Abstract
You use JWTs don’t you? Everyone does, right? But do you know how to generate, sign, and verify them? The purpose of this post is to demonstrate how to code all these operations.
Disclaimer
This post is solely informative. Critically think before using any information presented. Learn from it but ultimately make your own decisions at your own risk.
Requirements
I did all of the work for this post using the following major technologies. You may be able to do the same thing with different technologies or versions, but no guarantees.
- Java 11
- Maven 3.8.6 (Bundled with NetBeans)
Download
Visit my GitHub page https://github.com/mjremijan to see all of my open source projects. The code for this post is located at: https://github.com/mjremijan/thoth-jwt
Introduction
A JWT is a simple three-part string of encoded characters - header, payload, signature - separated by 2 “.” characters.
xxxxx.yyyyy.zzzzz
JWT technology has been around for years. Read all about the JWT specification on the Introduction to JSON Web Tokens at https://jwt.io/introduction. This blog focuses on the Java code to create and verify JWT values. There are 2 examples:
- JWT with Symmetric HMAC SHA256 Signature
- JWT with Asymmetric RSA SHA256 Signature
Let’s take a look at them.
JWT with Symmetric HMAC SHA256 Signature
Listing 1 shows the code and Listing 2 shows example output.
Listing 1 - JWT with Symmetric HMAC SHA256 Signature
1. package org.thoth.jwt.main;
2.
3. import java.util.Base64;
4. import javax.crypto.Mac;
5. import javax.crypto.spec.SecretKeySpec;
6.
7. /**
8. *
9. * @author Michael Remijan mjremijan@yahoo.com @mjremijan
10. */
11. public class SignatureWithSymmetricalHmacSha256Main
12. {
13. public static void main(String[] args) throws Exception
14. {
15. // JWT HEADER
16. //
17. // This is the xxxxx of a JWT xxxxx.yyyyy.zzzzz
18. //
19. // Given the following JSON document, encode it
20. // using Java as defined in the JWT specifications
21. String header = "{\"alg\":\"HS256\",\"typ\": \"JWT\"}";
22. String headerEncoded
23. = Base64.getUrlEncoder()
24. .withoutPadding()
25. .encodeToString(
26. header.getBytes()
27. );
28. String headerDecoded
29. = new String(
30. Base64.getUrlDecoder().decode(headerEncoded)
31. );
32.
33. System.out.printf("Header Plain : %s%n", header);
34. System.out.printf("Header Encoded : %s%n", headerEncoded);
35. System.out.printf("Header Decoded : %s%n", headerDecoded);
36.
37.
38. // JWT PAYLOAD
39. //
40. // This is the yyyyy of a JWT xxxxx.yyyyy.zzzzz
41. //
42. // Given the following JSON document, encode it
43. // using Java as defined in the JWT specifications
44. String payload = "{\"sub\":\"TMJR00001\",\"name\":\"Michael J. Remijan\",\"exp\":61475608800,\"iss\":\"info@wstutorial.com\",\"groups\":[\"user\",\"admin\"]}";
45. String payloadEncoded
46. = Base64.getUrlEncoder()
47. .withoutPadding()
48. .encodeToString(
49. payload.getBytes()
50. );
51.
52. String payloadDecoded
53. = new String(
54. Base64.getUrlDecoder().decode(payloadEncoded)
55. );
56.
57. System.out.printf("%n");
58. System.out.printf("Payload Plain : %s%n", payload);
59. System.out.printf("Payload Encoded : %s%n", payloadEncoded);
60. System.out.printf("Payload Decoded : %s%n", payloadDecoded);
61.
62.
63. // SIGNATURE / VERIFY
64. // This is the zzzzz of a JWT xxxxx.yyyyy.zzzzz
65. //
66. // Hash-based message authentication code(HMAC)
67. // is a specific type of message authentication code
68. // (MAC) involving a cryptographic hash function and
69. // a secret cryptographic key. As with any MAC, it
70. // may be used to simultaneously verify both the data
71. // integrity and authenticity of a message.
