Data encryption basics in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of AES encryption snippet

What is the output of this Unity C# code snippet that encrypts a string using AES and prints the Base64 result?

Unity

using System;
using System.Security.Cryptography;
using System.Text;

public class EncryptTest {
    public static void Main() {
        string original = "hello";
        byte[] key = Encoding.UTF8.GetBytes("1234567890123456");
        byte[] iv = Encoding.UTF8.GetBytes("abcdefghijklmnop");

        using (Aes aes = Aes.Create()) {
            aes.Key = key;
            aes.IV = iv;
            ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

            byte[] originalBytes = Encoding.UTF8.GetBytes(original);
            byte[] encrypted = encryptor.TransformFinalBlock(originalBytes, 0, originalBytes.Length);

            Console.WriteLine(Convert.ToBase64String(encrypted));
        }
    }
}

A U2FsdGVkX1+q7Q9r6XzQ7v6v6Zq5zqX1w==

B q7Q9r6XzQ7v6v6Zq5zqX1w==

C SyntaxError: missing semicolon

D hello

💡 Hint

Look at how AES encryption transforms the input bytes and outputs a Base64 string.

The code encrypts the string "hello" with AES using the given key and IV, then prints the encrypted bytes as Base64. Option B matches the expected Base64 output. Option B is a common OpenSSL prefix but not produced here. Option B is invalid syntax. Option B is the original string, not encrypted.

🧠 Conceptual

intermediate

Understanding symmetric vs asymmetric encryption

Which statement correctly describes the difference between symmetric and asymmetric encryption?

A Symmetric encryption uses one key for both encrypting and decrypting, while asymmetric uses a pair of keys.

B Symmetric encryption uses two keys, asymmetric uses one key only.

C Symmetric encryption is slower than asymmetric encryption.

D Asymmetric encryption cannot be used for data encryption, only for hashing.

💡 Hint

Think about how keys are used in each encryption type.

Symmetric encryption uses the same secret key to encrypt and decrypt data. Asymmetric encryption uses a public key to encrypt and a private key to decrypt. Option A correctly states this difference. Option A reverses the key usage. Option A is generally false; symmetric is faster. Option A is incorrect; asymmetric encryption is used for encrypting data.

🔧 Debug

advanced

Identify the error in this encryption code

What error will this Unity C# code produce when run?

Unity

using System;
using System.Security.Cryptography;
using System.Text;

public class EncryptDebug {
    public static void Main() {
        string text = "data";
        byte[] key = Encoding.UTF8.GetBytes("shortkey");
        byte[] iv = Encoding.UTF8.GetBytes("shortiv");

        using (Aes aes = Aes.Create()) {
            aes.Key = key;
            aes.IV = iv;
            ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

            byte[] bytes = Encoding.UTF8.GetBytes(text);
            byte[] encrypted = encryptor.TransformFinalBlock(bytes, 0, bytes.Length);

            Console.WriteLine(Convert.ToBase64String(encrypted));
        }
    }
}

A Output: "ZGF0YQ=="

B NullReferenceException because aes is null

C CryptographicException due to invalid key size

D IndexOutOfRangeException during TransformFinalBlock

💡 Hint

Check the length of the key and IV for AES requirements.

AES requires keys of specific lengths (128, 192, or 256 bits). The provided key and IV are too short (8 bytes each). This causes a CryptographicException about invalid key size. Option C is incorrect because aes is properly created. Option C is Base64 of "data" (no encryption). Option C is not triggered here.

📝 Syntax

advanced

Which code snippet correctly decrypts AES data?

Which of the following Unity C# code snippets correctly decrypts AES encrypted data stored in a byte array named encryptedData?

A

using (Aes aes = Aes.Create()) {
    aes.Key = key;
    aes.IV = iv;
    ICryptoTransform decryptor = aes.CreateDecryptor(aes.IV, aes.Key);
    byte[] decrypted = decryptor.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
    string result = Encoding.UTF8.GetString(decrypted);
    Console.WriteLine(result);
}

B

using (Aes aes = Aes.Create()) {
    aes.Key = key;
    aes.IV = iv;
    ICryptoTransform decryptor = aes.CreateEncryptor(aes.Key, aes.IV);
    byte[] decrypted = decryptor.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
    string result = Encoding.UTF8.GetString(decrypted);
    Console.WriteLine(result);
}

C

using (Aes aes = Aes.Create()) {
    aes.Key = key;
    aes.IV = iv;
    ICryptoTransform decryptor = aes.CreateDecryptor();
    byte[] decrypted = decryptor.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
    string result = Encoding.UTF8.GetString(decrypted);
    Console.WriteLine(result);
}

D

using (Aes aes = Aes.Create()) {
    aes.Key = key;
    aes.IV = iv;
    ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
    byte[] decrypted = decryptor.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
    string result = Encoding.UTF8.GetString(decrypted);
    Console.WriteLine(result);
}

💡 Hint

Check the method used to create the decryptor and the order of parameters.

Option D correctly uses CreateDecryptor with key and IV in correct order. Option D incorrectly uses CreateEncryptor for decryption. Option D calls CreateDecryptor without parameters, which uses default key and IV (likely incorrect). Option D swaps key and IV parameters, causing wrong decryption or error.

🚀 Application

expert

Calculate the length of encrypted data

Given a plaintext string of length 30 bytes encrypted with AES in CBC mode with PKCS7 padding, what is the length in bytes of the encrypted output?

A 32 bytes

B 30 bytes

C 48 bytes

D 16 bytes

💡 Hint

AES block size is 16 bytes. PKCS7 padding adds bytes to fill the last block.

