = listOf("Evil Notary")
-        require(!blacklist.contains(newNotary.name))
-
-        return true
-    }
 }

node/src/test/kotlin/node/services/NotaryChangeTests.kt

@@ -1,20 +1,24 @@
 package node.services
 
-import com.r3corda.contracts.DummyContract
+import com.r3corda.core.crypto.Party
-import com.r3corda.core.contracts.StateAndRef
+import com.r3corda.core.crypto.generateKeyPair
-import com.r3corda.core.contracts.StateRef
-import com.r3corda.core.contracts.TransactionState
-import com.r3corda.core.contracts.TransactionType
 import com.r3corda.core.testing.DUMMY_NOTARY
 import com.r3corda.core.testing.DUMMY_NOTARY_KEY
 import com.r3corda.node.internal.testing.MockNetwork
+import com.r3corda.node.internal.testing.issueMultiPartyState
 import com.r3corda.node.internal.testing.issueState
-import com.r3corda.node.services.transactions.NotaryService
+import com.r3corda.node.services.network.NetworkMapService
+import com.r3corda.node.services.transactions.SimpleNotaryService
 import org.junit.Before
 import org.junit.Test
+import protocols.NotaryChangeException
 import protocols.NotaryChangeProtocol
 import protocols.NotaryChangeProtocol.Instigator
+import protocols.NotaryChangeRefused
+import java.util.concurrent.ExecutionException
 import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
 
 class NotaryChangeTests {
     lateinit var net: MockNetwork
@@ -26,22 +30,22 @@ class NotaryChangeTests {
     @Before
     fun setup() {
         net = MockNetwork()
-        oldNotaryNode = net.createNotaryNode(DUMMY_NOTARY.name, DUMMY_NOTARY_KEY)
+        oldNotaryNode = net.createNode(
-        clientNodeA = net.createPartyNode(networkMapAddr = oldNotaryNode.info)
+                legalName = DUMMY_NOTARY.name,
-        clientNodeB = net.createPartyNode(networkMapAddr = oldNotaryNode.info)
+                keyPair = DUMMY_NOTARY_KEY,
-        newNotaryNode = net.createNode(networkMapAddress = oldNotaryNode.info, advertisedServices = NotaryService.Type)
+                advertisedServices = *arrayOf(NetworkMapService.Type, SimpleNotaryService.Type))
+        clientNodeA = net.createNode(networkMapAddress = oldNotaryNode.info)
+        clientNodeB = net.createNode(networkMapAddress = oldNotaryNode.info)
+        newNotaryNode = net.createNode(networkMapAddress = oldNotaryNode.info, advertisedServices = SimpleNotaryService.Type)
 
         net.runNetwork() // Clear network map registration messages
     }
 
     @Test
     fun `should change notary for a state with single participant`() {
-        val ref = issueState(clientNodeA, DUMMY_NOTARY).ref
+        val state = issueState(clientNodeA)
-        val state = clientNodeA.services.loadState(ref)
-
         val newNotary = newNotaryNode.info.identity
-
+        val protocol = Instigator(state, newNotary)
-        val protocol = Instigator(StateAndRef(state, ref), newNotary)
         val future = clientNodeA.smm.add(NotaryChangeProtocol.TOPIC_CHANGE, protocol)
 
         net.runNetwork()
@@ -52,21 +56,9 @@ class NotaryChangeTests {
 
     @Test
     fun `should change notary for a state with multiple participants`() {
-        val state = TransactionState(DummyContract.MultiOwnerState(0,
+        val state = issueMultiPartyState(clientNodeA, clientNodeB)
-                listOf(clientNodeA.info.identity.owningKey, clientNodeB.info.identity.owningKey)), DUMMY_NOTARY)
-
-        val tx = TransactionType.NotaryChange.Builder().withItems(state)
-        tx.signWith(clientNodeA.storage.myLegalIdentityKey)
-        tx.signWith(clientNodeB.storage.myLegalIdentityKey)
-        tx.signWith(DUMMY_NOTARY_KEY)
-        val stx = tx.toSignedTransaction()
-        clientNodeA.services.recordTransactions(listOf(stx))
-        clientNodeB.services.recordTransactions(listOf(stx))
-        val stateAndRef = StateAndRef(state, StateRef(stx.id, 0))
-
         val newNotary = newNotaryNode.info.identity
-
+        val protocol = Instigator(state, newNotary)
-        val protocol = Instigator(stateAndRef, newNotary)
         val future = clientNodeA.smm.add(NotaryChangeProtocol.TOPIC_CHANGE, protocol)
 
         net.runNetwork()
@@ -78,8 +70,21 @@ class NotaryChangeTests {
         assertEquals(loadedStateA, loadedStateB)
     }
 
+    @Test
+    fun `should throw when a participant refuses to change Notary`() {
+        val state = issueMultiPartyState(clientNodeA, clientNodeB)
+        val newEvilNotary = Party("Evil Notary", generateKeyPair().public)
+        val protocol = Instigator(state, newEvilNotary)
+        val future = clientNodeA.smm.add(NotaryChangeProtocol.TOPIC_CHANGE, protocol)
+
+        net.runNetwork()
+
+        val ex = assertFailsWith(ExecutionException::class) { future.get() }
+        val error = (ex.cause as NotaryChangeException).error
+        assertTrue(error is NotaryChangeRefused)
+    }
+
     // TODO: Add more test cases once we have a general protocol/service exception handling mechanism:
-    //       - A participant refuses to change Notary
     //       - A participant is offline/can't be found on the network
     //       - The requesting party is not a participant
     //       - The requesting party wants to change additional state fields