72. //
73. // A cryptographic hash function (CHF) is any function
74. // that can be used to map data of arbitrary size to
75. // a fixed-size number of n bits that has special
76. // properties desirable for a cryptographic application.
77. //
78. // For this example, the process will use the SHA256
79. // cryptographic hash function and a secret key
80. // to generate a signatureCreatedFromThisData (hash) of the JWT data.
81. // This signatureCreatedFromThisData can then be used to verify the
82. // JWT data has not been tampered.
83. //
84. // Typically the secret key is only available on the
85. // Authentication Server. The key is used to create the
86. // signatureCreatedFromThisData for the JWT. Clients will typically make
87. // an authentication request (HTTPS) to the Authentication
88. // server to verify a JWT. Clients cannot verify a JWT
89. // themselves because they do not have access to the
90. // secret key. However, if a Client is 100% trusted,
91. // The secret key can be shared with the Client so
92. // that the Client can do its own verification.
93. // WARNING: This means the Client will also be able
94. // to make new JWTs, which can be dangerous.
95. String algorithm = "HmacSHA256";
96. String secret = "thisismysupersecretkeywhichshouldonlybeontheauthenticationserver";
97. SecretKeySpec key = new SecretKeySpec(secret.getBytes(), algorithm);
98. Mac mac = Mac.getInstance(algorithm);
99. mac.init(key);
100. String signatureCreatedFromThisData
101. = headerEncoded + "." + payloadEncoded;
102. String signatureEncoded
103. = Base64.getUrlEncoder()
104. .withoutPadding()
105. .encodeToString(mac.doFinal(
106. signatureCreatedFromThisData.getBytes()
107. )
108. );
109.
110. System.out.printf("%n");
111. System.out.printf("Signature Algorithm : %s%n", algorithm);
112. System.out.printf("Signature Secret : %s%n", secret);
113. System.out.printf("Signaure Encoded :%s%n", signatureEncoded);
114. }
115. }
116.
Listing 2 - HMAC Example Output
Header Plain : {"alg":"HS256","typ": "JWT"}
Header Encoded : eyJhbGciOiJIUzI1NiIsInR5cCI6ICJKV1QifQ
Header Decoded : {"alg":"HS256","typ": "JWT"}
Payload Plain : {"sub":"TMJR00001","name":"Michael J. Remijan","exp":61475608800,"iss":"info@wstutorial.com","groups":["user","admin"]}
Payload Encoded : eyJzdWIiOiJUTUpSMDAwMDEiLCJuYW1lIjoiTWljaGFlbCBKLiBSZW1pamFuIiwiZXhwIjo2MTQ3NTYwODgwMCwiaXNzIjoiaW5mb0B3c3R1dG9yaWFsLmNvbSIsImdyb3VwcyI6WyJ1c2VyIiwiYWRtaW4iXX0
Payload Decoded : {"sub":"TMJR00001","name":"Michael J. Remijan","exp":61475608800,"iss":"info@wstutorial.com","groups":["user","admin"]}
Signature Algorithm : HmacSHA256
Signature Secret : thisismysupersecretkeywhichshouldonlybeontheauthenticationserver
Signaure Encoded :Xi6kVafrGX18FQIkNZuVJVBbmGbmEzI8cM-5G02S32A
Line #21 of Listing 1 starts the creation of the JWT header. This is the xxxxx part of a xxxxx.yyyyy.zzzzz JWT. As you can see, the code is simple. Use Base64.getUrlEncoder().withoutPadding() for encoding and Base64.getUrlDecoder() for decoding.
NOTE Make sure to use the
**.withoutPadding()**encoder. If not, trailing “=” characters will be added by the encoder to make the encoded string the necessary length. These trailing “=” are not allowed by the JWT specification so if you have them, other JWT decoders won’t be able to decode your JWT properly.
Line #44 of Listing 1 starts the creation of the JWT payload, typically user information, but in theory can be anything. This is the yyyyy part of a xxxxx.yyyyy.zzzzz JWT. As you can see, the code is simple. Use Base64.getUrlEncoder().withoutPadding() for encoding and Base64.getUrlDecoder() for decoding. See NOTE above about using the .withoutPadding() encoder.
