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 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

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package sun.security.rsa;

import java.math.BigInteger;

import java.security.*;

import java.security.spec.AlgorithmParameterSpec;

import java.security.spec.RSAKeyGenParameterSpec;

import sun.security.jca.JCAUtil;

import static sun.security.util.SecurityProviderConstants.DEF_RSA_KEY_SIZE;

/**

 * RSA keypair generation. Standard algorithm, minimum key length 512 bit.

 * We generate two random primes until we find two where phi is relative

 * prime to the public exponent. Default exponent is 65537. It has only bit 0

 * and bit 4 set, which makes it particularly efficient.

 *

 * @since   1.5

 * @author  Andreas Sterbenz

 */

    // public exponent to use

    private BigInteger publicExponent;

    // size of the key to generate, >= RSAKeyFactory.MIN_MODLEN

    private int keySize;

    // PRNG to use

    private SecureRandom random;

        // initialize to default in case the app does not call initialize()

    }

    // initialize the generator. See JCA doc

    public void initialize(int keySize, SecureRandom random) {

        try {

        }

    }

    // second initialize method. See JCA doc.

    public void initialize(AlgorithmParameterSpec params, SecureRandom random)

            throws InvalidAlgorithmParameterException {

        if (params instanceof RSAKeyGenParameterSpec == false) {

            throw new InvalidAlgorithmParameterException

                ("Params must be instance of RSAKeyGenParameterSpec");

        }

        RSAKeyGenParameterSpec rsaSpec = (RSAKeyGenParameterSpec)params;

        int tmpKeySize = rsaSpec.getKeysize();

        BigInteger tmpPublicExponent = rsaSpec.getPublicExponent();

        if (tmpPublicExponent == null) {

            tmpPublicExponent = RSAKeyGenParameterSpec.F4;

        } else {

            if (tmpPublicExponent.compareTo(RSAKeyGenParameterSpec.F0) < 0) {

                throw new InvalidAlgorithmParameterException

                        ("Public exponent must be 3 or larger");

            }

            if (tmpPublicExponent.bitLength() > tmpKeySize) {

                throw new InvalidAlgorithmParameterException

                        ("Public exponent must be smaller than key size");

            }

        }

        // do not allow unreasonably large key sizes, probably user error

        try {

            RSAKeyFactory.checkKeyLengths(tmpKeySize, tmpPublicExponent,

                512, 64 * 1024);

        } catch (InvalidKeyException e) {

            throw new InvalidAlgorithmParameterException(

                "Invalid key sizes", e);

        }

        this.keySize = tmpKeySize;

        this.publicExponent = tmpPublicExponent;

        this.random = random;

    }

    // generate the keypair. See JCA doc

    public KeyPair generateKeyPair() {

        // accommodate odd key sizes in case anybody wants to use them

        int lp = (keySize + 1) >> 1;

        int lq = keySize - lp;

        if (random == null) {

            random = JCAUtil.getSecureRandom();

        }

        BigInteger e = publicExponent;

        while (true) {

            // generate two random primes of size lp/lq

            BigInteger p = BigInteger.probablePrime(lp, random);

            BigInteger q, n;

            do {

                q = BigInteger.probablePrime(lq, random);

                // convention is for p > q

                if (p.compareTo(q) < 0) {

                    BigInteger tmp = p;

                    p = q;

                    q = tmp;

                }

                // modulus n = p * q

                n = p.multiply(q);

                // even with correctly sized p and q, there is a chance that

                // n will be one bit short. re-generate the smaller prime if so

            } while (n.bitLength() < keySize);

            // phi = (p - 1) * (q - 1) must be relative prime to e

            // otherwise RSA just won't work ;-)

            BigInteger p1 = p.subtract(BigInteger.ONE);

            BigInteger q1 = q.subtract(BigInteger.ONE);

            BigInteger phi = p1.multiply(q1);

            // generate new p and q until they work. typically

            // the first try will succeed when using F4

            if (e.gcd(phi).equals(BigInteger.ONE) == false) {

                continue;

            }

            // private exponent d is the inverse of e mod phi

            BigInteger d = e.modInverse(phi);

            // 1st prime exponent pe = d mod (p - 1)

            BigInteger pe = d.mod(p1);

            // 2nd prime exponent qe = d mod (q - 1)

            BigInteger qe = d.mod(q1);

            // crt coefficient coeff is the inverse of q mod p

            BigInteger coeff = q.modInverse(p);

            try {

                return new KeyPair(publicKey, privateKey);

            } catch (InvalidKeyException exc) {

                // invalid key exception only thrown for keys < 512 bit,

                // will not happen here

                throw new RuntimeException(exc);

            }

        }

    }

}