Minor: provide a random63BitValue() function and use it instead of the previously duplicated code. Update docs.

docs/source/protocol-state-machines.rst

@@ -140,8 +140,8 @@ Let's unpack what this code does:
 - Two of the classes are simply wrappers for parameters to the trade; things like what is being sold, what the price
   of the asset is, how much the buyer is willing to pay and so on. The ``myKeyPair`` field is simply the public key
   that the seller wishes the buyer to send the cash to. The session ID field is sent from buyer to seller when the
-  trade is being set up and is just a big random number. It's used to keep messages separated on the network, and stop
-  malicious entities trying to interfere with the message stream.
+  trade is being set up and is used to keep messages separated on the network, and stop malicious entities trying to
+  interfere with the message stream.
 - The other two classes define empty abstract classes called ``Buyer`` and ``Seller``. These inherit from a class
   called ``ProtocolStateMachine`` and provide two type parameters: the arguments class we just defined for each side
   and the type of the object that the protocol finally produces (this doesn't have to be identical for each side, even
@@ -149,6 +149,9 @@ Let's unpack what this code does:
 - Finally it simply defines a static method that creates an instance of an object that inherits from this base class
   and returns it, with a ``StateMachineManager`` as an instance. The Impl class will be defined below.
 
+.. note:: Session IDs keep different traffic streams separated, so for security they must be large and random enough
+to be unguessable. 63 bits is good enough.
+
 Alright, so using this protocol shouldn't be too hard: in the simplest case we can just pass in the details of the trade
 to either runBuyer or runSeller, depending on who we are, and then call ``.get()`` on the resulting future to block the
 calling thread until the protocol has finished. Or we could register a callback on the returned future that will be
@@ -179,7 +182,6 @@ Implementing the seller
       private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) : TwoPartyTradeProtocol() {
           companion object {
               val TRADE_TOPIC = "com.r3cev.protocols.trade"
-              fun makeSessionID() = Math.abs(SecureRandom.getInstanceStrong().nextLong())
           }
 
           class SellerImpl : Seller() {
@@ -206,10 +208,7 @@ Implementing the seller
 
 We start with a skeleton on which we will build the protocol. Putting things in a *companion object* in Kotlin is like
 declaring them as static members in Java. Here, we define a "topic" that will identify trade related messages that
-arrive at a node (see :doc:`messaging` for details), and a convenience function to pick a large random session ID.
-
-.. note:: Session IDs keep different traffic streams separated, so for security they must be large and random enough
-   to be unguessable. 63 bits is good enough.
+arrive at a node (see :doc:`messaging` for details).
 
 The runSeller and runBuyer methods simply start the state machines, passing in a reference to the classes and the topics
 each side will use.
@@ -240,7 +239,7 @@ Next we add some code to the ``SellerImpl.call`` method:
 
    .. sourcecode:: kotlin
 
-      val sessionID = makeSessionID()
+      val sessionID = random63BitValue()
 
       // Make the first message we'll send to kick off the protocol.
       val hello = SellerTradeInfo(args.assetToSell, args.price, args.myKeyPair.public, sessionID)

src/main/kotlin/contracts/protocols/TwoPartyTradeProtocol.kt

@@ -16,7 +16,6 @@ import core.serialization.deserialize
 import core.utilities.trace
 import java.security.KeyPair
 import java.security.PublicKey
-import java.security.SecureRandom
 
 /**
  * This asset trading protocol has two parties (B and S for buyer and seller) and the following steps:
@@ -73,7 +72,6 @@ abstract class TwoPartyTradeProtocol {
 private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) : TwoPartyTradeProtocol() {
     companion object {
         val TRADE_TOPIC = "com.r3cev.protocols.trade"
-        fun makeSessionID() = Math.abs(SecureRandom.getInstanceStrong().nextLong())
     }
 
     // This object is serialised to the network and is the first protocol message the seller sends to the buyer.
@@ -92,7 +90,7 @@ private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) :
     // learn more about the protocol state machine framework.
     class SellerImpl : Seller() {
         override fun call(args: SellerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
-            val sessionID = makeSessionID()
+            val sessionID = random63BitValue()
 
             // Make the first message we'll send to kick off the protocol.
             val hello = SellerTradeInfo(args.assetToSell, args.price, args.myKeyPair.public, sessionID)

src/main/kotlin/core/Utils.kt

@@ -13,6 +13,7 @@ import com.google.common.util.concurrent.ListenableFuture
 import com.google.common.util.concurrent.MoreExecutors
 import com.google.common.util.concurrent.SettableFuture
 import org.slf4j.Logger
+import java.security.SecureRandom
 import java.time.Duration
 import java.util.*
 import java.util.concurrent.Executor
@@ -44,6 +45,12 @@ val Int.hours: Duration get() = Duration.ofHours(this.toLong())
 val Int.minutes: Duration get() = Duration.ofMinutes(this.toLong())
 val Int.seconds: Duration get() = Duration.ofSeconds(this.toLong())
 
+/**
+ * Returns a random positive long generated using a secure RNG. This function sacrifies a bit of entropy in order to
+ * avoid potential bugs where the value is used in a context where negative numbers are not expected.
+ */
+fun random63BitValue(): Long = Math.abs(SecureRandom.getInstanceStrong().nextLong())
+
 fun <T> ListenableFuture<T>.whenComplete(executor: Executor? = null, body: () -> Unit) {
     addListener(Runnable { body() }, executor ?: MoreExecutors.directExecutor())
 }

src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt

@@ -12,15 +12,11 @@ import com.google.common.util.concurrent.MoreExecutors
 import contracts.Cash
 import contracts.CommercialPaper
 import contracts.protocols.TwoPartyTradeProtocol
-import core.ContractState
-import core.DOLLARS
-import core.StateAndRef
-import core.days
+import core.*
 import core.testutils.*
 import org.junit.After
 import org.junit.Before
 import org.junit.Test
-import java.security.SecureRandom
 import java.util.concurrent.Executors
 import java.util.logging.Formatter
 import java.util.logging.Level
@@ -77,7 +73,7 @@ class TwoPartyTradeProtocolTests : TestWithInMemoryNetwork() {
             val tpSeller = TwoPartyTradeProtocol.create(StateMachineManager(alicesServices, backgroundThread))
             val tpBuyer = TwoPartyTradeProtocol.create(StateMachineManager(bobsServices, backgroundThread))
 
-            val buyerSessionID = SecureRandom.getInstanceStrong().nextLong()
+            val buyerSessionID = random63BitValue()
 
             val aliceResult = tpSeller.runSeller(
                     bobsAddress,
@@ -134,7 +130,7 @@ class TwoPartyTradeProtocolTests : TestWithInMemoryNetwork() {
             val smmBuyer = StateMachineManager(bobsServices, MoreExecutors.directExecutor())
             val tpBuyer = TwoPartyTradeProtocol.create(smmBuyer)
 
-            val buyerSessionID = SecureRandom.getInstanceStrong().nextLong()
+            val buyerSessionID = random63BitValue()
 
             tpSeller.runSeller(
                     bobsAddress,