DES/3DES are not considered a strong cipher for modern applications. Currently, NIST recommends the usage of AES block ciphers instead.

Risk

Broken or deprecated ciphers have typically known weekness. A attacker might be able to brute force the secret key use for the encryption. The confidentiality and integrity of the information encrypted is at risk.

Vulnerable Code

DES DESalg = DES.Create();

// Create a string to encrypt. 
byte[] encrypted;
ICryptoTransform encryptor = DESalg.CreateEncryptor(key, zeroIV);

// Create the streams used for encryption. 
using (MemoryStream msEncrypt = new MemoryStream())
{
    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
    {
        using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
        {
            //Write all data to the stream.
            swEncrypt.Write(Data);
        }
        encrypted = msEncrypt.ToArray();
        return encrypted;
    }
}

Solution

Using bouncy castle:

    
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;

public static readonly int BlockBitSize = 128;
public static readonly int KeyBitSize = 256;

public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] key)
{
    //User Error Checks
    if (key == null || key.Length != KeyBitSize / 8)
    throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");

    if (secretMessage == null || secretMessage.Length == 0)
    throw new ArgumentException("Secret Message Required!", "secretMessage");

    //Using random nonce large enough not to repeat
    var nonce = new byte[NonceBitSize / 8];
    Random.NextBytes(nonce, 0, nonce.Length);

    var cipher = new GcmBlockCipher(new AesFastEngine());
    var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, new byte[0]);
    cipher.Init(true, parameters);

    //Generate Cipher Text With Auth Tag
    var cipherText = new byte[cipher.GetOutputSize(secretMessage.Length)];
    var len = cipher.ProcessBytes(secretMessage, 0, secretMessage.Length, cipherText, 0);
    cipher.DoFinal(cipherText, len);

    //Assemble Message
    using (var combinedStream = new MemoryStream())
    {
        using (var binaryWriter = new BinaryWriter(combinedStream))
        {
            //Prepend Nonce
            binaryWriter.Write(nonce);
            //Write Cipher Text
            binaryWriter.Write(cipherText);
        }
        return combinedStream.ToArray();
    }
}

Custom implementation of Encrypt and HMAC

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

public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] cryptKey, byte[] authKey, byte[] nonSecretPayload = null)
{
    //User Error Checks
    if (cryptKey == null || cryptKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "cryptKey");

    if (authKey == null || authKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "authKey");

    if (secretMessage == null || secretMessage.Length < 1)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

    byte[] cipherText;
    byte[] iv;

    using (var aes = new AesManaged {
        KeySize = KeyBitSize,
        BlockSize = BlockBitSize,
        Mode = CipherMode.CBC,
        Padding = PaddingMode.PKCS7
    })
    {
        //Use random IV
        aes.GenerateIV();
        iv = aes.IV;
        using (var encrypter = aes.CreateEncryptor(cryptKey, iv))
        using (var cipherStream = new MemoryStream())
        {
            using (var cryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
            using (var binaryWriter = new BinaryWriter(cryptoStream))
            {
            //Encrypt Data
            binaryWriter.Write(secretMessage);
            }
            cipherText = cipherStream.ToArray();
        }
    }

    //Assemble encrypted message and add authentication
    using (var hmac = new HMACSHA256(authKey))
    using (var encryptedStream = new MemoryStream())
    {
        using (var binaryWriter = new BinaryWriter(encryptedStream))
        {
            //Prepend IV
            binaryWriter.Write(iv);
            //Write Ciphertext
            binaryWriter.Write(cipherText);
            binaryWriter.Flush();
            //Authenticate all data
            var tag = hmac.ComputeHash(encryptedStream.ToArray());
            //Postpend tag
            binaryWriter.Write(tag);
        }
        return encryptedStream.ToArray();
    }
}

References

NIST Withdraws Outdated Data Encryption Standard
CWE-326: Inadequate Encryption Strength StackOverflow: Authenticated encryption example