fix potential overflow

build.gradle

@@ -1,7 +1,7 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
-    ext.kotlin_version = '1.3.11'
+    ext.kotlin_version = '1.3.61'
     repositories {
         jcenter()
     }

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.1.1-all.zip

knacl/build.gradle

@@ -1,7 +1,10 @@
 apply plugin: "kotlin"
 apply plugin: "maven"
 
+compileKotlin.kotlinOptions.jvmTarget = "1.8"
+compileTestKotlin.kotlinOptions.jvmTarget = "1.8"
+
 dependencies {
-    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
     testImplementation "junit:junit:4.12"
 }

knacl/src/main/kotlin/me/uport/knacl/NaClLowLevel.kt

@@ -5,6 +5,8 @@ import java.security.SecureRandom
 import kotlin.experimental.and
 import kotlin.experimental.or
 import kotlin.experimental.xor
+import kotlin.math.floor
+import kotlin.math.roundToLong
 
 /**
  * This is a port of the TweetNaCl library
@@ -817,7 +819,7 @@ internal object NaClLowLevel {
             carry = 0
             for (j in (i - 32) until (i - 12)) {
                 x[j] += carry - 16 * x[i] * L[j - (i - 32)]
-                carry = (x[j] + 128) shr 8
+                carry = floor((x[j].toDouble() + 128.0) / 256.0).roundToLong()
                 x[j] -= carry shl 8
             }
             ///XXX: check index arithmetic