PBKDF2WithHmacSHA1算法

     主要用于明文密码加密字符串存入数据库。由棱镜门思考。目前大部分企业中都是明文密码。一旦被攻破。危害非常大。现在主流加密技术是MD5加密。不过MD5的存在小概率碰撞(根据密码学的定义，如果内容不同的明文，通过散列算法得出的结果（密码学称为信息摘要）相同，就称为发生了“碰撞”。).如何生成md5碰撞的算法论文http://www.infosec.sdu.edu.cn/paper/md5-attack.pdf。一些黑客破获密码的方法是一种被称为“跑字典”的方法。有两种方法得到字典，一种是日常搜集的用做密码的字符串表，另一种是用排列组合方法生成的，先用MD5程序计算出这些字典项的MD5值，然后再用目标的MD5值在这个字典中检索。即使假设密码的最大长度为8，同时密码只能是字母和数字，共26+26+10=62个字符，排列组合出的字典的项数则是P(62,1)+P (62,2)….+P(62,8)，那也已经是一个很天文的数字了，存储这个字典就需要TB级的磁盘组，而且这种方法还有一个前提，就是能获得目标账户的 密码MD5值的情况下才可以。当如果用户的密码是弱密码的就很危险了。

　　PBKDF2WithHmacSHA1算法比MD5算法更安全。它可以同样密码在不同时间生成不同加密Hash。跑字典将无效。下面是算法Demo。

package hashpassword;
import java.security.SecureRandom;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.SecretKeyFactory;
import java.math.BigInteger;
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidKeySpecException;

/*
 * PBKDF2 salted password hashing.
 * Author: havoc AT defuse.ca
 * www: http://crackstation.net/hashing-security.htm
 */
public class PasswordHash
{
    public static final String PBKDF2_ALGORITHM = "PBKDF2WithHmacSHA1";

    // The following constants may be changed without breaking existing hashes.
    public static final int SALT_BYTE_SIZE = 24;
    public static final int HASH_BYTE_SIZE = 24;
    public static final int PBKDF2_ITERATIONS = 10;

    public static final int ITERATION_INDEX = 0;
    public static final int SALT_INDEX = 1;
    public static final int PBKDF2_INDEX = 2;

    public static String createHash(String password)
        throws NoSuchAlgorithmException, InvalidKeySpecException
    {
        return createHash(password.toCharArray());
    }

    /**
     * Returns a salted PBKDF2 hash of the password.
     * 返回一个加盐后的PBKDF2哈希密码
     * @param   password    the password to hash
     * @return              a salted PBKDF2 hash of the password
     */
    public static String createHash(char[] password)
        throws NoSuchAlgorithmException, InvalidKeySpecException
    {
        // Generate a random salt 随即盐序列
        SecureRandom random = new SecureRandom();
        byte[] salt = new byte[SALT_BYTE_SIZE];
        random.nextBytes(salt);

        // Hash the password  生成哈希密码
        byte[] hash = pbkdf2(password, salt, PBKDF2_ITERATIONS, HASH_BYTE_SIZE);
        // format iterations:salt:hash 格式化 迭代次数：盐：哈希
        return PBKDF2_ITERATIONS + ":" + toHex(salt) + ":" +  toHex(hash);
    }

    /**
     * Validates a password using a hash.
     *
     * @param   password        the password to check
     * @param   correctHash     the hash of the valid password
     * @return                  true if the password is correct, false if not
     */
    public static boolean validatePassword(String password, String correctHash)
        throws NoSuchAlgorithmException, InvalidKeySpecException
    {
        return validatePassword(password.toCharArray(), correctHash);
    }

    /**
     * Validates a password using a hash.
     *
     * @param   password        the password to check
     * @param   correctHash     the hash of the valid password
     * @return                  true if the password is correct, false if not
     */
    public static boolean validatePassword(char[] password, String correctHash)
        throws NoSuchAlgorithmException, InvalidKeySpecException
    {
        // Decode the hash into its parameters
        String[] params = correctHash.split(":");
        int iterations = Integer.parseInt(params[ITERATION_INDEX]);
        byte[] salt = fromHex(params[SALT_INDEX]);
        byte[] hash = fromHex(params[PBKDF2_INDEX]);
        // Compute the hash of the provided password, using the same salt, 
        // iteration count, and hash length
        byte[] testHash = pbkdf2(password, salt, iterations, hash.length);
        // Compare the hashes in constant time. The password is correct if
        // both hashes match.
        return slowEquals(hash, testHash);
    }