Line #95 of Listing 1 starts the creation of the JWT signature. This is the zzzzz part of a xxxxx.yyyyy.zzzzz JWT. Listing 1 is an example of using the "alg":"HS256" aka HmacSHA256 algorithm. This is a single-key, symmetric algorithm which relies on a user-generated secret value as seen on line #96. This secret typically is stored outside the application in some kind of configuration system (file, git, database, etc.). Staring with line #102, you see how the MAC is used to finish the hash and the Base64.getUrlEncoder().withoutPadding() is used to encode the hash.
You’ll notice that after signing, there is no more code in Listing 1. Where’s the code showing how to verify a JWT? Well with a single-key, symmetric algorithm like HmacSHA256, the signing and verifying steps are exactly the same. To verify, the signature needs to be generated again and compared with the zzzzz part of a xxxxx.yyyyy.zzzzz JWT.
That’s it for JWT with Symmetric HMAC SHA256 Signature.
JWT with Asymmetric RSA SHA256 Signature
Listing 3 shows the code and Listing 4 shows example output.
Listing 3 - JWT with Asymmetric RSA SHA256 Signature
1. package org.thoth.jwt.main;
2.
3. import java.security.KeyPair;
4. import java.security.KeyPairGenerator;
5. import java.security.PrivateKey;
6. import java.security.PublicKey;
7. import java.security.Signature;
8. import java.util.Base64;
9.
10. /**
11. *
12. * @author Michael Remijan mjremijan@yahoo.com @mjremijan
13. */
14. public class SignatureWithAsymmetricalRsaSha256Main
15. {
16. public static void main(String[] args) throws Exception
17. {
18. // JWT HEADER
19. //
20. // This is the xxxxx of a JWT xxxxx.yyyyy.zzzzz
21. //
22. // Given the following JSON document, encode it
23. // using Java as defined in the JWT specifications
24. String header = "{\"alg\":\"RS256\",\"typ\": \"JWT\"}";
25. String headerEncoded
26. = Base64.getUrlEncoder()
27. .withoutPadding()
28. .encodeToString(
29. header.getBytes()
30. );
31. String headerDecoded
32. = new String(
33. Base64.getUrlDecoder().decode(headerEncoded)
34. );
35.
36. System.out.printf("Header Plain : %s%n", header);
37. System.out.printf("Header Encoded : %s%n", headerEncoded);
38. System.out.printf("Header Decoded : %s%n", headerDecoded);
39.
40.
41. // JWT PAYLOAD
42. //
43. // This is the yyyyy of a JWT xxxxx.yyyyy.zzzzz
44. //
45. // Given the following JSON document, encode it
46. // using Java as defined in the JWT specifications
47. String payload = "{\"sub\":\"TMJR00001\",\"name\":\"Michael J. Remijan\",\"exp\":61475608800,\"iss\":\"info@wstutorial.com\",\"groups\":[\"user\",\"admin\"]}";
48. String payloadEncoded
49. = Base64.getUrlEncoder()
50. .withoutPadding()
51. .encodeToString(
52. payload.getBytes()
53. );
54.
55. String payloadDecoded
56. = new String(
57. Base64.getUrlDecoder().decode(payloadEncoded)
58. );
59.
60. System.out.printf("%n");
61. System.out.printf("Payload Plain : %s%n", payload);
62. System.out.printf("Payload Encoded : %s%n", payloadEncoded);
63. System.out.printf("Payload Decoded : %s%n", payloadDecoded);
64.
65.
66. // SIGNATURE
67. //
68. // This is the zzzzz of a JWT xxxxx.yyyyy.zzzzz
69. //
70. // RSA (Rivest--Shamir--Adleman) is a public-key cryptosystem
71. // that is widely used for secure data transmission.
72. // In a public-key cryptosystem, the public key is used for
73. // encryption and the private key is used for decryption. The
74. // private key is also used for creating digital signatures
75. // of data and the public key is used for verifying the
76. // digital signature.
