I wrote a program to encrypt and decrypt Strings and am having a small issue with it...
Code:
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:
Result: abcdefghijklmnopzqrstuvzwxyz encrypted to ablmqrghistcdnopqefjkuaqblmq
Result: ablmqrghistcdnopqefjkuaqblmq decrypted to abcdefghijklmnopeqrstuaebcde
It did not work, please check your implementation!
As you can see, it works for all letters but v-z and I'm not sure where I'm going wrong at. I ommitted some of the code, but left in probably more than I should have since I have no idea at all where I'm going wrong at.
