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C# AES 256 bits Encryption Library with Salt

, 9 May 2014 Public Domain
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A C# universal AES Encryption Library.

Core Encryption Code

using System.Security.Cryptography;
using System.IO;

Encryption

public byte[] AES_Encrypt(byte[] bytesToBeEncrypted, byte[] passwordBytes)
{
    byte[] encryptedBytes = null;

    // Set your salt here, change it to meet your flavor:
    // The salt bytes must be at least 8 bytes.
    byte[] saltBytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };

    using (MemoryStream ms = new MemoryStream())
    {
        using (RijndaelManaged AES = new RijndaelManaged())
        {
            AES.KeySize = 256;
            AES.BlockSize = 128;

            var key = new Rfc2898DeriveBytes(passwordBytes, saltBytes, 1000);
            AES.Key = key.GetBytes(AES.KeySize / 8);
            AES.IV = key.GetBytes(AES.BlockSize / 8);

            AES.Mode = CipherMode.CBC;

            using (var cs = new CryptoStream(ms, AES.CreateEncryptor(), CryptoStreamMode.Write))
            {
                cs.Write(bytesToBeEncrypted, 0, bytesToBeEncrypted.Length);
                cs.Close();
            }
            encryptedBytes = ms.ToArray();
        }
    }

    return encryptedBytes;
}

Decryption

public byte[] AES_Decrypt(byte[] bytesToBeDecrypted, byte[] passwordBytes)
{
    byte[] decryptedBytes = null;

    // Set your salt here, change it to meet your flavor:
    // The salt bytes must be at least 8 bytes.
    byte[] saltBytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };

    using (MemoryStream ms = new MemoryStream())
    {
        using (RijndaelManaged AES = new RijndaelManaged())
        {
            AES.KeySize = 256;
            AES.BlockSize = 128;

            var key = new Rfc2898DeriveBytes(passwordBytes, saltBytes, 1000);
            AES.Key = key.GetBytes(AES.KeySize / 8);
            AES.IV = key.GetBytes(AES.BlockSize / 8);

            AES.Mode = CipherMode.CBC;

            using (var cs = new CryptoStream(ms, AES.CreateDecryptor(), CryptoStreamMode.Write))
            {
                cs.Write(bytesToBeDecrypted, 0, bytesToBeDecrypted.Length);
                cs.Close();
            }
            decryptedBytes = ms.ToArray();
        }
    }

    return decryptedBytes;
}

Example of Encrypting String

Encrypt String

public string EncryptText(string input, string password)
{
    // Get the bytes of the string
    byte[] bytesToBeEncrypted = Encoding.UTF8.GetBytes(input);
    byte[] passwordBytes = Encoding.UTF8.GetBytes(password);

    // Hash the password with SHA256
    passwordBytes = SHA256.Create().ComputeHash(passwordBytes);

    byte[] bytesEncrypted = AES_Encrypt(bytesToBeEncrypted, passwordBytes);

    string result = Convert.ToBase64String(bytesEncrypted);

    return result;
}

Decrypt String

public string DecryptText(string input, string password)
{
    // Get the bytes of the string
    byte[] bytesToBeDecrypted = Convert.FromBase64String(input);
    byte[] passwordBytes = Encoding.UTF8.GetBytes(password);
    passwordBytes = SHA256.Create().ComputeHash(passwordBytes);

    byte[] bytesDecrypted = AES_Decrypt(bytesToBeDecrypted, passwordBytes);

    string result = Encoding.UTF8.GetString(bytesDecrypted);

    return result;
}

You'll notice that the encrypted string is stored in base64 encoded mode. For those who is not familiar with base64 encoding, you may want to read Base What? A Practical Introduction to Base Encoding[^]

Example of Encrypting File

Encrypt File

public void EncryptFile()
{
    string file = "C:\\SampleFile.DLL";
    string password = "abcd1234";

    byte[] bytesToBeEncrypted = File.ReadAllBytes(file);
    byte[] passwordBytes = Encoding.UTF8.GetBytes(password);

    // Hash the password with SHA256
    passwordBytes = SHA256.Create().ComputeHash(passwordBytes);

    byte[] bytesEncrypted = AES_Encrypt(bytesToBeEncrypted, passwordBytes);

    string fileEncrypted = "C:\\SampleFileEncrypted.DLL";

    File.WriteAllBytes(fileEncrypted, bytesEncrypted);
}

Decrypt File

public void DecryptFile()
{
    string fileEncrypted = "C:\\SampleFileEncrypted.DLL";
    string password = "abcd1234";

    byte[] bytesToBeDecrypted = File.ReadAllBytes(fileEncrypted);
    byte[] passwordBytes = Encoding.UTF8.GetBytes(password);
    passwordBytes = SHA256.Create().ComputeHash(passwordBytes);

    byte[] bytesDecrypted = AES_Decrypt(bytesToBeDecrypted, passwordBytes);

    string file = "C:\\SampleFile.DLL";
    File.WriteAllBytes(file, bytesDecrypted);
}

Getting Randomized Encryption Result with Salt

If we encrypt the same context (i.e. string of "Hello World") for 10 times, the encrypted results will be the same. What if we want the results different from each time it is encrypted?

What I do is appending a random salt bytes in front of the original bytes before encryption, and remove it after decryption.

