HmacSHA256 Signature in Java

Carvia Tech | July 25, 2020 | 4 min read | 15 views


In this tutorial we will learn how to generate Hmac256 signature in Java using standard library, Google Guava and Apache commons codec library.

MAC is a Message Authentication Code algorithm, which provides a way to check integrity of information transmitted over or stored in an unreliable medium. Typically, message authentication codes are used between two parties that share a secret key in order to validate information transmitted between these parties.

A MAC mechanism that is based on cryptographic hash functions is referred to as HMAC. HMAC can be used with any cryptographic hash function, e.g., SHA256 or SHA384, in combination with a secret shared key. HMAC is specified in RFC 2104.

Most commonly used HMAC implementations are:

  • HmacMD5

  • HmacSHA1

  • HmacSHA256

Now we will create HmacSHA256 signature using 3 different libraries - Java Standard Library, Google Guava and Apache Commons Codec.

1. Using JDK Standard Library

Java/Android has everything in cryptography libraries that is required to generate a Hmac256.

HmacUtils.java
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;

import static java.nio.charset.StandardCharsets.UTF_8;

public class HmacUtils {

    String generateHmac256(String message, byte[] key) throws InvalidKeyException, NoSuchAlgorithmException {
        byte[] bytes = hmac("HmacSHA256", key, message.getBytes());
        return bytesToHex(bytes);   (1)
    }

    byte[] hmac(String algorithm, byte[] key, byte[] message) throws NoSuchAlgorithmException, InvalidKeyException {
        Mac mac = Mac.getInstance(algorithm);
        mac.init(new SecretKeySpec(key, algorithm));
        return mac.doFinal(message);
    }

    String bytesToHex(byte[] bytes) {   (2)
        final char[] hexArray = "0123456789abcdef".toCharArray();
        char[] hexChars = new char[bytes.length * 2];
        for (int j = 0, v; j < bytes.length; j++) {
            v = bytes[j] & 0xFF;
            hexChars[j * 2] = hexArray[v >>> 4];
            hexChars[j * 2 + 1] = hexArray[v & 0x0F];
        }
        return new String(hexChars);
    }

     public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeyException {
        String valueToDigest = "The quick brown fox jumps over the lazy dog";
        byte[] key = "secret".getBytes();

        HmacUtils hm = new HmacUtils();
        String messageDigest = hm.generateHmac256(valueToDigest, key);
        System.out.println(messageDigest);
    }
}
1 Default return type for Mac function is byte array, so we need to convert it to Hex format.
2 Our custom method to convert a byte array to hex, optionally we can use Apache Commons Codec’s Hex utility to convert byte array to hex in single line.

Converting byte[] to hex

There are so many libraries in Java ecosystem that can convert a byte array to hex encoded string:

a) Apache Commons Codec

You can use org.apache.commons.codec.binary.Hex class to convert given byte array to Hex encoded string using single line code:

Apache Commons Codec’s Hex utility
import org.apache.commons.codec.binary.Hex;
...
String hexHmac = Hex.encodeHexString(bytes);

b) Spring Security

If you are using Spring Security in your project, you can use org.springframework.security.crypto.codec.Hex utility to encode and decode byte array into string and vice versa.

import org.springframework.security.crypto.codec.Hex;
...
String hexValue = new String(Hex.encode(bytes));

c) Google Guava

Google Guava provides BaseEncoding for handlign encoding and decoding to Base16 format.

import com.google.common.io.BaseEncoding;
...
String hexValue = BaseEncoding.base16().lowerCase().encode(bytes);  (1)
1 Default is Upper case hex caharacters.

d) Bouncy Castle

If you are already using Bouncy Castle security framework, then you can use this Hex util:

import org.bouncycastle.util.encoders.Hex;
...
String hexValue = Hex.toHexString(bytes);

You can also follow this stackoverflow discussion.

2. Using Google Guava

Google Guava library provides static hashing related utilities in class com.google.common.hash.Hashing.

build.gradle
dependencies {
// https://mvnrepository.com/artifact/com.google.guava/guava
    compile group: 'com.google.guava', name: 'guava', version: '29.0-jre'
}
Hmac256 using Google Guava
import com.google.common.hash.Hashing;

public class HMacTest {

    public void hmac256() {
        String valueToDigest = "The quick brown fox jumps over the lazy dog";
        byte[] key = "secret".getBytes();

        String gHmac = Hashing.hmacSha256(key)
                .newHasher()    (1)
                .putString(valueToDigest, UTF_8)
                .hash()
                .toString();    (2)
        System.out.println(gHmac);
    }
}
1 A hasher can be used only once and must be created for next Hmac256 calculation.
2 This will convert the byte array to hex encoded string
Output
54cd5b827c0ec938fa072a29b177469c843317b095591dc846767aa338bac600

3. Apache Commons Codec

Apache Commons Codec provides a utility class for Hmac operations.

org.apache.commons.codec.digest.HmacUtils

build.gradle
dependencies {
    // https://mvnrepository.com/artifact/commons-codec/commons-codec
    compile group: 'commons-codec', name: 'commons-codec', version: '1.14'
}
Hmac256 using Apache Commons Codec
import org.apache.commons.codec.digest.HmacAlgorithms;
import org.apache.commons.codec.digest.HmacUtils;

public class HMacTest {

    public void hmac256() {
        String valueToDigest = "The quick brown fox jumps over the lazy dog";
        byte[] key = "secret".getBytes();

        HmacUtils hm256 = new HmacUtils(HmacAlgorithms.HMAC_SHA_256, key);
        //hm256 object can be used again and again
        String hmac = hm256.hmacHex(valueToDigest);
        System.out.println(hmac);
    }
}
Output
54cd5b827c0ec938fa072a29b177469c843317b095591dc846767aa338bac600

Similarly, you can compute Hmac256 for the entire file.

Hmac256 for a file
 String hexPom = hm256.hmacHex(new File("pom.xml"));

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