Implemented InvCipher function for AES

aes/src/commonMain/kotlin/com/mooncloak/kodetools/aes/InvCipher.kt

@@ -0,0 +1,38 @@
+package com.mooncloak.kodetools.aes
+
+/**
+ * Represents the general decryption function for executing the AES (Advanced Encryption Standard).
+ *
+ * @param [input] The data input, which is a block represented as a linear array of 16 bytes.
+ *
+ * @param [numberOfRounds] The number of rounds, "Nr", for the instance. This value is dependent on the key bit size.
+ * The following values are supported: AES-128: `10`, for AES-192: `12`, AES-256: `14`.
+ *
+ * @param [roundKeys] The keys for each round. This value must be of size [numberOfRounds]. A round key is a block that
+ * is usually represented as a sequence of four words (16 bytes). The "KEYEXPANSION()" function, defined by the AES
+ * Specification, takes the block cipher key as input and generates the round keys as output.
+ *
+ * @see [AES Specification](https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197-upd1.pdf)
+ */
+internal fun InvCipher(
+    input: ByteArray,
+    numberOfRounds: Int,
+    roundKeys: Array<ByteArray>
+): ByteArray {
+    val state = State(input)
+
+    state.addRoundKey(roundKeys.copyOfRange(fromIndex = 4 * numberOfRounds, toIndex = 4 * numberOfRounds + 3))
+
+    for (round in (numberOfRounds - 1) downTo 1) {
+        state.invShiftRows()
+        state.invSubBytes()
+        state.addRoundKey(roundKeys.copyOfRange(fromIndex = 4 * round, toIndex = 4 * round + 3))
+        state.invMixColumns()
+    }
+
+    state.invShiftRows()
+    state.invSubBytes()
+    state.addRoundKey(roundKeys.copyOfRange(fromIndex = 0, toIndex = 4))
+
+    return state.output()
+}

aes/src/commonMain/kotlin/com/mooncloak/kodetools/aes/SBox.kt

@@ -20,4 +20,7 @@ internal val sbox: Array<UByteArray> = arrayOf(
     ubyteArrayOf(0xe1u, 0xf8u, 0x98u, 0x11u, 0x69u, 0xd9u, 0x8eu, 0x94u, 0x9bu, 0x1eu, 0x87u, 0xe9u, 0xceu, 0x55u, 0x28u, 0xdfu),
     ubyteArrayOf(0x8cu, 0xa1u, 0x89u, 0x0du, 0xbfu, 0xe6u, 0x42u, 0x68u, 0x41u, 0x99u, 0x2du, 0x0fu, 0xb0u, 0x54u, 0xbbu, 0x16u)
 )
+
+@OptIn(ExperimentalUnsignedTypes::class)
+internal val invSbox: Array<UByteArray> = TODO()
 //@formatter:on

aes/src/commonMain/kotlin/com/mooncloak/kodetools/aes/State.kt

@@ -130,7 +130,7 @@ internal inline fun State.addRoundKey(roundKey: Array<ByteArray>): State {
 }
 
 @OptIn(ExperimentalUnsignedTypes::class)
-internal fun State.subBytes(): State {
+internal inline fun State.subBytes(): State {
     for (r in 0..3) {
         for (c in 0..3) {
             val x = this[r, c].toInt() and 0x0F
@@ -142,3 +142,54 @@ internal fun State.subBytes(): State {
 
     return this
 }
+
+internal inline fun State.invShiftRows(): State {
+    for (r in 1..3) {
+        val temp = ByteArray(4)
+
+        for (c in 0..3) {
+            temp[c] = this[r, (c - r + 4) % 4]
+        }
+
+        for (c in 0..3) {
+            this[r, c] = temp[c]
+        }
+    }
+
+    return this
+}
+
+@OptIn(ExperimentalUnsignedTypes::class)
+internal inline fun State.invSubBytes(): State {
+    for (r in 0..3) {
+        for (c in 0..3) {
+            this[r, c] = invSbox[this[r, c].toInt() ushr 4][this[r, c].toInt() and 0x0f].toByte()
+        }
+    }
+
+    return this
+}
+
+internal inline fun State.invMixColumns(): State {
+    for (c in 0..3) {
+        val a = IntArray(4)
+        val b = IntArray(4)
+
+        for (i in 0..3) {
+            a[i] = this[i, c].toInt() and 0xff
+        }
+
+        //@formatter:off
+        b[0] = a[0].multiplyGF256(0x0e) xor a[1].multiplyGF256(0x0b) xor a[2].multiplyGF256(0x0d) xor a[3].multiplyGF256(0x09)
+        b[1] = a[0].multiplyGF256(0x09) xor a[1].multiplyGF256(0x0e) xor a[2].multiplyGF256( 0x0b) xor a[3].multiplyGF256( 0x0d)
+        b[2] = a[0].multiplyGF256(0x0d) xor a[1].multiplyGF256( 0x09) xor a[2].multiplyGF256( 0x0e) xor a[3].multiplyGF256( 0x0b)
+        b[3] = a[0].multiplyGF256( 0x0b) xor a[1].multiplyGF256( 0x0d) xor a[2].multiplyGF256( 0x09) xor a[3].multiplyGF256(0x0e)
+        //@formatter:on
+
+        for (i in 0..3) {
+            this[i, c] = b[i].toByte()
+        }
+    }
+
+    return this
+}