Example of Appending Randomized Salt Before Encrypting a String

public static string EncryptString(string text, string password)
{
    byte[] baPwd = Encoding.UTF8.GetBytes(password);

    // Hash the password with SHA256
    byte[] baPwdHash = SHA256Managed.Create().ComputeHash(baPwd);

    byte[] baText = Encoding.UTF8.GetBytes(text);

    byte[] baSalt = GetRandomBytes();
    byte[] baEncrypted = new byte[baSalt.Length + baText.Length];

    // Combine Salt + Text
    for (int i = 0; i < baSalt.Length; i++)
        baEncrypted[i] = baSalt[i];
    for (int i = 0; i < baText.Length; i++)
        baEncrypted[i + baSalt.Length] = baText[i];

    baEncrypted = AES_Encrypt(baEncrypted, baPwdHash);

    string result = Convert.ToBase64String(baEncrypted);
    return result;
}

Example of Removing the Salt after Decryption

public static string DecryptString(string text, string password)
{
    byte[] baPwd = Encoding.UTF8.GetBytes(password);

    // Hash the password with SHA256
    byte[] baPwdHash = SHA256Managed.Create().ComputeHash(baPwd);

    byte[] baText = Convert.FromBase64String(text);

    byte[] baDecrypted = AES_Decrypt(baText, baPwdHash);

    // Remove salt
    int saltLength = GetSaltLength();
    byte[] baResult = new byte[baDecrypted.Length - saltLength];
    for (int i = 0; i < baResult.Length; i++)
        baResult[i] = baDecrypted[i + saltLength];

    string result = Encoding.UTF8.GetString(baResult);
    return result;
}

Code for getting random bytes

public static byte[] GetRandomBytes()
{
    int saltLength = GetSaltLength();
    byte[] ba = new byte[saltLength];
    RNGCryptoServiceProvider.Create().GetBytes(ba);
    return ba;
}

public static int GetSaltLength()
{
    return 8;
}

Another way of getting random bytes is by using System.Random. However, System.Random is strongly not recommended to be used in cryptography. This is because System.Random is not a true random. The changes of the value are following a specific sequence and pattern and it is predictable. RNGCrypto is truly randomize and the generated values does not follow a pattern and it is unpredictable.

About Storing Password in System.String

If you are working this on a desktop pc application, you are recommended not to store the password in plain string. Strings cannot be manually disposed. It will be disposed by Garbage Collector, however, it is hard to tell how long will the string continue to stay in memory before it is completely dispose. At the time it remains in memory, it is easily retrievable. One of the suggested method is by capturing the password key strokes and store it into byte array (or list of bytes) and wipe off the byte array immediately after it is used (for example, fill the byte array with 0 (zero), etc).

Another option is by using System.Security.SecureString. System.Security.SecureString can be destroyed manually at any time. The value stored within System.Security.SecureString is encrypted.

Example of Using SecureString

using System.Security;
using System.Runtime.InteropServices; 

Storing text into SecureString:

SecureString secureString = new SecureString();
secureString.AppendChar('h');
secureString.AppendChar('e');
secureString.AppendChar('l');
secureString.AppendChar('l');
secureString.AppendChar('o'); 

Retrieving data from SecureString

byte[] secureStringBytes = null;
// Convert System.SecureString to Pointer
IntPtr unmanagedBytes = Marshal.SecureStringToGlobalAllocAnsi(secureString);
try
{
    unsafe
    {
        byte* byteArray = (byte*)unmanagedBytes.ToPointer();
        // Find the end of the string
        byte* pEnd = byteArray;
        while (*pEnd++ != 0) { }
        // Length is effectively the difference here (note we're 1 past end) 
        int length = (int)((pEnd - byteArray) - 1);
        secureStringBytes = new byte[length];
        for (int i = 0; i < length; ++i)
        {
            // Work with data in byte array as necessary, via pointers, here
            secureStringBytes[i] = *(byteArray + i);
        }
    }
}
finally
{
    // This will completely remove the data from memory
    Marshal.ZeroFreeGlobalAllocAnsi(unmanagedBytes);
} 
return secureStringBytes; 

Retriving data from SecureString involves executing unsafe code (for handling unmanaged bytes). Therefore you have to allow your project to be built with unsafe code allowed. This option is available at your project's Properties settings.

Or else you will receive error message:

Unsafe code may only appear if compiling with /unsafe 

License

This article, along with any associated source code and files, is licensed under A Public Domain dedication

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About the Author

adriancs
Software Developer
Malaysia Malaysia
Writing programs is an art.

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QuestionMessage Removed Pin
Ritik7562-Feb-16 16:28
memberRitik7562-Feb-16 16:28 
QuestionHow to implement file encryption and decryption? Pin
Member 1227869624-Jan-16 7:26
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AnswerRe: How to implement file encryption and decryption? Pin
adriancs24-Jan-16 15:17
professionaladriancs24-Jan-16 15:17 
QuestionHow to remove randomized salt? Pin
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memberMember 1227869622-Jan-16 8:04 
Questionabout salt Pin
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QuestionDecrypting SecureString Pin
chriha16-Oct-15 6:36
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QuestionWhy OFB mode not working in visual studio 2015 Pin
vishalgpt4-Oct-15 20:41
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QuestionTwice SHA256 Pin
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AnswerRe: Twice SHA256 Pin
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AnswerRe: Twice SHA256 Pin
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QuestionAES 128 ECB Pin
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AnswerRe: AES 128 ECB Pin
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Questioncs.Close() error Pin
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AnswerRe: cs.Close() error Pin
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AnswerRe: cs.Close() error Pin
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BugRe: cs.Close() error Pin
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AnswerRe: cs.Close() error Pin
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AnswerRe: cs.Close() error Pin
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GeneralRe: cs.Close() error Pin
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AnswerRe: cs.Close() error Pin
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GeneralMy vote of 5 Pin
Dirkster9910-Jun-15 14:08
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SuggestionNice Code But It Need More Optimizing Pin
Ahmed Aljaff25-Apr-15 1:28
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GeneralRe: Nice Code But It Need More Optimizing Pin
adriancs4-May-15 23:30
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GeneralRe: Nice Code But It Need More Optimizing Pin
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