Buena práctica de vector de inicialización AES

por mi pregunta Aes Encryption… Faltando una pieza importante, ahora he aprendido que mi suposición para crear un cifrado reversible en una cadena estaba un poco fuera de lugar. Ahora tengo

    public static byte[] EncryptString(string toEncrypt, byte[] encryptionKey)
    {
        var toEncryptBytes = Encoding.UTF8.GetBytes(toEncrypt);
        using (var provider = new AesCryptoServiceProvider())
        {
            provider.Key = encryptionKey;
            provider.Mode = CipherMode.CBC;
            provider.Padding = PaddingMode.PKCS7;
            using (var encryptor = provider.CreateEncryptor(provider.Key, provider.IV))
            {
                using (var ms = new MemoryStream())
                {
                    using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                    {
                        cs.Write(toEncryptBytes, 0, toEncryptBytes.Length);
                        cs.FlushFinalBlock();
                    }
                    return ms.ToArray();
                }
            }
        }
    }

y esto produce resultados consistentes; sin embargo, no podré descifrar sin conocer / establecer el vector de inicialización. Realmente no quiero pasar tres valores a este método (on para el IV), lo que me deja con la codificación rígida del IV o derivarlo de la clave. Me gustaría saber si esta es una buena práctica, o si hará que el valor cifrado sea vulnerable a los ataques de alguna manera … ¿O realmente estoy pensando demasiado en esto y debería simplemente codificar el IV?

ACTUALIZAR Por sugerencia de Iridium, probé algo como esto en su lugar:

    public static byte[] EncryptString(string toEncrypt, byte[] encryptionKey)
    {
        if (string.IsNullOrEmpty(toEncrypt)) throw new ArgumentException("toEncrypt");
        if (encryptionKey == null || encryptionKey.Length == 0) throw new ArgumentException("encryptionKey");
        var toEncryptBytes = Encoding.UTF8.GetBytes(toEncrypt);
        using (var provider = new AesCryptoServiceProvider())
        {
            provider.Key = encryptionKey;
            provider.Mode = CipherMode.CBC;
            provider.Padding = PaddingMode.PKCS7;
            using (var encryptor = provider.CreateEncryptor(provider.Key, provider.IV))
            {
                using (var ms = new MemoryStream())
                {
                    ms.Write(provider.IV, 0, 16);
                    using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                    {
                        cs.Write(toEncryptBytes, 0, toEncryptBytes.Length);
                        cs.FlushFinalBlock();
                    }
                    return ms.ToArray();
                }
            }
        }
    }

    public static string DecryptString(byte[] encryptedString, byte[] encryptionKey)
    {
        using (var provider = new AesCryptoServiceProvider())
        {
            provider.Key = encryptionKey;
            provider.Mode = CipherMode.CBC;
            provider.Padding = PaddingMode.PKCS7;
            using (var ms = new MemoryStream(encryptedString))
            {
                byte[] buffer;
                ms.Read(buffer, 0, 16);
                provider.IV = buffer;
                using (var decryptor = provider.CreateDecryptor(provider.Key, provider.IV))
                {
                    using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                    {
                        byte[] decrypted = new byte[encryptedString.Length];
                        var byteCount = cs.Read(decrypted, 0, encryptedString.Length);
                        return Encoding.UTF8.GetString(decrypted, 0, byteCount);
                    }
                }
            }
        }
    }

sin embargo, esto muestra algo extraño en mi prueba unitaria:

    [TestMethod]
    public void EncryptionClosedLoopTest()
    {
        var roundtrip = "This is the data I am encrypting.  There are many like it but this is my encryption.";
        var encrypted = Encryption.EncryptString(roundtrip, encryptionKey);
        var decrypted = Encryption.DecryptString(encrypted, encryptionKey);
        Assert.IsTrue(roundtrip == decrypted);
    }

mi texto descifrado aparece como «92ʪ F», hpv0 que estoy cifrando. Hay muchos como él, pero este es mi cifrado», lo que parece casi correcto pero, por supuesto, completamente incorrecto. Sin embargo, parece que estoy cerca. ¿Me falta un desplazamiento en el flujo de memoria?

