diff --git a/client/rpc/src/main/kotlin/net/corda/client/rpc/internal/RPCClientProxyHandler.kt b/client/rpc/src/main/kotlin/net/corda/client/rpc/internal/RPCClientProxyHandler.kt
index 03cc1bec69..dce8fb99dd 100644
--- a/client/rpc/src/main/kotlin/net/corda/client/rpc/internal/RPCClientProxyHandler.kt
+++ b/client/rpc/src/main/kotlin/net/corda/client/rpc/internal/RPCClientProxyHandler.kt
@@ -10,7 +10,7 @@ import com.google.common.cache.RemovalCause
 import com.google.common.cache.RemovalListener
 import com.google.common.util.concurrent.SettableFuture
 import com.google.common.util.concurrent.ThreadFactoryBuilder
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.crypto.random63BitValue
 import net.corda.core.getOrThrow
 import net.corda.core.internal.LazyPool
diff --git a/core/src/main/kotlin/net/corda/core/Utils.kt b/core/src/main/kotlin/net/corda/core/Utils.kt
index 94ef3ac8f5..11121de66c 100644
--- a/core/src/main/kotlin/net/corda/core/Utils.kt
+++ b/core/src/main/kotlin/net/corda/core/Utils.kt
@@ -25,12 +25,10 @@ import java.util.concurrent.CompletableFuture
 import java.util.concurrent.ExecutionException
 import java.util.concurrent.Future
 import java.util.concurrent.TimeUnit
-import java.util.concurrent.locks.ReentrantLock
 import java.util.zip.Deflater
 import java.util.zip.ZipEntry
 import java.util.zip.ZipInputStream
 import java.util.zip.ZipOutputStream
-import kotlin.concurrent.withLock
 
 // TODO: Review by EOY2016 if we ever found these utilities helpful.
 val Int.bd: BigDecimal get() = BigDecimal(this)
@@ -163,33 +161,6 @@ fun <T> logElapsedTime(label: String, logger: Logger? = null, body: () -> T): T
 
 fun <T> Logger.logElapsedTime(label: String, body: () -> T): T = logElapsedTime(label, this, body)
 
-/**
- * A threadbox is a simple utility that makes it harder to forget to take a lock before accessing some shared state.
- * Simply define a private class to hold the data that must be grouped under the same lock, and then pass the only
- * instance to the ThreadBox constructor. You can now use the [locked] method with a lambda to take the lock in a
- * way that ensures it'll be released if there's an exception.
- *
- * Note that this technique is not infallible: if you capture a reference to the fields in another lambda which then
- * gets stored and invoked later, there may still be unsafe multi-threaded access going on, so watch out for that.
- * This is just a simple guard rail that makes it harder to slip up.
- *
- * Example:
- *
- * private class MutableState { var i = 5 }
- * private val state = ThreadBox(MutableState())
- *
- * val ii = state.locked { i }
- */
-class ThreadBox<out T>(val content: T, val lock: ReentrantLock = ReentrantLock()) {
-    inline fun <R> locked(body: T.() -> R): R = lock.withLock { body(content) }
-    inline fun <R> alreadyLocked(body: T.() -> R): R {
-        check(lock.isHeldByCurrentThread, { "Expected $lock to already be locked." })
-        return body(content)
-    }
-
-    fun checkNotLocked() = check(!lock.isHeldByCurrentThread)
-}
-
 /**
  * Given a path to a zip file, extracts it to the given directory.
  */
diff --git a/core/src/main/kotlin/net/corda/core/internal/ThreadBox.kt b/core/src/main/kotlin/net/corda/core/internal/ThreadBox.kt
new file mode 100644
index 0000000000..eedd576694
--- /dev/null
+++ b/core/src/main/kotlin/net/corda/core/internal/ThreadBox.kt
@@ -0,0 +1,32 @@
+package net.corda.core.internal
+
+import java.util.concurrent.locks.ReentrantLock
+import kotlin.concurrent.withLock
+
+/**
+ * A threadbox is a simple utility that makes it harder to forget to take a lock before accessing some shared state.
+ * Simply define a private class to hold the data that must be grouped under the same lock, and then pass the only
+ * instance to the ThreadBox constructor. You can now use the [locked] method with a lambda to take the lock in a
+ * way that ensures it'll be released if there's an exception.
+ *
+ * Note that this technique is not infallible: if you capture a reference to the fields in another lambda which then
+ * gets stored and invoked later, there may still be unsafe multi-threaded access going on, so watch out for that.
+ * This is just a simple guard rail that makes it harder to slip up.
+ *
+ * Example:
+ *```
+ * private class MutableState { var i = 5 }
+ * private val state = ThreadBox(MutableState())
+ *
+ * val ii = state.locked { i }
+ * ```
+ */
+class ThreadBox<out T>(val content: T, val lock: ReentrantLock = ReentrantLock()) {
+    inline fun <R> locked(body: T.() -> R): R = lock.withLock { body(content) }
+    inline fun <R> alreadyLocked(body: T.() -> R): R {
+        check(lock.isHeldByCurrentThread, { "Expected $lock to already be locked." })
+        return body(content)
+    }
+
+    fun checkNotLocked(): Unit = check(!lock.isHeldByCurrentThread)
+}
diff --git a/node/src/main/kotlin/net/corda/node/services/events/NodeSchedulerService.kt b/node/src/main/kotlin/net/corda/node/services/events/NodeSchedulerService.kt
index b69906afbe..2ec91e357e 100644
--- a/node/src/main/kotlin/net/corda/node/services/events/NodeSchedulerService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/events/NodeSchedulerService.kt
@@ -1,7 +1,7 @@
 package net.corda.node.services.events
 
