Thread: How do I convert a char into a BigInteger and vice versa?

I'm trying to do an encryption. I have this message: "hello world"

I am supposed to encrypt and decrypt that character by character.

So, the letter 'h' as a char may equal 97, and so on.

I want to make 97 a BigInteger, and then perform arithmetic on that to encrypt it.

I can turn the String into a char array.
I can turn the char array into an int array.
I cannot turn the int array into a BigInteger array.

Mind you, I need to use the BigInteger.modPow(e,n) function in order to perform the encryption, so it must ultimately become a BigInteger.

And, even if I did make the conversion, how would I convert it back?

Looked everywhere online for over an hour, couldn't find anything that makes sense.

Thanks in advance.

Okay, for encryption, we use this formula:

c = m^e mod n
m is the "message".
We need a BigInteger.modPow(e,n) to calculate this.

So, the 'm' must be a BigInteger.

I intend to convert each character of the String into a BigInteger and perform the arithmetic, creating an array of BigIntegers the size of the length of the original message, where each entry in the array represents a character of the message.

And then convert back from that. Make sense?

--- Update ---

Though I'm sure there's a simpler way to do it character by character!

Thanks for the response!

I found this:

BigInteger(String val)
Translates the decimal String representation of a BigInteger into a BigInteger.

But I don't really know what the "decimal String representation is", and that doesn't seem to convert character by character.


I also found this:

BigInteger(byte[] val)
Translates a byte array containing the two's-complement binary representation of a BigInteger into a BigInteger.

But the two's complement binary representation kind of scares me. Plus I don't even know what a Byte array is....though it seems like that's the one I should use.

But...I'm creating a single BigInteger, not an array of BigIntegers, right? How exactly does that represent the String?

So I can pass the BigInteger function a String....but the String has to contain numbers within it?

But I want to pass a String that is a message, not a String of numbers.

Do I need to take the message, make it an array of chars, make the chars into ints, and then make the char array into a String? That seems pretty convoluted, and it doesn't convert character by character.

Yeah, I made that test program and it worked only when I passed a String like "45" or "439", not when I passed "hello" or "world".

I also have this code, which is converting character to int value and then that to a String of digits for the constructor, but it won't compile:

public void Encrypt()
        {
                int[] numArray = new int[message.length()];
                char[] charArray = new char[message.length()];
 
                charArray = message.toCharArray();
 
                System.out.println("Encrypted message: ");
 
                for (int i = 0; i < message.length(); i++)
                {
                        numArray[i] = charArray[i];
                }
 
                String str = Arrays.toString(numArray);
                BigInteger BI = new BigInteger(str);
 
 
        }

I got it compiling, but now it has this error during runtime:

HTML Code:

Encrypted message:
Exception in thread "main" java.lang.NumberFormatException: For input string: "[104, 1"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.lang.Integer.parseInt(Integer.java:492)
        at java.math.BigInteger.<init>(BigInteger.java:338)
        at java.math.BigInteger.<init>(BigInteger.java:476)
        at RSACipher1.Encrypt(RSACipher1.java:134)
        at RSACipher1.main(RSACipher1.java:44)

And yeah, I'll do some testing right now.

--- Update ---

And this is line 44:

BigInteger BI = new BigInteger(str);

--- Update ---

Okay, and I tried outputting the String "str" to the screen...and I got this:
str = [108, 111, 107]

So obviously that's no good lol.

But then how am I supposed to be making these conversions???

Okay, I finally got the little bastard to work.

Thanks for all the help.