// Copyright (C) 2011 - Will Glozer.  All rights reserved.

package com.lambdaworks.crypto;

import java.security.GeneralSecurityException;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

import static java.lang.System.arraycopy;

/**
 * An implementation of the Password-Based Key Derivation Function as specified
 * in RFC 2898.
 *
 * @author  Will Glozer
 */
public class PBKDF {
    /**
     * Implementation of PBKDF2 (RFC2898).
     *
     * @param   alg     HMAC algorithm to use.
     * @param   P       Password.
     * @param   S       Salt.
     * @param   c       Iteration count.
     * @param   dkLen   Intended length, in octets, of the derived key.
     *
     * @return  The derived key.
     *
     * @throws  GeneralSecurityException
     */
    public static byte[] pbkdf2(String alg, byte[] P, byte[] S, int c, int dkLen) throws GeneralSecurityException {
        Mac mac = Mac.getInstance(alg);
        mac.init(new SecretKeySpec(P, alg));
        byte[] DK = new byte[dkLen];
        pbkdf2(mac, S, c, DK, dkLen);
        return DK;
    }

    /**
     * Implementation of PBKDF2 (RFC2898).
     *
     * @param   mac     Pre-initialized {@link Mac} instance to use.
     * @param   S       Salt.
     * @param   c       Iteration count.
     * @param   DK      Byte array that derived key will be placed in.
     * @param   dkLen   Intended length, in octets, of the derived key.
     *
     * @throws  GeneralSecurityException
     */
    public static void pbkdf2(Mac mac, byte[] S, int c, byte[] DK, int dkLen) throws GeneralSecurityException {
        int hLen = mac.getMacLength();

        if (dkLen > (Math.pow(2, 32) - 1) * hLen) {
            throw new GeneralSecurityException("Requested key length too long");
        }

        byte[] U      = new byte[hLen];
        byte[] T      = new byte[hLen];
        byte[] block1 = new byte[S.length + 4];

        int l = (int) Math.ceil((double) dkLen / hLen);
        int r = dkLen - (l - 1) * hLen;

        arraycopy(S, 0, block1, 0, S.length);

        for (int i = 1; i <= l; i++) {
            block1[S.length + 0] = (byte) (i >> 24 & 0xff);
            block1[S.length + 1] = (byte) (i >> 16 & 0xff);
            block1[S.length + 2] = (byte) (i >> 8  & 0xff);
            block1[S.length + 3] = (byte) (i >> 0  & 0xff);

            mac.update(block1);
            mac.doFinal(U, 0);
            arraycopy(U, 0, T, 0, hLen);

            for (int j = 1; j < c; j++) {
                mac.update(U);
                mac.doFinal(U, 0);

                for (int k = 0; k < hLen; k++) {
                    T[k] ^= U[k];
                }
            }

            arraycopy(T, 0, DK, (i - 1) * hLen, (i == l ? r : hLen));
        }
    }
}