77. //
78. // A cryptographic hash function (CHF) is any function
79. // that can be used to map data of arbitrary size to
80. // a fixed-size number of n bits that has special
81. // properties desirable for a cryptographic application.
82. //
83. // For this example, the process will use the SHA256
84. // cryptographic hash function along with a public/private
85. // keypair and the RSA encryption algorithm to generate
86. // a signature for the JWT.
87. //
88. // The private key is used for creating the signature.
89. //
90. KeyPairGenerator keyGenerator = KeyPairGenerator.getInstance("RSA");
91. keyGenerator.initialize(1024);
92. KeyPair kp = keyGenerator.genKeyPair();
93. PublicKey publicKey = (PublicKey) kp.getPublic();
94. PrivateKey privateKey = (PrivateKey) kp.getPrivate();
95. String algorithm = "SHA256withRSA";
96. String signatureCreatedFromThisData
97. = headerEncoded + "." + payloadEncoded;
98.
99. Signature privateSignature
100. = Signature.getInstance(algorithm);
101. privateSignature.initSign(privateKey);
102.
103. System.out.printf("%n");
104. System.out.printf("Algorithm : %s%n", algorithm);
105. System.out.printf("Public Key : %s%n", Base64.getEncoder().encodeToString(publicKey.getEncoded()));
106. System.out.printf("Private Key : %s%n", Base64.getEncoder().encodeToString(privateKey.getEncoded()));
107.
108. privateSignature.update(signatureCreatedFromThisData.getBytes());
109. String signatureEncoded
110. = Base64.getUrlEncoder()
111. .withoutPadding()
112. .encodeToString(
113. privateSignature.sign()
114. );
115. System.out.printf("%n");
116. System.out.printf("Signaure Encoded : %s%n", signatureEncoded);
117.
118. // VERIFY
119. // This is the zzzzz of a JWT xxxxx.yyyyy.zzzzz
120. //
121. // The public key is used for verifying the signature.
122. //
123. // Becuase the public key is used for creating a signature,
124. // it safe to distribute the public key to Clients so
125. // that Clients can verify the JWT signature without
126. // having to ask the Authentication Server for verification
127. //
128.
129. Signature publicSignature = Signature.getInstance(algorithm);
130. publicSignature.initVerify(publicKey);
131. publicSignature.update(signatureCreatedFromThisData.getBytes());
132. boolean verified = publicSignature.verify(
133. Base64.getUrlDecoder().decode(signatureEncoded)
134. );
135. System.out.printf("Signature Verified (t/f) : %b%n", verified);
136. }
137. }
138.
Listing 4 - RSA Example Output
Header Plain : {"alg":"RS256","typ": "JWT"}
Header Encoded : eyJhbGciOiJSUzI1NiIsInR5cCI6ICJKV1QifQ
Header Decoded : {"alg":"RS256","typ": "JWT"}
Payload Plain : {"sub":"TMJR00001","name":"Michael J. Remijan","exp":61475608800,"iss":"info@wstutorial.com","groups":["user","admin"]}
Payload Encoded : eyJzdWIiOiJUTUpSMDAwMDEiLCJuYW1lIjoiTWljaGFlbCBKLiBSZW1pamFuIiwiZXhwIjo2MTQ3NTYwODgwMCwiaXNzIjoiaW5mb0B3c3R1dG9yaWFsLmNvbSIsImdyb3VwcyI6WyJ1c2VyIiwiYWRtaW4iXX0
Payload Decoded : {"sub":"TMJR00001","name":"Michael J. Remijan","exp":61475608800,"iss":"info@wstutorial.com","groups":["user","admin"]}
Algorithm : SHA256withRSA
Public Key : MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDRxw6Ncvsx0/kDYKwA6pLwn3hSbRdYFBOv1aiBomF7lPfOPfqaTgN2yPN6hErlLAP2d+94ig4uXv7MROXlsn8n7jdr2g5yo/kC92RJwALpffzBlWh29hEadiznWp2u0b0h++Cn4HJejfJpZOek6wurBL/7K2Y2TELOg8eg1uipEwIDAQAB
Private Key : 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
Signaure Encoded : HO4FLrLDt4ObECVWRiUGIGUimU1M70Y9aILT5op0UkV-kbEx8AqjCsLTh-Y1zOAisvFmuH5LYRw1wQyncQ5uEUWJYcoeldr-1_uFlpD2LqUy-QZfng8e6pxXOopL8Of_OcNEOqRijmI_dob8Gf0UnT7GQWpGTl32cIuuIFDeRHo
Signature Verified (t/f) : true
Line #24 of Listing 3 starts the creation of the JWT header. This is the xxxxx part of a xxxxx.yyyyy.zzzzz JWT. As you can see, the code is simple. Use Base64.getUrlEncoder().withoutPadding() for encoding and Base64.getUrlDecoder() for decoding.
