Data commit

Task/Vigen-re-cipher-Cryptanalysis/Kotlin/vigen-re-cipher-cryptanalysis.kotlin

@@ -0,0 +1,102 @@
+// version 1.1.3
+
+val encoded =
+    "MOMUD EKAPV TQEFM OEVHP AJMII CDCTI FGYAG JSPXY ALUYM NSMYH" +
+    "VUXJE LEPXJ FXGCM JHKDZ RYICU HYPUS PGIGM OIYHF WHTCQ KMLRD" +
+    "ITLXZ LJFVQ GHOLW CUHLO MDSOE KTALU VYLNZ RFGBX PHVGA LWQIS" +
+    "FGRPH JOOFW GUBYI LAPLA LCAFA AMKLG CETDW VOELJ IKGJB XPHVG" +
+    "ALWQC SNWBU BYHCU HKOCE XJEYK BQKVY KIIEH GRLGH XEOLW AWFOJ" +
+    "ILOVV RHPKD WIHKN ATUHN VRYAQ DIVHX FHRZV QWMWV LGSHN NLVZS" +
+    "JLAKI FHXUF XJLXM TBLQV RXXHR FZXGV LRAJI EXPRV OSMNP KEPDT" +
+    "LPRWM JAZPK LQUZA ALGZX GVLKL GJTUI ITDSU REZXJ ERXZS HMPST" +
+    "MTEOE PAPJH SMFNB YVQUZ AALGA YDNMP AQOWT UHDBV TSMUE UIMVH" +
+    "QGVRW AEFSP EMPVE PKXZY WLKJA GWALT VYYOB YIXOK IHPDS EVLEV" +
+    "RVSGB JOGYW FHKBL GLXYA MVKIS KIEHY IMAPX UOISK PVAGN MZHPW" +
+    "TTZPV XFCCD TUHJH WLAPF YULTB UXJLN SIJVV YOVDJ SOLXG TGRVO" +
+    "SFRII CTMKO JFCQF KTINQ BWVHG TENLH HOGCS PSFPV GJOKM SIFPR" +
+    "ZPAAS ATPTZ FTPPD PORRF TAXZP KALQA WMIUD BWNCT LEFKO ZQDLX" +
+    "BUXJL ASIMR PNMBF ZCYLV WAPVF QRHZV ZGZEF KBYIO OFXYE VOWGB" +
+    "BXVCB XBAWG LQKCM ICRRX MACUO IKHQU AJEGL OIJHH XPVZW JEWBA" +
+    "FWAML ZZRXJ EKAHV FASMU LVVUT TGK"
+
+val freq = doubleArrayOf(
+    0.08167, 0.01492, 0.02782, 0.04253, 0.12702, 0.02228, 0.02015,
+    0.06094, 0.06966, 0.00153, 0.00772, 0.04025, 0.02406, 0.06749,
+    0.07507, 0.01929, 0.00095, 0.05987, 0.06327, 0.09056, 0.02758,
+    0.00978, 0.02360, 0.00150, 0.01974, 0.00074
+)
+
+fun bestMatch(a: DoubleArray): Int {
+    val sum = a.sum()
+    var bestFit = 1e100
+    var bestRotate = 0
+    for (rotate in 0..25) {
+        var fit = 0.0
+        for (i in 0..25) {
+            val d = a[(i + rotate) % 26] / sum - freq[i]
+            fit += d * d / freq[i]
+        }
+        if (fit < bestFit) {
+            bestFit = fit
+            bestRotate = rotate
+        }
+    }
+    return bestRotate
+}
+
+fun freqEveryNth(msg: IntArray, key: CharArray): Double {
+    val len = msg.size
+    val interval = key.size
+    val out = DoubleArray(26)
+    val accu = DoubleArray(26)
+    for (j in 0 until interval) {
+        out.fill(0.0)
+        for (i in j until len step interval) out[msg[i]]++
+        val rot = bestMatch(out)
+        key[j] = (rot + 65).toChar()
+        for (i in 0..25) accu[i] += out[(i + rot) % 26]
+    }
+    val sum = accu.sum()
+    var ret = 0.0
+    for (i in 0..25) {
+        val d = accu[i] / sum - freq[i]
+        ret += d * d / freq[i]
+    }
+    return ret
+}
+
+fun decrypt(text: String, key: String): String {
+    val sb = StringBuilder()
+    var ki = 0
+    for (c in text) {
+        if (c !in 'A'..'Z') continue
+        val ci = (c.toInt() - key[ki].toInt() +  26) % 26
+        sb.append((ci + 65).toChar())
+        ki = (ki + 1) % key.length
+    }
+    return sb.toString()
+}
+
+fun main(args: Array<String>) {
+    val enc = encoded.replace(" ", "")
+    val txt = IntArray(enc.length) { enc[it] - 'A' }
+    var bestFit = 1e100
+    var bestKey = ""
+    val f = "%f    %2d     %s"
+    println("  Fit     Length   Key")
+    for (j in 1..26) {
+        val key = CharArray(j)
+        val fit = freqEveryNth(txt, key)
+        val sKey = key.joinToString("")
+        print(f.format(fit, j, sKey))
+        if (fit < bestFit) {
+           bestFit = fit
+           bestKey = sKey
+           print(" <--- best so far")
+        }
+        println()
+    }
+    println()
+    println("Best key : $bestKey")
+    println("\nDecrypted text:\n${decrypt(enc, bestKey)}")
+}