 import com.google.common.util.concurrent.SettableFuture
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.contracts.SchedulableState
 import net.corda.core.contracts.ScheduledActivity
 import net.corda.core.contracts.ScheduledStateRef
diff --git a/node/src/main/kotlin/net/corda/node/services/keys/E2ETestKeyManagementService.kt b/node/src/main/kotlin/net/corda/node/services/keys/E2ETestKeyManagementService.kt
index cd09d34e3c..c41398a236 100644
--- a/node/src/main/kotlin/net/corda/node/services/keys/E2ETestKeyManagementService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/keys/E2ETestKeyManagementService.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.keys
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.crypto.DigitalSignature
 import net.corda.core.crypto.generateKeyPair
 import net.corda.core.crypto.keys
diff --git a/node/src/main/kotlin/net/corda/node/services/keys/PersistentKeyManagementService.kt b/node/src/main/kotlin/net/corda/node/services/keys/PersistentKeyManagementService.kt
index 19108a422c..d145003723 100644
--- a/node/src/main/kotlin/net/corda/node/services/keys/PersistentKeyManagementService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/keys/PersistentKeyManagementService.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.keys
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.crypto.DigitalSignature
 import net.corda.core.crypto.generateKeyPair
 import net.corda.core.crypto.keys
diff --git a/node/src/main/kotlin/net/corda/node/services/messaging/ArtemisMessagingServer.kt b/node/src/main/kotlin/net/corda/node/services/messaging/ArtemisMessagingServer.kt
index 221460f952..a598dded88 100644
--- a/node/src/main/kotlin/net/corda/node/services/messaging/ArtemisMessagingServer.kt
+++ b/node/src/main/kotlin/net/corda/node/services/messaging/ArtemisMessagingServer.kt
@@ -7,6 +7,7 @@ import net.corda.core.*
 import net.corda.core.crypto.*
 import net.corda.core.crypto.X509Utilities.CORDA_CLIENT_TLS
 import net.corda.core.crypto.X509Utilities.CORDA_ROOT_CA
+import net.corda.core.internal.ThreadBox
 import net.corda.core.internal.div
 import net.corda.core.node.NodeInfo
 import net.corda.core.node.services.NetworkMapCache
diff --git a/node/src/main/kotlin/net/corda/node/services/messaging/NodeMessagingClient.kt b/node/src/main/kotlin/net/corda/node/services/messaging/NodeMessagingClient.kt
index 81f9a47701..463a89002b 100644
--- a/node/src/main/kotlin/net/corda/node/services/messaging/NodeMessagingClient.kt
+++ b/node/src/main/kotlin/net/corda/node/services/messaging/NodeMessagingClient.kt
@@ -1,10 +1,10 @@
 package net.corda.node.services.messaging
 
 import com.google.common.util.concurrent.ListenableFuture
-import net.corda.core.ThreadBox
 import net.corda.core.andForget
 import net.corda.core.crypto.random63BitValue
 import net.corda.core.getOrThrow
+import net.corda.core.internal.ThreadBox
 import net.corda.core.messaging.CordaRPCOps
 import net.corda.core.messaging.MessageRecipients
 import net.corda.core.messaging.RPCOps
@@ -468,8 +468,8 @@ class NodeMessagingClient(override val config: NodeConfiguration,
     }
 
