## 1. Introduction

Encryption is a cornerstone of modern data security, safeguarding information from unauthorized access. Among the many encryption standards, AES (Advanced Encryption Standard) stands out due to its balance of efficiency, simplicity, and robust security. It operates on blocks of data with fixed sizes of 128 bits and supports key sizes of 128, 192, or 256 bits, making it versatile for various security needs.

In this tutorial, we’ll cover the basics of AES, set up a Kotlin project for AES encryption and decryption, and finally, demonstrate how to use these implementations in a simple Kotlin application.

## 2. Understanding AES

AES is celebrated for its efficiency across both software and hardware implementations. Its design simplicity, coupled with robust security features, has propelled its widespread adoption in numerous security protocols and systems globally.

** As a symmetric encryption algorithm, AES uses the same key for both encryption and decryption**. This facilitates secure data exchange in an array of applications.

Let’s look at some **key features of AES**:

- Block Cipher: Operates on a fixed block size of 128 bits
- Symmetric Key: Uses the same key for encryption and decryption
- Security: Offers strong security against various cryptographic attacks
- Flexibility: Supports key sizes of 128, 192, or 256 bits

## 3. Implementing AES Encryption and Decryption

Let’s understand how to generate an AES Key and use it either to encrypt or decrypt data.

### 3.1. Generating an AES Key

First, we need to generate a symmetric AES key. We can use the *KeyGenerator* class from *javax.crypto* for this purpose:

```
fun generateAESKey(keySize: Int = 256): SecretKey {
val keyGenerator = KeyGenerator.getInstance("AES")
keyGenerator.init(keySize)
return keyGenerator.generateKey()
}
```

Notice how **we can tweak the ***keySize* parameter to fit our needs if necessary.

### 3.2. AES Encryption

In AES encryption/decryption, the “AES/CBC/PKCS5Padding” string specifies the algorithm, mode of operation, and padding scheme. CBC (Cipher Block Chaining) mode uses an Initialization Vector (IV) for added security, while PKCS5Padding ensures the last block is properly padded.

Let’s implement our encryption logic using the *Cipher* class:

```
fun aesEncrypt(data: ByteArray, secretKey: SecretKey): ByteArray {
val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
val ivParameterSpec = IvParameterSpec(ByteArray(16)) // Use a secure IV in production
cipher.init(Cipher.ENCRYPT_MODE, secretKey, ivParameterSpec)
return cipher.doFinal(data)
}
```

A secure Initialization Vector (IV) should be randomly generated for each encryption operation to enhance security. It’s also **important that we store this IV, as it needs to be shared with the decryption logic**. Lastly, *doFinal()* executes the encryption or decryption operation.

### 3.3. AES Decryption

Following encryption, let’s implement the decryption logic:

```
fun aesDecrypt(encryptedData: ByteArray, secretKey: SecretKey): ByteArray {
val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
val ivParameterSpec = IvParameterSpec(ByteArray(16)) // Use the same IV as used in encryption
cipher.init(Cipher.DECRYPT_MODE, secretKey, ivParameterSpec)
return cipher.doFinal(encryptedData)
}
```

This function illustrates how to reverse the encryption process to retrieve the original data. By initializing the *Cipher* object in *DECRYPT_MODE* with the same IV used for encryption, we ensure the integrity of the decryption process, enabling the secure retrieval of original information from the encrypted bytes.

**This step is crucial for validating the symmetric nature of AES, where the same key is instrumental in both locking and unlocking the encrypted data**.

## 4. Unit Testing AES Encryption and Decryption

We’ll write a JUnit test case that encrypts and then decrypts a piece of text, asserting that the decrypted text matches the original:

```
@Test
fun `Given text when encrypted and decrypted should return original text`() {
val originalText = "Hello Kotlin AES Encryption!"
val secretKey = generateAESKey(256)
val encryptedData = aesEncrypt(originalText.toByteArray(), secretKey)
val decryptedData = aesDecrypt(encryptedData, secretKey)
val decryptedText = String(decryptedData)
assertEquals(originalText, decryptedText, "The decrypted text does not match the original")
}
```

Running this test will validate the AES encryption and decryption process. **This test also showcases the symmetric nature of AES**, meaning that the same key used to encrypt a message can also decrypt it.

## 5. Conclusion

In this article, we explored AES encryption and decryption in Kotlin. We covered the basics of AES, how to set up a Kotlin project for cryptographic operations, and implemented AES encryption and decryption. We also demonstrated these implementations through a simple example of encrypting and decrypting text in a JUnit test.

When implementing AES in production applications, it’s crucial to manage keys securely, use secure IVs, and adhere to best practices for cryptographic security. As always, the code used in this article is available over on GitHub.