Solución

Otras respuestas

Great input from folks. I took the combined answers from ankurpatel and Konstantin and cleaned it up and added some convenient method overrides. This works as of June 2019 in .NET Core 2.2.

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

private const int AesKeySize = 16;

public static void Main()
{
    // the data to encrypt
    var message = "Here is some data to encrypt!";

    // create KeySize character key
    var key = "g(KMDu(EEw63.*V`";

    // encrypt the string to a string
    var encrypted = AesEncrypt(message, key);

    // decrypt the string to a string.
    var decrypted = AesDecrypt(encrypted, key);

    // display the original data and the decrypted data
    Console.WriteLine($"Original:   text: {encrypted}");
    Console.WriteLine($"Round Trip: text: {decrypted}");
}

static string AesEncrypt(string data, string key)
{
    return AesEncrypt(data, Encoding.UTF8.GetBytes(key));
}

static string AesDecrypt(string data, string key)
{
    return AesDecrypt(data, Encoding.UTF8.GetBytes(key));
}

static string AesEncrypt(string data, byte[] key)
{
    return Convert.ToBase64String(AesEncrypt(Encoding.UTF8.GetBytes(data), key));
}

static string AesDecrypt(string data, byte[] key)
{
    return Encoding.UTF8.GetString(AesDecrypt(Convert.FromBase64String(data), key));
}

static byte[] AesEncrypt(byte[] data, byte[] key)
{
    if (data == null || data.Length <= 0)
    {
        throw new ArgumentNullException($"{nameof(data)} cannot be empty");
    }

    if (key == null || key.Length != AesKeySize)
    {
        throw new ArgumentException($"{nameof(key)} must be length of {AesKeySize}");
    }

    using (var aes = new AesCryptoServiceProvider
    {
        Key = key,
        Mode = CipherMode.CBC,
        Padding = PaddingMode.PKCS7
    })
    {
        aes.GenerateIV();
        var iv = aes.IV;
        using (var encrypter = aes.CreateEncryptor(aes.Key, iv))
        using (var cipherStream = new MemoryStream())
        {
            using (var tCryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
            using (var tBinaryWriter = new BinaryWriter(tCryptoStream))
            {
                // prepend IV to data
                cipherStream.Write(iv);
                tBinaryWriter.Write(data);
                tCryptoStream.FlushFinalBlock();
            }
            var cipherBytes = cipherStream.ToArray();

            return cipherBytes;
        }
    }
}

static byte[] AesDecrypt(byte[] data, byte[] key)
{
    if (data == null || data.Length <= 0)
    {
        throw new ArgumentNullException($"{nameof(data)} cannot be empty");
    }

    if (key == null || key.Length != AesKeySize)
    {
        throw new ArgumentException($"{nameof(key)} must be length of {AesKeySize}");
    }

    using (var aes = new AesCryptoServiceProvider
    {
        Key = key,
        Mode = CipherMode.CBC,
        Padding = PaddingMode.PKCS7
    })
    {
        // get first KeySize bytes of IV and use it to decrypt
        var iv = new byte[AesKeySize];
        Array.Copy(data, 0, iv, 0, iv.Length);

        using (var ms = new MemoryStream())
        {
            using (var cs = new CryptoStream(ms, aes.CreateDecryptor(aes.Key, iv), CryptoStreamMode.Write))
            using (var binaryWriter = new BinaryWriter(cs))
            {
                // decrypt cipher text from data, starting just past the IV
                binaryWriter.Write(
                    data,
                    iv.Length,
                    data.Length - iv.Length
                );
            }

            var dataBytes = ms.ToArray();

            return dataBytes;
        }
    }
}