     private fun sendWithRetry(retryCount: Int, address: String, message: ClientMessage, retryId: Long) {
-        fun randomiseDuplicateId(message: ClientMessage) {
-            message.putStringProperty(HDR_DUPLICATE_DETECTION_ID, SimpleString(UUID.randomUUID().toString()))
+        fun ClientMessage.randomiseDuplicateId() {
+            putStringProperty(HDR_DUPLICATE_DETECTION_ID, SimpleString(UUID.randomUUID().toString()))
         }
 
         log.trace { "Attempting to retry #$retryCount message delivery for $retryId" }
@@ -479,7 +479,7 @@ class NodeMessagingClient(override val config: NodeConfiguration,
             return
         }
 
-        randomiseDuplicateId(message)
+        message.randomiseDuplicateId()
 
         state.locked {
             log.trace { "Retry #$retryCount sending message $message to $address for $retryId" }
diff --git a/node/src/main/kotlin/net/corda/node/services/network/NetworkMapService.kt b/node/src/main/kotlin/net/corda/node/services/network/NetworkMapService.kt
index 7ee02601b5..b5d5aa729b 100644
--- a/node/src/main/kotlin/net/corda/node/services/network/NetworkMapService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/network/NetworkMapService.kt
@@ -1,7 +1,7 @@
 package net.corda.node.services.network
 
 import com.google.common.annotations.VisibleForTesting
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.crypto.DigitalSignature
 import net.corda.core.crypto.SignedData
 import net.corda.core.crypto.isFulfilledBy
diff --git a/node/src/main/kotlin/net/corda/node/services/network/PersistentNetworkMapService.kt b/node/src/main/kotlin/net/corda/node/services/network/PersistentNetworkMapService.kt
index 2220d94a19..1cb7de5292 100644
--- a/node/src/main/kotlin/net/corda/node/services/network/PersistentNetworkMapService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/network/PersistentNetworkMapService.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.network
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.identity.PartyAndCertificate
 import net.corda.core.messaging.SingleMessageRecipient
 import net.corda.node.services.api.ServiceHubInternal
diff --git a/node/src/main/kotlin/net/corda/node/services/persistence/DBTransactionMappingStorage.kt b/node/src/main/kotlin/net/corda/node/services/persistence/DBTransactionMappingStorage.kt
index b9c376b768..9b455f9544 100644
--- a/node/src/main/kotlin/net/corda/node/services/persistence/DBTransactionMappingStorage.kt
+++ b/node/src/main/kotlin/net/corda/node/services/persistence/DBTransactionMappingStorage.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.persistence
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.bufferUntilSubscribed
 import net.corda.core.crypto.SecureHash
 import net.corda.core.flows.StateMachineRunId
diff --git a/node/src/main/kotlin/net/corda/node/services/persistence/InMemoryStateMachineRecordedTransactionMappingStorage.kt b/node/src/main/kotlin/net/corda/node/services/persistence/InMemoryStateMachineRecordedTransactionMappingStorage.kt
index 168fee3bc8..8c4a684a11 100644
--- a/node/src/main/kotlin/net/corda/node/services/persistence/InMemoryStateMachineRecordedTransactionMappingStorage.kt
+++ b/node/src/main/kotlin/net/corda/node/services/persistence/InMemoryStateMachineRecordedTransactionMappingStorage.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.persistence
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.bufferUntilSubscribed
 import net.corda.core.crypto.SecureHash
 import net.corda.core.flows.StateMachineRunId
diff --git a/node/src/main/kotlin/net/corda/node/services/statemachine/StateMachineManager.kt b/node/src/main/kotlin/net/corda/node/services/statemachine/StateMachineManager.kt
index 3104a3b9b2..6ee71cb3c1 100644
--- a/node/src/main/kotlin/net/corda/node/services/statemachine/StateMachineManager.kt
+++ b/node/src/main/kotlin/net/corda/node/services/statemachine/StateMachineManager.kt
@@ -8,7 +8,7 @@ import com.esotericsoftware.kryo.KryoException
 import com.google.common.collect.HashMultimap
 import com.google.common.util.concurrent.ListenableFuture
 import com.google.common.util.concurrent.MoreExecutors
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.bufferUntilSubscribed
 import net.corda.core.crypto.SecureHash
 import net.corda.core.crypto.random63BitValue
diff --git a/node/src/main/kotlin/net/corda/node/services/transactions/PersistentUniquenessProvider.kt b/node/src/main/kotlin/net/corda/node/services/transactions/PersistentUniquenessProvider.kt
index f6419d2c24..4f94975f5e 100644
--- a/node/src/main/kotlin/net/corda/node/services/transactions/PersistentUniquenessProvider.kt
+++ b/node/src/main/kotlin/net/corda/node/services/transactions/PersistentUniquenessProvider.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.transactions
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.contracts.StateRef
 import net.corda.core.crypto.SecureHash
 import net.corda.core.identity.Party
diff --git a/node/src/main/kotlin/net/corda/node/services/vault/HibernateVaultQueryImpl.kt b/node/src/main/kotlin/net/corda/node/services/vault/HibernateVaultQueryImpl.kt
index 929312f753..c7cb8b6647 100644
--- a/node/src/main/kotlin/net/corda/node/services/vault/HibernateVaultQueryImpl.kt
+++ b/node/src/main/kotlin/net/corda/node/services/vault/HibernateVaultQueryImpl.kt
@@ -1,6 +1,6 @@
 package net.corda.node.services.vault
 
