.NET AES/Rijndael — inconsistent decryption when reusing decryptor

Question

I've created a class for encrypting and decrypting using AES.

public class AesEncryptionProvider {
    #region Fields

    // Encryption key
    private static readonly byte[] s_key = new byte[32] {
        // Omitted...
    };

    // Initialization vector
    private static readonly byte[] s_iv = new byte[16] {
        // Omitted...
    };

    private AesCryptoServiceProvider m_provider;
    private ICryptoTransform m_encryptor;
    private ICryptoTransform m_decryptor;

    #endregion

    #region Constructors

    private AesEncryptionProvider () {
        m_provider = new AesCryptoServiceProvider();
        m_encryptor = m_provider.CreateEncryptor(s_key, s_iv);
        m_decryptor = m_provider.CreateDecryptor(s_key, s_iv);
    }

    static AesEncryptionProvider () {
        Instance = new AesEncryptionProvider();
    }

    #endregion

    #region Properties

    public static AesEncryptionProvider Instance { get; private set; }

    #endregion

    #region Methods

    public string Encrypt (string value) {
        if (string.IsNullOrEmpty(value)) {
            throw new ArgumentException("Value required.");
        }

        return Convert.ToBase64String(
            Transform(
                Encoding.UTF8.GetBytes(value),
                m_encryptor));
    }

    public string Decrypt (string value) {
        if (string.IsNullOrEmpty(value)) {
            throw new ArgumentException("Value required.");
        }

        return Encoding.UTF8.GetString(
            Transform(
                Convert.FromBase64String(value),
                m_decryptor));
    }

    #endregion

    #region Private methods

    private byte[] Transform (byte[] input, ICryptoTransform transform) {
        byte[] output;
        using (MemoryStream memory = new MemoryStream()) {
            using (CryptoStream crypto = new CryptoStream(
                memory,
                transform,
                CryptoStreamMode.Write
            )) {
                crypto.Write(input, 0, input.Length);
                crypto.FlushFinalBlock();

                output = memory.ToArray();
            }
        }
        return output;
    }

    #endregion
}

As you can see, in both cases I'm writing to a MemoryStream via a CryptoStream. If I create a new decryptor via m_provider.CreateDecyptor(s_key, s_iv) on every call to Decrypt it works just fine.

What has gone wrong here? Why is the decryptor behaving as if its forgotten the IV? Is there something that the call to StreamReader.ReadToEnd() is doing that helps m_decryptor function correctly?

I would like to avoid either of the two "working" approaches I listed here as there is a performance hit on both and this is a very critical path. Thanks in advance.

Answer 1

Ok, I admit I have no idea why this works, but change AesCryptoServiceProvider to AesManaged and voila.

I also recommend making your class implement IDisposable as it contains three member variables which implement it. See below for code changes:

public class AesEncryptionProvider : IDisposable
{
    #region Fields

    // Encryption key
    private static readonly byte[] s_key = new byte[32] {
        // Omitted...
    };

    // Initialization vector
    private static readonly byte[] s_iv = new byte[16] {
        // Omitted...
    };

    private readonly AesManaged m_provider;
    private readonly ICryptoTransform m_encryptor;
    private readonly ICryptoTransform m_decryptor;

    #endregion

    #region Constructors

    private AesEncryptionProvider()
    {
        m_provider = new AesManaged();
        m_encryptor = m_provider.CreateEncryptor(s_key, s_iv);
        m_decryptor = m_provider.CreateDecryptor(s_key, s_iv);
    }

    public void Dispose()
    {
        m_decryptor.Dispose();
        m_encryptor.Dispose();
        m_provider.Dispose();
        GC.SuppressFinalize(this);
    }

    static AesEncryptionProvider()
    {
        Instance = new AesEncryptionProvider();
    }

    #endregion

    #region Properties

    public static AesEncryptionProvider Instance { get; private set; }

    #endregion

    #region Methods

    public string Encrypt(string value)
    {
        if (string.IsNullOrEmpty(value))
        {
            throw new ArgumentException("Value required.");
        }

        return Convert.ToBase64String(
            Transform(
                Encoding.UTF8.GetBytes(value),
                m_encryptor));
    }

    public string Decrypt(string value)
    {
        if (string.IsNullOrEmpty(value))
        {
            throw new ArgumentException("Value required.");
        }

        return Encoding.UTF8.GetString(
            Transform(
                Convert.FromBase64String(value),
                m_decryptor));
    }

    #endregion

    #region Private methods

    private byte[] Transform(byte[] input, ICryptoTransform transform)
    {
        byte[] output;
        using (MemoryStream memory = new MemoryStream())
        {
            using (CryptoStream crypto = new CryptoStream(
                memory,
                transform,
                CryptoStreamMode.Write
            ))
            {
                crypto.Write(input, 0, input.Length);
                crypto.FlushFinalBlock();

                output = memory.ToArray();
            }
        }
        return output;
    }

    #endregion
}