Quickly understand the commonly used message summarization algorithms, and no longer have to worry about the thorough inquiry of the interviewer

interviewer ： Tell me about your common encryption algorithms ？

Encryption algorithms are usually divided into two types ： Symmetric encryption and Asymmetric encryption . among , Symmetric encryption algorithm uses the same key for encryption and decryption ; Asymmetric encryption algorithm uses different keys for encryption and decryption , It is divided into public key and private key . Besides , There is also a class called Message digest algorithm , It is an irreversible algorithm for summarizing data .

This time, let's look at the message digest algorithm .

Message digest algorithm

The message digest algorithm is an algorithm that combines inputs of any length to produce pseudo-random results of fixed length . In information security , There are many important applications , Message digest algorithm is used to realize , E.g. digital signature 、 Message authentication code .

For any given data , Message digest algorithms can easily calculate the digest results . It is difficult to get a known summary result , To calculate the original data . Without changing the summary results , It is not feasible to modify the data content . For two different data , There is only a very low probability that the same summary results will be produced .

The common symmetric encryption algorithms are ：MD5 Algorithm 、SHA.

MD5 Algorithm

MD5 Algorithm （Message Digest 5） It's a cryptographic hash function , Produce a 128 Hash value of bits , You can use a length of 32 The hexadecimal string of represents .

MD5 The algorithm was developed by American cryptologists Ronald Linn Rivest（ This big guy invented RSA Algorithm R） The design of the , On 1992 Open in , To replace the previous MD4 Algorithm （ And before that MD3 Algorithm 、MD2 Algorithm ）.

MD5 The algorithm divides the original data into groups 512 Bit size , Then each group is divided into 16 individual 32 Group the seats , And then with the pre-defined 4 A magic number performs a series of bit operations , Finally, I got four 32 A group of , Put these four 32 After the concatenation of bit groups, a 128 Bit hash value .

We use it Java Write an example ：

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import java.math.BigInteger;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Md5Util {

    private static final String MD5 = "MD5";
    private static final Charset CHARSET = StandardCharsets.UTF_8;

    public static String build(String input) throws NoSuchAlgorithmException {
        MessageDigest md5 = MessageDigest.getInstance(MD5);
        byte[] digest = md5.digest(input.getBytes(CHARSET));
        return new BigInteger(1, digest).toString(16);
    }

    public static void main(String[] args) throws NoSuchAlgorithmException {
        String msg = " I like you , Can you be my girl friend? ？";
        System.out.println(" original text ：" + msg);
        System.out.println(" Abstract ：" + build(msg));
    }
}

     

      
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The operation results are as follows ：

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 original text ： I like you , Can you be my girl friend? ？
 Abstract ：f9fa148f2cfffda9b5e15a9e5bf34b66

     

      
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2004 year ,MD5 The algorithm proved unable to prevent collision attack , So it doesn't apply to security certification .
2009 year , Xie Tao and Feng Dengguo of the Chinese Academy of Sciences only used $2^{20.96}$ The complexity of the collision algorithm , It's broken MD5 Collision resistance of , This attack only takes a few seconds to run on a normal computer .
2011 year ,IETF（Internet Engineering Task Force, Internet Engineering Task Force ） Release RFC 6151, prohibit MD5 Used as key hash message authentication code .
All in all ,MD5 It's no longer safe , Don't use it any more .

SHA

SHA（Secure Hash Algorithm, Secure hash algorithm ） Is a family of cryptographic hash functions , yes FIPS（Federal Information Processing Standards, Federal Information Processing Standards ） Authenticated secure hash algorithm .

SHA The family contains a set of algorithms that have evolved over time , Yes 1993 Published in SHA-0、1995 Published in SHA-1、2001 Published in SHA-2、2015 Published in SHA-3.

Due to MD5 A successful solution , And right SHA-0 and SHA-1 There is a theoretical solution to the problem , Therefore, it is recommended to use SHA-2, Or safer SHA-3.

We use it Java Write a SHA-2 Example ：

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import java.math.BigInteger;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class ShaUtil {
    private static final String SHA_256 = "SHA-256";
    private static final String SHA_512 = "SHA-512";
    private static final String SHA3_256 = "SHA3-256";
    private static final String SHA3_512 = "SHA3-512";
    private static final Charset CHARSET = StandardCharsets.UTF_8;

    public static String build(String input, String algorithm) throws NoSuchAlgorithmException {
        MessageDigest md5 = MessageDigest.getInstance(algorithm);
        byte[] digest = md5.digest(input.getBytes(CHARSET));
        return new BigInteger(1, digest).toString(16);
    }

    public static void main(String[] args) throws NoSuchAlgorithmException {
        String msg = " I like you , Can you be my girl friend? ？";
        System.out.println(" original text ：" + msg);
        System.out.println("SHA2-256 Abstract ：" + build(msg, SHA_256));
        System.out.println("SHA2-512 Abstract ：" + build(msg, SHA_512));
        System.out.println("SHA3-256 Abstract ：" + build(msg, SHA3_256));
        System.out.println("SHA3-512 Abstract ：" + build(msg, SHA3_512));
    }
}

     

      
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The operation results are as follows ：

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 original text ： I like you , Can you be my girl friend? ？
SHA2-256 Abstract ：b6da8ee261f2b852c1140cf181e8d64b180ca6c884651ddb871bdff25afc836b
SHA2-512 Abstract ：d65f455eb38a565fae8e7c3ea6dbc005612071d5e57b688f32674e9641ab9aa6f056381222ba47cc973c86380f24fd10f4078ad7bfd3d498210d721734740a5a
SHA3-256 Abstract ：fc5f1427fc5a1bb2f231eec52fdaa5ac84652730143a3c7598dc2148ccd05cec
SHA3-512 Abstract ：5d8ba707c40c39f37c8cffd2eabf8da8d6d4ede70c697402a5e5ea6228c5710c3d76a6abbc1d46413bfced66280f72621feac12ce3ef49aed60902091ca1979f

     

      
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JDK8 The following versions do not support SHA-3, So when you run the above code, there will be NoSuchAlgorithmException: SHA3-256 MessageDigest not available abnormal .

summary

The message digest algorithm is an algorithm that combines inputs of any length to produce pseudo-random results of fixed length . The common symmetric encryption algorithms are ：MD5 Algorithm 、SHA.

MD5 The algorithm should not be used any more .SHA In the family , Recommended SHA-2, Or safer SHA-3.