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Java implementation of scrypt
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// Copyright (C) 2011 - Will Glozer.  All rights reserved.

package com.lambdaworks.crypto;

import java.io.UnsupportedEncodingException;
import java.security.GeneralSecurityException;
import java.security.SecureRandom;

import static com.lambdaworks.crypto.Base64.*;

/**
 * Simple {@link SCrypt} interface for hashing passwords using the
 * scrypt key derivation function
 * and comparing a plain text password to a hashed one. The hashed output is an
 * extended implementation of the Modular Crypt Format that also includes the scrypt
 * algorithm parameters.
 *
 * Format: $s0$PARAMS$SALT$KEY.
 *
 * 
 * PARAMS
32-bit hex integer containing log2(N) (16 bits), r (8 bits), and p (8 bits)
 * SALT
base64-encoded salt
 * KEY
base64-encoded derived key
 * 
 *
 * s0 identifies version 0 of the scrypt format, using a 128-bit salt and 256-bit derived key.
 *
 * @author  Will Glozer
 */
public class SCryptUtil {
    /**
     * Hash the supplied plaintext password and generate output in the format described
     * in {@link SCryptUtil}.
     *
     * @param passwd    Password.
     * @param N         CPU cost parameter.
     * @param r         Memory cost parameter.
     * @param p         Parallelization parameter.
     *
     * @return The hashed password.
     */
    public static String scrypt(String passwd, int N, int r, int p) {
        try {
            byte[] salt = new byte[16];
            SecureRandom.getInstance("SHA1PRNG").nextBytes(salt);

            byte[] derived = SCrypt.scrypt(passwd.getBytes("UTF-8"), salt, N, r, p, 32);

            String params = Integer.toString(log2(N) << 16 | r << 8 | p, 16);

            StringBuilder sb = new StringBuilder((salt.length + derived.length) * 2);
            sb.append("$s0$").append(params).append('$');
            sb.append(encodeToChar(salt, false)).append('$');
            sb.append(encodeToChar(derived, false));

            return sb.toString();
        } catch (UnsupportedEncodingException e) {
            throw new IllegalStateException("JVM doesn't support UTF-8?");
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException("JVM doesn't support SHA1PRNG or HMAC_SHA256?");
        }
    }

    /**
     * Compare the supplied plaintext password to a hashed password.
     *
     * @param   passwd  Plaintext password.
     * @param   hashed  scrypt hashed password.
     *
     * @return true if passwd matches hashed value.
     */
    public static boolean check(String passwd, String hashed) {
        try {
            String[] parts = hashed.split("\\$");

            if (parts.length != 5 || !parts[1].equals("s0")) {
                throw new IllegalArgumentException("Invalid hashed value");
            }

            int params = Integer.parseInt(parts[2], 16);
            byte[] salt = decodeFast(parts[3].toCharArray());
            byte[] derived0 = decodeFast(parts[4].toCharArray());

            int N = (int) Math.pow(2, params >> 16 & 0xff);
            int r = params >> 8 & 0x0f;
            int p = params      & 0x0f;

            byte[] derived1 = SCrypt.scrypt(passwd.getBytes("UTF-8"), salt, N, r, p, 32);

            if (derived0.length != derived1.length) return false;

            int result = 0;
            for (int i = 0; i < derived0.length; i++) {
                result |= derived0[i] ^ derived1[i];
            }
            return result == 0;
        } catch (UnsupportedEncodingException e) {
            throw new IllegalStateException("JVM doesn't support UTF-8?");
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException("JVM doesn't support SHA1PRNG or HMAC_SHA256?");
        }
    }

    private static int log2(int n) {
        int log = 0;
        if ((n & 0xffff0000 ) != 0) { n >>>= 16; log = 16; }
        if (n >= 256) { n >>>= 8; log += 8; }
        if (n >= 16 ) { n >>>= 4; log += 4; }
        if (n >= 4  ) { n >>>= 2; log += 2; }
        return log + (n >>> 1);
    }
}