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

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    Default 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.


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

    I am supposed to encrypt and decrypt that character by character.
    I don't understand why you are using BigInteger if the task does it one character at a time.
    What is done with the encrypted character?
    How are the encrypted characters saved? In an array?
    If you don't understand my answer, don't ignore it, ask a question.

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

    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!

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

    What does the BigInteger constructor take to create an instance of BigInteger?
    What technique can you use to convert a character to what the constructor takes?
    If you don't understand my answer, don't ignore it, ask a question.

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

    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?

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

    BigInteger(String val)
    Translates the decimal String representation of a BigInteger into a BigInteger.
    That looks promising. Go read the details for that one.

    Try some experiments using Strings of decimal digits: "56" or "65" and see what it does when you use some of the methods to add etc two BIs
    If you don't understand my answer, don't ignore it, ask a question.

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

    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.

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

    pass the BigInteger function a String....but the String has to contain numbers within it
    Is that what the doc said? Did you write a small program to create two BIs , add them together and print out the results to see how they worked?

    The next step will be to convert a character to its int value and then that to a String of digits for the constructor.
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    Default Re: How do I convert a char into a BigInteger and vice versa?

    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);
     
     
            }

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

    it won't compile:
    Please copy the full text of the error message and paste it here. It has important info about the error.

    Have you tried a simple test program creating some BIs and using some methods and printing out the results?
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    Default Re: How do I convert a char into a BigInteger and vice versa?

    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???

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

    how am I supposed to be making these conversions???
    I suppose, one char at a time.

    For testing I'd use a very short String, 1 or two characters, so the results can be easily verified manually.
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    Default Re: How do I convert a char into a BigInteger and vice versa?

    Okay, I finally got the little bastard to work.

    Thanks for all the help.

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

    Glad you got it going.
    If you don't understand my answer, don't ignore it, ask a question.

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