Bael 4805 Decode a JWT Token in Java (#10535)

* BAEL-4805 Added the code for Decoding a JWT Token in Java tutorial

* BAEL-4805 Added newline at end of test file to conform to standards

* BAEL-4805 renamed test class to conform to naming standard for unit tests

* BAEL-4805 removed @DisplayName annotations as they are redundant

* BAEL-4805 changed assertion to be the user name in payload section to be more meaningful to reader

Co-authored-by: Liam Garvie <liamgarvie@Liams-MacBook-Pro.local>

jjwt/pom.xml

@@ -41,6 +41,12 @@
             <artifactId>jjwt</artifactId>
             <version>${jjwt.version}</version>
         </dependency>
+
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <properties>

jjwt/src/main/java/io/jsonwebtoken/jjwtfun/util/JWTDecoderUtil.java

@@ -0,0 +1,46 @@
+package io.jsonwebtoken.jjwtfun.util;
+
+import io.jsonwebtoken.SignatureAlgorithm;
+import io.jsonwebtoken.impl.crypto.DefaultJwtSignatureValidator;
+
+import javax.crypto.spec.SecretKeySpec;
+import java.util.Base64;
+
+import static io.jsonwebtoken.SignatureAlgorithm.HS256;
+
+public class JWTDecoderUtil {
+
+    public static String decodeJWTToken(String token) {
+        Base64.Decoder decoder = Base64.getDecoder();
+
+        String[] chunks = token.split("\\.");
+
+        String header = new String(decoder.decode(chunks[0]));
+        String payload = new String(decoder.decode(chunks[1]));
+
+        return header + " " + payload;
+    }
+
+    public static String decodeJWTToken(String token, String secretKey) throws Exception {
+        Base64.Decoder decoder = Base64.getDecoder();
+
+        String[] chunks = token.split("\\.");
+
+        String header = new String(decoder.decode(chunks[0]));
+        String payload = new String(decoder.decode(chunks[1]));
+
+        String tokenWithoutSignature = chunks[0] + "." + chunks[1];
+        String signature = chunks[2];
+
+        SignatureAlgorithm sa = HS256;
+        SecretKeySpec secretKeySpec = new SecretKeySpec(secretKey.getBytes(), sa.getJcaName());
+
+        DefaultJwtSignatureValidator validator = new DefaultJwtSignatureValidator(sa, secretKeySpec);
+
+        if (!validator.isValid(tokenWithoutSignature, signature)) {
+            throw new Exception("Could not verify JWT token integrity!");
+        }
+
+        return header + " " + payload;
+    }
+}

jjwt/src/test/java/io/jsonwebtoken/jjwtfun/util/JWTDecoderUtilUnitTest.java

@@ -0,0 +1,32 @@
+package io.jsonwebtoken.jjwtfun.util;
+
+import io.jsonwebtoken.SignatureAlgorithm;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class JWTDecoderUtilUnitTest {
+
+    private final static String SIMPLE_TOKEN = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkJhZWxkdW5nIFVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9";
+    private final static String SIGNED_TOKEN = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkJhZWxkdW5nIFVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9.qH7Zj_m3kY69kxhaQXTa-ivIpytKXXjZc1ZSmapZnGE";
+
+    @Test
+    void givenSimpleToken_whenDecoding_thenStringOfHeaderPayloadAreReturned() {
+        assertThat(JWTDecoderUtil.decodeJWTToken(SIMPLE_TOKEN))
+          .contains(SignatureAlgorithm.HS256.getValue());
+    }
+
+    @Test
+    void givenSignedToken_whenDecodingWithInvalidSecret_thenIntegrityIsNotValidated() {
+        assertThatThrownBy(() -> JWTDecoderUtil.decodeJWTToken(SIGNED_TOKEN, "BAD_SECRET"))
+          .hasMessage("Could not verify JWT token integrity!");
+    }
+
+    @Test
+    void givenSignedToken_whenDecodingWithValidSecret_thenIntegrityIsValidated() throws Exception {
+        assertThat(JWTDecoderUtil.decodeJWTToken(SIGNED_TOKEN, "MySecretKey"))
+          .contains("Baeldung User");
+    }
+}