Add XteaKey inline class with ZERO "constant" and isZero method

Unfortunately we can't use the compiler to guarantee k isn't changed,
though making it internal will help. When the JVM (and Kotlin) get value
types, we might be able to improve on this (e.g. by making it an inline
class of four integers).

Signed-off-by: Graham <gpe@openrs2.dev>

build.gradle.kts

@@ -56,7 +56,7 @@ allprojects {
     tasks.withType<KotlinCompile> {
         kotlinOptions {
             jvmTarget = "11"
-            freeCompilerArgs = listOf("-Xjsr305=strict")
+            freeCompilerArgs = listOf("-Xinline-classes", "-Xjsr305=strict")
         }
     }
 

crypto/src/main/java/dev/openrs2/crypto/Xtea.kt

@@ -7,8 +7,17 @@ private const val ROUNDS = 32
 private const val BLOCK_SIZE = 8
 private const val BLOCK_SIZE_MASK = BLOCK_SIZE - 1
 
-fun ByteBuf.xteaEncrypt(index: Int, length: Int, key: IntArray) {
+inline class XteaKey(internal val k: IntArray) {
-    require(key.size == 4)
+    val isZero: Boolean
+        get() = k[0] == 0 && k[1] == 0 && k[2] == 0 && k[3] == 0
+
+    companion object {
+        val ZERO = XteaKey(IntArray(4))
+    }
+}
+
+fun ByteBuf.xteaEncrypt(index: Int, length: Int, key: XteaKey) {
+    require(key.k.size == 4)
 
     val end = index + (length and BLOCK_SIZE_MASK.inv())
     for (i in index until end step BLOCK_SIZE) {
@@ -17,9 +26,9 @@ fun ByteBuf.xteaEncrypt(index: Int, length: Int, key: IntArray) {
         var v1 = getInt(i + 4)
 
         for (j in 0 until ROUNDS) {
-            v0 += (((v1 shl 4) xor (v1 ushr 5)) + v1) xor (sum + key[sum and 3])
+            v0 += (((v1 shl 4) xor (v1 ushr 5)) + v1) xor (sum + key.k[sum and 3])
             sum += GOLDEN_RATIO
-            v1 += (((v0 shl 4) xor (v0 ushr 5)) + v0) xor (sum + key[(sum ushr 11) and 3])
+            v1 += (((v0 shl 4) xor (v0 ushr 5)) + v0) xor (sum + key.k[(sum ushr 11) and 3])
         }
 
         setInt(i, v0)
@@ -27,8 +36,8 @@ fun ByteBuf.xteaEncrypt(index: Int, length: Int, key: IntArray) {
     }
 }
 
-fun ByteBuf.xteaDecrypt(index: Int, length: Int, key: IntArray) {
+fun ByteBuf.xteaDecrypt(index: Int, length: Int, key: XteaKey) {
-    require(key.size == 4)
+    require(key.k.size == 4)
 
     val end = index + (length and BLOCK_SIZE_MASK.inv())
     for (i in index until end step BLOCK_SIZE) {
@@ -38,9 +47,9 @@ fun ByteBuf.xteaDecrypt(index: Int, length: Int, key: IntArray) {
         var v1 = getInt(i + 4)
 
         for (j in 0 until ROUNDS) {
-            v1 -= (((v0 shl 4) xor (v0 ushr 5)) + v0) xor (sum + key[(sum ushr 11) and 3])
+            v1 -= (((v0 shl 4) xor (v0 ushr 5)) + v0) xor (sum + key.k[(sum ushr 11) and 3])
             sum -= GOLDEN_RATIO
-            v0 -= (((v1 shl 4) xor (v1 ushr 5)) + v1) xor (sum + key[sum and 3])
+            v0 -= (((v1 shl 4) xor (v1 ushr 5)) + v1) xor (sum + key.k[sum and 3])
         }
 
         setInt(i, v0)

crypto/src/test/java/dev/openrs2/crypto/XteaTest.kt

@@ -8,9 +8,9 @@ import kotlin.test.assertEquals
 
 object XteaTest {
     private class TestVector(key: String, plaintext: String, ciphertext: String) {
-        val key = IntArray(4) {
+        val key = XteaKey(IntArray(4) {
             Integer.parseUnsignedInt(key, it * 8, it * 8 + 8, 16)
-        }
+        })
         val plaintext: ByteArray = ByteBufUtil.decodeHexDump(plaintext)
         val ciphertext: ByteArray = ByteBufUtil.decodeHexDump(ciphertext)
     }