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Thread: Big Integer help

  1. #1
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    Lightbulb Big Integer help

    Hi all,

    I'm currently working on a RSA Encryption using Fast Exponential technique. As the numbers I'm working with will be very huge, I am forced to use BigInteger.

    As this is the first time I'm using Big Integer, I encountered quite a number of issues. I'll try my best to explain the errors I'm facing:

    1. I had done this step: BigInteger d;
    So I'm currently trying to get d = 0;
    However, I keep getting an error required: BigInteger, found int.

    When I changed it to d = new BigInteger(0) , I get this error:

    java:19: error: constructor BigInteger in class BigInteger cannot be applied to given types;

    d = new BigInteger(0);
    ^
    required: no arguments
    found: int
    reason: actual and formal argument lists differ in length


    Part of Code:
    public class BigFastExponential
    {
    BigInteger e; // public exponenet: all has this
    BigInteger d; // private key: only key owner has this
    BigInteger n; // public modulus: all has this

    /**
    * Create a LongRSA object for a public key holder,
    * with just e and n.
    */
    public BigFastExponential(BigInteger public_e, BigInteger public_n)
    {
    e = public_e;
    n = public_n;
    d = new BigInteger(0);
    }



    2. Similar to above, I tried the following if (d == 0).

    I also tried :
    BigInteger = result;
    result = 1;

    This is the error I received:

    java:57: error: incomparable types: BigInteger and int
    if (d == 0) {
    ^
    BigFastExponential.java:77: error: incompatible types
    result = 1;
    ^
    required: BigInteger
    found: int


    3. I'm also getting some erros about the operators for BigIntegers. All these used to work when I was using LONG instead of BigInteger.

    Error:
    BigFastExponential.java:79: error: bad operand types for binary operator '>'
    while (e > 0)
    ^
    first type: BigInteger
    second type: int
    BigFastExponential.java:81: error: bad operand types for binary operator '&'
    if ((e & 1) == 1) // & = bitwise &&
    ^
    first type: BigInteger
    second type: int
    BigFastExponential.java:83: error: bad operand types for binary operator '*'
    result = (result * b) % n;
    ^
    first type: BigInteger
    second type: BigInteger
    BigFastExponential.java:85: error: bad operand types for binary operator '>>'
    e = e >> 1;
    ^
    first type: BigInteger
    second type: int
    BigFastExponential.java:86: error: bad operand types for binary operator '*'
    b = (b * b) % n;
    ^
    first type: BigInteger
    second type: BigInteger



    Code segment:
    public BigInteger decrypt(BigInteger c) {
    if (d == 0) {
    throw new IllegalStateException("No private key value set.");
    }
    BigInteger M;
    M = modpow(c, d, n);
    return M;
    }


    public static final BigInteger modpow(BigInteger b, BigInteger e, BigInteger n)
    {
    BigInteger result;
    result = 1;

    while (e > 0)
    {
    if ((e & 1) == 1) // & = bitwise &&
    {
    result = (result * b) % n;
    }
    e = e >> 1;
    b = (b * b) % n;
    }

    return result;
    }


    4. Last but not least, I did my research and I was told that to get input as BigInteger, I have to use scanner.nextBigInteger();

    I did so but I received a very weird error:

    BigFastExponential.java:104: error: incompatible types
    n = user_input.nextBigInteger();
    ^
    required: BigInteger
    found: java.math.BigInteger


    Are they different ?

    Code Segment:

    public static void main(String[] args)
    {
    BigInteger p;
    BigInteger q;
    BigInteger e;
    BigInteger n;
    BigInteger d;
    BigInteger M;

    // Lab 2 - (2)(a) Bob uses PublicAlice key to encrypt a plaintext message M
    Scanner user_input = new Scanner (System.in);
    System.out.print("Enter value of n: "); // n = value of pq. Bob does NOT know p and q, only n
    n = user_input.nextBigInteger();

    System.out.print("Enter value of e: "); // (pq/n,e) = Public Alice Key
    e = user_input.nextBigInteger();

    System.out.print("Enter value of d: "); // for decryption using Private Alice Key
    d = user_input.nextBigInteger();


    Thanks in advance for all your help ! Sorry for the long list of questions on my very first post !

    Warmest Regards,
    Zeke


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Big Integer help

    BigInteger is a class, not a primitive like an int. You have to use the class's methods to compare the contents of the class.

    BigFastExponential.java:104: error: incompatible types
    I don't get that error. What imports or definitions do you have? Do you have your own version of BigInteger?
    Last edited by Norm; April 17th, 2012 at 10:01 AM.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Big Integer help

    Oh. Turns out I there was some corruption in the library java.util.BigInteger I tried to import. I figured out the rest already. Thanks !

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