June 2018 Update

Task/Vigen-re-cipher-Cryptanalysis/Go/vigen-re-cipher-cryptanalysis.go

@@ -0,0 +1,119 @@
+package main
+
+import (
+    "fmt"
+    "strings"
+)
+
+var 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"
+
+var freq = [26]float64{
+    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,
+}
+
+func sum(a []float64) (sum float64) {
+    for _, f := range a {
+        sum += f
+    }
+    return
+}
+
+func bestMatch(a []float64) int {
+    sum := sum(a)
+    bestFit, bestRotate := 1e100, 0
+    for rotate := 0; rotate < 26; rotate++ {
+        fit := 0.0
+        for i := 0; i < 26; i++ {
+            d := a[(i+rotate)%26]/sum - freq[i]
+            fit += d * d / freq[i]
+        }
+        if fit < bestFit {
+            bestFit, bestRotate = fit, rotate
+        }
+    }
+    return bestRotate
+}
+
+func freqEveryNth(msg []int, key []byte) float64 {
+    l := len(msg)
+    interval := len(key)
+    out := make([]float64, 26)
+    accu := make([]float64, 26)
+    for j := 0; j < interval; j++ {
+        for k := 0; k < 26; k++ {
+            out[k] = 0.0
+        }
+        for i := j; i < l; i += interval {
+            out[msg[i]]++
+        }
+        rot := bestMatch(out)
+        key[j] = byte(rot + 65)
+        for i := 0; i < 26; i++ {
+            accu[i] += out[(i+rot)%26]
+        }
+    }
+    sum := sum(accu)
+    ret := 0.0
+    for i := 0; i < 26; i++ {
+        d := accu[i]/sum - freq[i]
+        ret += d * d / freq[i]
+    }
+    return ret
+}
+
+func decrypt(text, key string) string {
+    var sb strings.Builder
+    ki := 0
+    for _, c := range text {
+        if c < 'A' || c > 'Z' {
+            continue
+        }
+        ci := (c - rune(key[ki]) + 26) % 26
+        sb.WriteRune(ci + 65)
+        ki = (ki + 1) % len(key)
+    }
+    return sb.String()
+}
+
+func main() {
+    enc := strings.Replace(encoded, " ", "", -1)
+    txt := make([]int, len(enc))
+    for i := 0; i < len(txt); i++ {
+        txt[i] = int(enc[i] - 'A')
+    }
+    bestFit, bestKey := 1e100, ""
+    fmt.Println("  Fit     Length   Key")
+    for j := 1; j <= 26; j++ {
+        key := make([]byte, j)
+        fit := freqEveryNth(txt, key)
+        sKey := string(key)
+        fmt.Printf("%f    %2d     %s", fit, j, sKey)
+        if fit < bestFit {
+            bestFit, bestKey = fit, sKey
+            fmt.Print(" <--- best so far")
+        }
+        fmt.Println()
+    }
+    fmt.Println("\nBest key :", bestKey)
+    fmt.Printf("\nDecrypted text:\n%s\n", decrypt(enc, bestKey))
+}