Task: Vigenère cipher_Cryptanalysis





Java: public class Vig{
static String encodedMessage =
    "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";
 
final static double freq[] = {
    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
};
 

public static void main(String[] args) {
    int lenghtOfEncodedMessage = encodedMessage.length();
    char[] encoded = new char [lenghtOfEncodedMessage] ;
    char[] key =  new char [lenghtOfEncodedMessage] ;

    encodedMessage.getChars(0, lenghtOfEncodedMessage, encoded, 0);
    int txt[] = new int[lenghtOfEncodedMessage];
    int len = 0, j;

    double fit, best_fit = 1e100;
 
    for (j = 0; j < lenghtOfEncodedMessage; j++)
        if (Character.isUpperCase(encoded[j]))
            txt[len++] = encoded[j] - 'A';
 
    for (j = 1; j < 30; j++) {
        fit = freq_every_nth(txt, len, j, key);
        System.out.printf("%f, key length: %2d ", fit, j);
            System.out.print(key);
        if (fit < best_fit) {
            best_fit = fit;
            System.out.print(" <--- best so far");
        }
        System.out.print("\n");

    }
}


    static String decrypt(String text, final String key) {
        String res = "";
        text = text.toUpperCase();
        for (int i = 0, j = 0; i < text.length(); i++) {
            char c = text.charAt(i);
            if (c < 'A' || c > 'Z') continue;
            res += (char)((c - key.charAt(j) + 26) % 26 + 'A');
            j = ++j % key.length();
        }
        return res;
    }

static int best_match(final double []a, final double []b) {
    double sum = 0, fit, d, best_fit = 1e100;
    int i, rotate, best_rotate = 0;
    for (i = 0; i < 26; i++)
        sum += a[i];
    for (rotate = 0; rotate < 26; rotate++) {
        fit = 0;
        for (i = 0; i < 26; i++) {
            d = a[(i + rotate) % 26] / sum - b[i];
            fit += d * d / b[i];
        }
 
        if (fit < best_fit) {
            best_fit = fit;
            best_rotate = rotate;
        }
    }
 
    return best_rotate;
}
 
static double freq_every_nth(final int []msg, int len, int interval, char[] key) {
    double sum, d, ret;
    double  [] accu = new double [26];
    double  [] out = new double [26];
    int i, j, rot;
 
    for (j = 0; j < interval; j++) {
        for (i = 0; i < 26; i++)
            out[i] = 0;
        for (i = j; i < len; i += interval)
            out[msg[i]]++;
	rot = best_match(out, freq);
	try{
            key[j] = (char)(rot + 'A');
	} catch (Exception e) {
		System.out.print(e.getMessage());
	}
        for (i = 0; i < 26; i++)
            accu[i] += out[(i + rot) % 26];
    }
 
    for (i = 0, sum = 0; i < 26; i++)
        sum += accu[i];
 
    for (i = 0, ret = 0; i < 26; i++) {
        d = accu[i] / sum - freq[i];
        ret += d * d / freq[i];
    }
 
    key[interval] = '\0';
    return ret;
}
 
}