Add Afffine cipher for Kotlin

code/cryptography/affine_cipher/AffineKotlin.kt

@@ -0,0 +1,192 @@
+import java.util.*
+
+/**
+ * Created by Phuwarin on 10/1/2018
+ */
+
+object AffineKotlin {
+
+    private val SCANNER = Scanner(System.`in`)
+    private const val ALPHABET_SIZE = 26
+
+    private val command: Int
+        get() = SCANNER.nextInt()
+
+    @JvmStatic
+    fun main(args: Array<String>) {
+        do {
+            print("Type 1 for encryption and 2 for decryption then Enter >> ")
+            val command = command
+            when (command) {
+                1 -> encryption()
+                2 -> decryption()
+                else -> {
+                    println("Command error, pls try again.")
+                }
+            }
+        } while (command != 1 && command != 2)
+    }
+
+    private fun decryption() {
+        var a: Int
+        var aInverse = 0
+        var isAOk = false
+
+        print("Enter cipherText which want to decryption >> ")
+        val cipherText = SCANNER.next()
+
+        while (!isAOk) {
+            print("Enter key 'a' >> ")
+            a = SCANNER.nextInt()
+            if (gcd(a, ALPHABET_SIZE) == 1) {
+                isAOk = true
+                aInverse = findInverse(a.toDouble(), ALPHABET_SIZE.toDouble())
+            } else {
+                println("'a' is not ok, pls try again.")
+            }
+        }
+
+        print("Enter key 'b' >> ")
+        val b: Int = SCANNER.nextInt()
+        decrypt(aInverse, b, cipherText)
+    }
+
+    private fun decrypt(aInverse: Int, b: Int, cipherText: String?) {
+        if (cipherText == null || cipherText.isEmpty()) {
+            println("Plaintext has a problem. Bye bye :)")
+            return
+        }
+
+        val cipherText = cipherText.toLowerCase()
+        val plaintext = StringBuilder()
+        var z: Int
+        var j: Int
+
+        for (i in 0 until cipherText.length) {
+            val agent = cipherText[i]
+            z = aInverse * (agent.toInt() - 97 - b)
+            j = if (z < 0) minusMod(z, ALPHABET_SIZE) else z % ALPHABET_SIZE
+            plaintext.append(('A'.toInt() + j).toChar())
+        }
+        println("Plaintext: $plaintext")
+    }
+
+    private fun minusMod(minus: Int, mod: Int): Int {
+        val a = Math.abs(minus)
+        return mod * (a / mod + 1) - a
+    }
+
+    private fun findInverse(firstNumber: Double, anotherNumber: Double): Int {
+        var a1: Int
+        var b1: Int
+        var a2: Int
+        var b2: Int
+        var r: Int
+        var q: Int
+        var a2Temp: Int
+        var b2Temp: Int
+        var n1: Int
+        var n2: Int
+        val max: Int
+
+        if (firstNumber > anotherNumber) {
+            max = firstNumber.toInt()
+            n1 = firstNumber.toInt()
+            n2 = anotherNumber.toInt()
+        } else {
+            max = anotherNumber.toInt()
+            n1 = anotherNumber.toInt()
+            n2 = firstNumber.toInt()
+        }
+
+        b2 = 1
+        a1 = b2
+        a2 = 0
+        b1 = a2
+        a2Temp = a2
+        b2Temp = b2
+
+        r = n1 % n2
+        q = n1 / n2
+
+        while (r != 0) {
+            n1 = n2
+            n2 = r
+            a2 = a1 - q * a2
+            b2 = b1 - q * b2
+            a1 = a2Temp
+            b1 = b2Temp
+            a2Temp = a2
+            b2Temp = b2
+            r = n1 % n2
+            q = n1 / n2
+        }
+
+        val result: Int
+        result = if (firstNumber == max.toDouble()) {
+            if (a2 < 0) {
+                (a2 - anotherNumber * Math.floor(a2 / anotherNumber)).toInt()
+            } else {
+                a2
+            }
+        } else {
+            if (b2 < 0) {
+                (b2 - anotherNumber * Math.floor(b2 / anotherNumber)).toInt()
+            } else
+                b2
+        }
+        return result
+    }
+
+    private fun encryption() {
+        var isAOk = false
+        var isBOk = false
+        var a = 0
+        var b = 0
+
+        print("Enter plaintext which want to encryption >> ")
+        val plaintext = SCANNER.next()
+
+        while (!isAOk) {
+            print("Choose 'a' but gcd (a,26) must equal 1\n" + "'a' might be {1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25} >> ")
+            a = SCANNER.nextInt()
+            if (gcd(a, ALPHABET_SIZE) == 1) {
+                isAOk = true
+            } else {
+                println("'a' is not ok, pls try again.")
+            }
+        }
+
+        while (!isBOk) {
+            print("Choose 'b' which number you want but not equal 1 >> ")
+            b = SCANNER.nextInt()
+            if (b != 1) {
+                isBOk = true
+            } else {
+                println("'b' is not ok, pls try again.")
+            }
+        }
+        encrypt(a, b, plaintext)
+    }
+
+    private fun gcd(a: Int, b: Int): Int {
+        return if (b == 0) a else gcd(b, a % b)
+    }
+
+    private fun encrypt(a: Int, b: Int, plaintext: String?) {
+        if (plaintext == null || plaintext.isEmpty()) {
+            println("Plaintext has a problem. Bye bye :)")
+            return
+        }
+
+        val plaintext = plaintext.toLowerCase()
+        val cipherText = StringBuilder()
+
+        for (i in 0 until plaintext.length) {
+            val agent = plaintext[i]
+            val value = (a * (agent.toInt() - 97) + b) % ALPHABET_SIZE
+            cipherText.append((value + 97).toChar())
+        }
+        println("cipherText: $cipherText")
+    }
+}