AES block size is 16 bytes. The plaintext is 30 bytes, which is 1 full block (16 bytes) plus 14 bytes. PKCS7 padding adds 2 bytes to fill the last block to 16 bytes. So total encrypted length is 32 bytes (2 blocks). Option A is correct. Option A ignores padding. Option A is too large. Option A is just one block size.

Practice

1. What is the main purpose of encryption in Unity programming?

easy

A. To create graphics effects

B. To convert readable data into a secret code to protect it

C. To make the game run faster

D. To store data in plain text files

Solution

1. Step 1: Understand what encryption does

   Encryption changes readable data into a secret code so others cannot understand it easily.

2. Step 2: Identify the purpose in Unity

   In Unity, encryption is used to protect sensitive information like passwords or personal data.

3. Final Answer:

   To convert readable data into a secret code to protect it -> Option B

4. Quick Check:

   Encryption = data protection [OK]

Hint: Encryption means hiding data by coding it [OK]

Common Mistakes:

• Thinking encryption makes the game faster
• Confusing encryption with graphics or speed
• Believing encryption stores data as plain text

2. Which of the following is the correct way to declare an AES encryption object in Unity C#?

easy

A. Aes aes = Create.Aes();

B. aes = new AES();

C. Aes aes = Aes.Create();

D. Aes aes = new Aes();

Solution

1. Step 1: Recall AES object creation in C#

   In C#, AES encryption objects are created using the static method Aes.Create().

2. Step 2: Check syntax correctness

   Aes aes = Aes.Create(); uses correct syntax: Aes aes = Aes.Create(); Options A, B, and C use incorrect constructors or methods.

3. Final Answer:

   Aes aes = Aes.Create(); -> Option C

4. Quick Check:

   Aes.Create() creates AES object [OK]

Hint: Use Aes.Create() to make AES object [OK]

Common Mistakes:

• Trying to use 'new AES()' which is invalid
• Using wrong capitalization like 'AES' instead of 'Aes'
• Calling non-existent methods like Create.Aes()

3. Consider this Unity C# code snippet for AES encryption:

using System.Security.Cryptography;
using System.Text;

Aes aes = Aes.Create();
aes.Key = Encoding.UTF8.GetBytes("1234567890123456");
aes.IV = Encoding.UTF8.GetBytes("abcdefghijklmnop");

byte[] encrypted;
using (var encryptor = aes.CreateEncryptor())
{
    byte[] input = Encoding.UTF8.GetBytes("HelloWorld");
    encrypted = encryptor.TransformFinalBlock(input, 0, input.Length);
}

string result = Convert.ToBase64String(encrypted);
Console.WriteLine(result);

What will be the output type of result?

medium

A. An error because of wrong key size

B. The original string 'HelloWorld'

C. A byte array of the encrypted data

D. A base64 encoded string representing encrypted data

Solution

1. Step 1: Understand encryption output

   The encrypted data is a byte array, which is then converted to a base64 string for easy display.

2. Step 2: Analyze the code's final output

   The code uses Convert.ToBase64String(encrypted) to get a string representation of encrypted bytes, so result is a base64 string.

3. Final Answer:

   A base64 encoded string representing encrypted data -> Option D

4. Quick Check:

   Encrypted bytes -> base64 string [OK]

Hint: Encrypted bytes are shown as base64 strings [OK]

Common Mistakes:

• Thinking output is original text
• Expecting byte array instead of string
• Assuming key size error without checking length

4. This Unity C# code tries to decrypt data but throws an exception:

byte[] encryptedData = ...;
Aes aes = Aes.Create();
aes.Key = Encoding.UTF8.GetBytes("shortkey");
aes.IV = Encoding.UTF8.GetBytes("shortiv");

using (var decryptor = aes.CreateDecryptor())
{
    byte[] decrypted = decryptor.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
}

What is the main problem causing the error?

medium

A. The key and IV sizes are too short for AES

B. The encryptedData array is empty

C. CreateDecryptor() cannot be used without a password

D. TransformFinalBlock requires a string input

Solution

1. Step 1: Check key and IV sizes for AES

   AES requires keys and IVs of specific lengths (e.g., 16 bytes for AES-128). The given strings "shortkey" and "shortiv" are too short.

2. Step 2: Understand error cause

   Using keys or IVs with incorrect length causes exceptions during encryption/decryption.

3. Final Answer:

   The key and IV sizes are too short for AES -> Option A

4. Quick Check:

   Key/IV length must match AES requirements [OK]

Hint: AES keys and IVs must be exact length [OK]

Common Mistakes:

• Assuming encryptedData is empty without checking
• Thinking CreateDecryptor needs password parameter
• Believing TransformFinalBlock accepts strings directly

5. You want to securely store user passwords in Unity using AES encryption. Which approach is best practice?

hard

A. Encrypt passwords with AES and store the encrypted string with a unique IV per user

B. Store passwords as plain text for easy retrieval

C. Use a fixed key and IV for all users to simplify encryption

D. Encrypt passwords with AES but reuse the same IV for all users

Solution

1. Step 1: Understand secure password storage

   Passwords should never be stored as plain text. Encryption with AES is good, but IV must be unique per encryption to prevent attacks.

2. Step 2: Evaluate options for key and IV usage

   Using a unique IV per user with AES encryption improves security. Reusing IV or fixed keys weakens protection.

3. Final Answer:

   Encrypt passwords with AES and store the encrypted string with a unique IV per user -> Option A

4. Quick Check:

   Unique IV + AES = secure password storage [OK]

Hint: Always use unique IVs with AES encryption [OK]

Common Mistakes:

• Storing passwords as plain text
• Reusing IVs for all encryptions
• Using fixed keys for all users