NOTE Make sure to use the
**.withoutPadding()**encoder. If not, trailing “=” characters will be added by the encoder to make the encoded string the necessary length. These trailing “=” are not allowed by the JWT specification so if you have them, other JWT decoders won’t be able to decode your JWT properly.
Line #47 of Listing 3 starts the creation of the JWT payload, typically user information, but in theory can be anything. This is the yyyyy part of a xxxxx.yyyyy.zzzzz JWT. As you can see, the code is simple. Use Base64.getUrlEncoder().withoutPadding() for encoding and Base64.getUrlDecoder() for decoding. See NOTE above about using the .withoutPadding() encoder.
Line #90 of Listing 3 starts the creation of the JWT signature. This is the zzzzz part of a xxxxx.yyyyy.zzzzz JWT. Listing 3 is an example of using the "alg":"RS256" aka SHA256withRSA algorithm. This is a two-key, asymmetric algorithm which relies on a public/private keypair created on Line #92. A Signature object is created on line #99 and it is initialized with the private key. Staring with line #108, see how the Signature is used to create a signature and the Base64.getUrlEncoder().withoutPadding() is used to encode the signature.
NOTE The public/private keypair will need to be generated outside the application and kept in some kind of configuration store (file, git, database, etc.). This is an exercise left up to you.
Line #129 of Listing 3 starts the verify process. A Client may verify a JWT it receives from an Authentication server to guard against tampering while in transit. To verify a JWT created using an asymmetric RSA SHA256 signature, the Client will need the public key. This typically is not a problem since public keys are designed to be giving away. Line #132 demonstrates the call to .verify().
That’s it for JWT with Asymmetric RSA SHA256 Signature.
Summary
Most of this blog is the code. Review the code, top to bottom, it is not overly complicated. But now you know how to create and verify JWT values using both a Symmetric HMAC SHA256 Signature and an Asymmetric RSA SHA256 Signature.
Enjoy!
References
Introduction to JSON Web Tokens. (n.d.). JWT. https://jwt.io/introduction.
Debugger. (n.d.). JWT. https://jwt.io/#debugger-io
Alx. (2017, December). Create jwt in java using Public key rsa. https://wstutorial.com/misc/jwt-java-public-key-rsa.html
Base64 withoutPadding Encoding of string or byte array (Java8). (n.d.). MakeInJava Tutorials. https://makeinjava.com/base64-withoutpadding-encoding-string-byte-array-java8/
Poulsen, Søren. (n.d.). Calculate HMAC-Sha256 with Java. https://sorenpoulsen.com/calculate-hmac-sha256-with-java
Dommerholt, Niels. (2016, December 28). Example of RSA generation, sign, verify, encryption, decryption and keystores in Java. https://gist.github.com/nielsutrecht/855f3bef0cf559d8d23e94e2aecd4ede1
HMAC. (2023, January 1). Wikipedia. https://en.wikipedia.org/wiki/HMAC
Cryptographic hash function. (2023, January 10). Wikipedia. https://en.wikipedia.org/wiki/Cryptographic_hash_function
Hash function. (2023, February 1). Wikipedia. https://en.wikipedia.org/wiki/Hash_function