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.bufferUntilSubscribed
 import net.corda.core.contracts.ContractState
 import net.corda.core.contracts.StateAndRef
diff --git a/node/src/main/kotlin/net/corda/node/services/vault/NodeVaultService.kt b/node/src/main/kotlin/net/corda/node/services/vault/NodeVaultService.kt
index 773772ad5f..3a7dc5ad0e 100644
--- a/node/src/main/kotlin/net/corda/node/services/vault/NodeVaultService.kt
+++ b/node/src/main/kotlin/net/corda/node/services/vault/NodeVaultService.kt
@@ -12,7 +12,7 @@ import io.requery.kotlin.notNull
 import io.requery.query.RowExpression
 import net.corda.contracts.asset.Cash
 import net.corda.contracts.asset.OnLedgerAsset
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.bufferUntilSubscribed
 import net.corda.core.contracts.*
 import net.corda.core.crypto.SecureHash
diff --git a/samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt b/samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt
index 01bcebfb17..d75ab5f5c8 100644
--- a/samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt
+++ b/samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt
@@ -8,7 +8,7 @@ import net.corda.contracts.Tenor
 import net.corda.contracts.math.CubicSplineInterpolator
 import net.corda.contracts.math.Interpolator
 import net.corda.contracts.math.InterpolatorFactory
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.contracts.Command
 import net.corda.core.crypto.DigitalSignature
 import net.corda.core.crypto.MerkleTreeException
diff --git a/test-utils/src/main/kotlin/net/corda/testing/driver/Driver.kt b/test-utils/src/main/kotlin/net/corda/testing/driver/Driver.kt
index d75150b844..a056e03e7e 100644
--- a/test-utils/src/main/kotlin/net/corda/testing/driver/Driver.kt
+++ b/test-utils/src/main/kotlin/net/corda/testing/driver/Driver.kt
@@ -15,6 +15,7 @@ import net.corda.core.crypto.X509Utilities
 import net.corda.core.crypto.appendToCommonName
 import net.corda.core.crypto.commonName
 import net.corda.core.identity.Party
+import net.corda.core.internal.ThreadBox
 import net.corda.core.internal.div
 import net.corda.core.internal.times
 import net.corda.core.messaging.CordaRPCOps
diff --git a/test-utils/src/main/kotlin/net/corda/testing/node/InMemoryMessagingNetwork.kt b/test-utils/src/main/kotlin/net/corda/testing/node/InMemoryMessagingNetwork.kt
index 2c9601b48f..92e420aa7d 100644
--- a/test-utils/src/main/kotlin/net/corda/testing/node/InMemoryMessagingNetwork.kt
+++ b/test-utils/src/main/kotlin/net/corda/testing/node/InMemoryMessagingNetwork.kt
@@ -3,7 +3,7 @@ package net.corda.testing.node
 import com.google.common.util.concurrent.Futures
 import com.google.common.util.concurrent.ListenableFuture
 import com.google.common.util.concurrent.SettableFuture
-import net.corda.core.ThreadBox
+import net.corda.core.internal.ThreadBox
 import net.corda.core.crypto.X509Utilities
 import net.corda.core.getOrThrow
 import net.corda.core.messaging.AllPossibleRecipients