Java：加密问题

我写了一个程序来加密和解密字符串，我有一个小问题

代码：

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正如你所看到的，它适用于所有字母，但v-z除外，我不确定我错在哪里。我写了一些代码，但留下的可能比我应该留下的更多，因为我根本不知道我在哪里出错。

我已经解决了我的问题，不是在26之前修改，而是在27之前修改，以使最后几个字母正常工作。

问题不在于解密，可能还有其他问题，但在于加密。e和z均加密为“q”，b和w均加密为“b”，26个字符中的9个字符未加密为其他字符。

    public static void main(String[] args) {
        System.out.println("Welcome the small-scale RSA Testing Tool!");
        System.out.println("========================================= \n");
            System.out.println("Please type two relatively small prime numbers");
            Scanner ob=new Scanner(System.in);
            System.out.print("p: ");
            int p = ob.nextInt();

            System.out.print("\nq: ");
            int q = ob.nextInt();

            System.out.println("\nn = p * q = " + p * q);

            int t = (p-1) * (q-1);
            System.out.println("t = (p-1) * (q-1) = " + t);

            System.out.println("Please type an e that is relatively prime with t=" + t +" : ");

            int e = ob.nextInt();

            int keys[] = rsaKeyGen(p, q, e);

            System.out.println("Public Keys: n = " + keys[0] + " and e = " + keys[1] + "(published)");
            System.out.println("Private Key: d = " + keys[2] + " (You should keep this secret!)");

            System.out.println("Testing the encryption/decryption using the above generated keys: ");

            else
                System.out.print("Please type a text to encrypt (0 to exit): ");
                String theString = ob.next(); //gets the string typed in
                char[] ciphertext= new char[theString.length()]; 
                char[] normaltext= new char[theString.length()];
                char[] letters = new char[]{'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
                BigDecimal y = null;
                BigDecimal z = null;
                for (int j=0; j<theString.length();j++) { //for each char in theString
                    char c = theString.charAt(j);
                    int index = c- 'a'; //subtracts value of a from c
                    y = rsaEncrypt(new BigDecimal(index), new BigDecimal(keys[1]), new BigDecimal(keys[0])); //encrypt
                    z = rsaDecrypt(y, new BigDecimal(keys[2]), new BigDecimal(keys[0])); //decrypt
                    ciphertext[j]=letters[y.intValue()%26];//mod 25 incase the value of y is bigger than letters in letters
                    normaltext[j]=letters[z.intValue()%26]; 
                }
                String newString = new String(ciphertext);
                System.out.println("Result: "+ theString +" encrypted to " + newString); // prints out encryption
                System.out.println("");

                String decryptString = new String (normaltext);

                System.out.println("Result: "+newString+" decrypted to " + decryptString);

                if(decryptString.matches(theString))    System.out.println("It works!");
                else        System.out.println("It did not work, please check your implementation!");








        }

    public static int[] egcd(int k, int j){
        List<Integer> quotients      = new ArrayList<Integer>();
        List<Integer> remainders    = new ArrayList<Integer>();
        List<Integer> xs = new ArrayList<Integer>();
        List<Integer> ys = new ArrayList<Integer>();

        int oldK = k;

        int quotient     = k / j;
        int remainder   = k % j;
        int gcd = remainder==0?j:1;
        while (remainder != 0){
            quotients.add(new Integer(quotient));
            remainders.add(new Integer(remainder));
            k = j;
            j = remainder;
            quotient     = k / j;
            remainder    = k % j;
            gcd = j;
        }

        int result[] = new int[2];
        result[0] = gcd;

        if(gcd != 1) {
            System.out.println("These two numbers "+ j + " and " + k+ " are not relatively prime.");
            System.exit(0);

        }
        xs.add(1);
        ys.add(0);

        int y = 1;
        int x = 0 - ((int)quotients.remove(quotients.size()-1)) * 1;
        int oldY = y;
        int oldX = x;

        while(quotients.size() > 0){
            y = x;
            x = oldY - ((int)quotients.remove(quotients.size()-1)) * oldX;
            oldY = y;
            oldX = x;
        }


        result[1] = mod(new BigDecimal(x), new BigDecimal(oldK)).intValue();

        return result;
    }

    public static int[] rsaKeyGen(int p, int q, int e){
        int n, d, t;
        int keys[] = new int[3];
        keys[0] = n = p * q;
        keys[1] = e;
        t = (p-1)*(q-1);
        int gcdx[] = egcd(t, e);
        keys[2] = gcdx[1];

        return keys;
    }

    public static BigDecimal rsaEncrypt(BigDecimal x, BigDecimal e, BigDecimal n){
        System.out.println("Encrypting " + x + " with e = " + e + " and n = " + n);
        return mod(x.pow(e.intValue()), n);
    }

    public static BigDecimal rsaDecrypt(BigDecimal y, BigDecimal d, BigDecimal n){
        System.out.println("Decrypting " + y + " with d = " + d + " and n = " + n);
        return  mod(y.pow(d.intValue()), n);
    }

    public static BigDecimal mod(BigDecimal a, BigDecimal n){
        if(a.compareTo(new BigDecimal(0)) == -1)
            return n.subtract(a.negate().remainder(n));
        else
            return a.remainder(n);
    }
}

    Result: abcdefghijklmnopzqrstuvzwxyz encrypted to ablmqrghistcdnopqefjkuaqblmq

Result: ablmqrghistcdnopqefjkuaqblmq decrypted to abcdefghijklmnopeqrstuaebcde
It did not work, please check your implementation!