Refactor JavaScript decryption into C# - AES 128

Question

I need to take working JavaScript that decrypts a message and convert it into C#. I have the decryption information (the "decrypt" variable below) which looks like: AES-128:<salt>:<iv>:<key>. Here's the JavaScript:

function decodeString(message, decrypt) {
    var parts = decrypt.split(':', 4);
    var salt = CryptoJS.enc.Hex.parse(parts[1]);
    var iv = CryptoJS.enc.Hex.parse(parts[2]);
    var key = CryptoJS.PBKDF2(parts[3], salt, { keySize: 128/32, iterations: 100 });
    try {
        message = message.replace(/\s+/g, '');
        var d = CryptoJS.AES.decrypt(message, key, { iv: iv, mode: CryptoJS.mode.CBC, padding: CryptoJS.pad.Pkcs7 });
        message = d.toString(CryptoJS.enc.Utf8);
    } catch (e) {
        console.error("Encountered a problem decrypting and encrypted page!");
        console.log(e);
    }
    
    return(message);       
}

Here's what I have in C#, but I get an exception on the CreateDecryptor call.

using System.Security.Cryptography;

private string DecodeString(string message, string decrypt)
{
    string[] parts = decrypt.ToString().Split(':');
    byte[] salt = Encoding.UTF8.GetBytes(parts[1]);
    byte[] iv = Encoding.UTF8.GetBytes(parts[2]);
    var pbkdf2 = new Rfc2898DeriveBytes(parts[3], salt, 100);
    int numKeyBytes = 128;  // Not sure this is correct
    byte[] key = pbkdf2.GetBytes(numKeyBytes);

    string plainText = null;
    using (AesManaged aes = new AesManaged())
    {
        aes.KeySize = numKeyBytes;  // Not sure if this is correct
        aes.BlockSize = 128;    // Defaults to 128, but not sure this is correct
        aes.Mode = CipherMode.CBC;
        aes.Padding = PaddingMode.PKCS7;

        try
        {
            // The below line has the following exception:
            //    The specified key is not a valid size for this algorithm.
            //    Parameter name: key
            using (var decrypter = aes.CreateDecryptor(key, iv))
            using (var plainTextStream = new MemoryStream())
            {
                using (var decrypterStream = new CryptoStream(plainTextStream, decrypter, CryptoStreamMode.Write))
                using (var binaryWriter = new BinaryWriter(decrypterStream))
                {
                    string encryptedText = Regex.Replace(message, @"\s+", "");
                    binaryWriter.Write(encryptedText);
                }
                byte[] plainTextBytes = plainTextStream.ToArray();
                plainText = Encoding.UTF8.GetString(plainTextBytes);
            }
        }
        catch (Exception ex)
        {
            log.Error("Unable to decrypt message.", ex);
        }

        return plainText;
    }
}

Any suggestions would be appreciated!

Topaco · Accepted Answer · 2021-03-22T21:01:30.960

In the C# code there are the following issues:

Salt and IV must be hex decoded (and not UTF8 encoded).
numKeyBytes specifies the key size in bytes and is therefore 16 (and not 128) for AES-128.
aes.KeySize specifies the key size in bits and is therefore numKeyBytes * 8 (and not numKeyBytes), but can alternatively be omitted.
For aes.BlockSize, aes.Mode and aes.Padding the default values are used (128, CBC, PKCS7), so they do not need to be specified explicitly.
encryptedText must be Base64 decoded.

A possible implementation is:

private string Decrypt(string message, string decrypt)
{
    string[] parts = decrypt.ToString().Split(':');
    byte[] salt = StringToByteArray(parts[1]);                                                              // Hex decode salt                                            
    byte[] iv = StringToByteArray(parts[2]);                                                                // Hex dedoce IV                                           
    var pbkdf2 = new Rfc2898DeriveBytes(parts[3], salt, 100);
    int numKeyBytes = 16;                                                                                   // AES-128 key size in bytes: 16
    byte[] key = pbkdf2.GetBytes(numKeyBytes);

    string plainText = null;
    using (AesManaged aes = new AesManaged())
    {
        aes.KeySize = 192;
        try
        {
            string encryptedText = Regex.Replace(message, @"\s+", "");
            using (var decrypter = aes.CreateDecryptor(key, iv))
            using (MemoryStream msDecrypt = new MemoryStream(Convert.FromBase64String(encryptedText)))      // Base64 decode ciphertext
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decrypter, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                    {
                        plainText = srDecrypt.ReadToEnd();
                    }
                }
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine("Unable to decrypt message.", ex);
        }

        return plainText;
    }
}

where StringToByteArray() is from here.

I changed the Stream part to be analogous to this example, but the original implementation works as well, so this change is optional.

Test: Both codes return for the data

message = "YhyXEjjNAnRUUONwVzlha59tRoWkeEwTkOtSKOicRd/iBKkGgIp+DeWmvEXxAU53";
decrypt = "AES-128:30313233343536373839303132333435:35343332313039383736353433323130:my passphrase";

the plaintext:

The quick brown fox jumps over the lazy dog

Simply fabulous! Worked like a charm. Thank you so much for taking the time to create that response. I have learned that even though I have 30 years in software, I have plenty to learn about cryptography (and, no doubt, other areas as well). — Kirk Liemohn, Mar 23 '21 at 15:38

Refactor JavaScript decryption into C# - AES 128

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