    /**
     * Compares two byte arrays in length-constant time. This comparison method
     * is used so that password hashes cannot be extracted from an on-line 
     * system using a timing attack and then attacked off-line.
     * 
     * @param   a       the first byte array
     * @param   b       the second byte array 
     * @return          true if both byte arrays are the same, false if not
     */
    private static boolean slowEquals(byte[] a, byte[] b)
    {
        int diff = a.length ^ b.length;
        for(int i = 0; i < a.length && i < b.length; i++)
            diff |= a[i] ^ b[i];
        return diff == 0;
    }

    /**
     *  Computes the PBKDF2 hash of a password.
     *  计算PBKDF2哈希密码
     * @param   password    the password to hash.   需要加密的明文密码
     * @param   salt        the salt                盐增加调味 增加密码破解难度
     * @param   iterations  the iteration count (slowness factor)  迭代次数
     * @param   bytes       the length of the hash to compute in bytes  计算密码后的 哈希长度
     * @return              the PBDKF2 hash of the password
     */
    private static byte[] pbkdf2(char[] password, byte[] salt, int iterations, int bytes)
        throws NoSuchAlgorithmException, InvalidKeySpecException
    {
        PBEKeySpec spec = new PBEKeySpec(password, salt, iterations, bytes * 8);
        SecretKeyFactory skf = SecretKeyFactory.getInstance(PBKDF2_ALGORITHM);
        return skf.generateSecret(spec).getEncoded();
    }

    /**
     * Converts a string of hexadecimal characters into a byte array.
     *
     * @param   hex         the hex string
     * @return              the hex string decoded into a byte array
     */
    private static byte[] fromHex(String hex)
    {
        byte[] binary = new byte[hex.length() / 2];
        for(int i = 0; i < binary.length; i++)
        {
            binary[i] = (byte)Integer.parseInt(hex.substring(2*i, 2*i+2), 16);
        }
        return binary;
    }

    /**
     * Converts a byte array into a hexadecimal string.
     *
     * @param   array       the byte array to convert
     * @return              a length*2 character string encoding the byte array
     */
    private static String toHex(byte[] array)
    {
        BigInteger bi = new BigInteger(1, array);
        String hex = bi.toString(16);
        int paddingLength = (array.length * 2) - hex.length();
        if(paddingLength > 0) 
            return String.format("%0" + paddingLength + "d", 0) + hex;
        else
            return hex;
    }

    /**
     * Tests the basic functionality of the PasswordHash class
     *
     * @param   args        ignored
     */
    public static void main(String[] args)
    {
        try
        {
            // Print out 10 hashes
            for(int i = 0; i < 10; i++)
                System.out.println(PasswordHash.createHash("p\r\nassw0Rd!"));

            // Test password validation
            boolean failure = false;
            System.out.println("Running tests...");
            for(int i = 0; i < 100; i++)
            {
                String password = ""+i;
                String hash = createHash(password);
                String secondHash = createHash(password);
                if(hash.equals(secondHash)) {
                    System.out.println("FAILURE: TWO HASHES ARE EQUAL!");
                    failure = true;
                }
                String wrongPassword = ""+(i+1);
                if(validatePassword(wrongPassword, hash)) {
                    System.out.println("FAILURE: WRONG PASSWORD ACCEPTED!");
                    failure = true;
                }
                if(!validatePassword(password, hash)) {
                    System.out.println("FAILURE: GOOD PASSWORD NOT ACCEPTED!");
                    failure = true;
                }
            }
            if(failure)
                System.out.println("TESTS FAILED!");
            else
                System.out.println("TESTS PASSED!");
        }
        catch(Exception ex)
        {
            System.out.println("ERROR: " + ex);
        }
    }

}

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