Resolving the javax.crypto.AEADBadTagException: Fixing Tag Mismatch in Java Cryptography

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Understanding the Problem

The specific error, Tag mismatch, occurs during the decryption phase when the provided key or initialization vector (IV) doesn’t match the values used during encryption. This can happen for various reasons, and identifying the root cause requires careful examination of the code.

Key Scenario

In the scenario presented, a Kotlin application was sending ciphertext to a Java application for decryption. The decryption failed with the Tag mismatch exception. This hints that either the key, IV, or the ciphertext itself is not consistent between the two platforms. Let’s break down the solution step-by-step.

Step-by-Step Solution

Here’s how to resolve the exception, ensuring the correct key and IV are used during the encryption and decryption processes.

1. Verify the Key Length

Encryption algorithms have specific requirements for key lengths. For AES, ensure that your keys are either 128, 192, or 256 bits long. In this case, 128 bits were needed, but there may have been discrepancies in how the key was generated and utilized.

Code Review

In the Kotlin app, the key is generated and trimmed to 16 bytes (128 bits) before being passed to the cipher:

[[See Video to Reveal this Text or Code Snippet]]

In the Java app, a similar approach was expected but the actual key used was not consistent:

[[See Video to Reveal this Text or Code Snippet]]

2. Ensure Consistent Key and IV

In cryptography, it is crucial that both the key and IV used in encryption are exactly the same during decryption. Validate that:

The same hashing method is utilized to derive the keys in both applications.

The initialization vector (IV) is set to the same value in both the Kotlin and Java apps.

Effective Approach

Double-check how the hashing function is implemented in both applications:

Make sure that the same parameters and order of operations are followed.

Confirm that the IV length is correct (12 bytes for AES-GCM) and is being reused appropriately.

3. Debugging the Ciphertext

Use debugging tools or simply add logging to capture the values of the ciphertext being sent from the Kotlin app to the Java app. Ensure that the full, unaltered ciphertext is being transmitted and used for decryption.

4. Common Pitfall - Key Trimming

In the example provided, the developer realized they were trimming the userKey after it was assigned to ukeyString. This could lead to unintended consequences, as the key that was actually used during encryption may not match what was utilized for decryption.

Solution Implementation

After making these checks, adjustments, and ensuring consistency between operations in both applications, the decryption should succeed without throwing the Tag mismatch exception.

Conclusion