如何正确一致地从字符串中获取字节进行AES加密

我目前正在C#中进行AES实现。加密方法有两个参数：一个字符串和一个密码。我将获取提供的字符串并将其转换为字节数组，以便稍后使用它将数据写入具有BinaryWriter的流。

问题是，当我使用Convert.FromBase64String(string)时，我会得到FormatException: Invalid length.，而当我使用Encoding.UTF8.GetBytes(string)时，我的解密方法会抛出无效的PKCS7.PPadding异常。

最近几天我一直在努力解决这个问题。我在stackoverflow.com和其他网站上读到了几乎无限的问题，但我仍然不知道解决这个问题最可靠的方法是什么。

此程序中使用的字符串仅限于句子（例如"需要加密的东西"）和数字（例如"12345"）。

提前谢谢你，这是我现在的代码：

    public class AESProvider {
    public byte[] EncryptStringToBytes_Aes(string plainText, string Key)
    {
        // Check arguments. 
        if (plainText == null || plainText.Length <= 0)
            throw new ArgumentNullException("plainText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        byte[] plainTextInBytes = Convert.FromBase64String(plainText);
        byte[] encrypted;
        //Create an Aes object
        //with the specified key and IV.
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.GenerateIV();
            byte[] IV = aesAlg.IV;
            //The Salt will be the first 8 bytes of the IV.
            byte[] theSalt = new byte[8];
            Array.Copy(IV,theSalt,8);
            //A key for AES is generated by expanding the password using the following method.
            Rfc2898DeriveBytes keyGen = new Rfc2898DeriveBytes(Key,theSalt);
            byte[] aesKey = keyGen.GetBytes(16);
            aesAlg.Key = aesKey;
            // Create a decrytor to perform the stream transform.
            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, IV);
            // Create the streams used for encryption. 
            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (BinaryWriter swEncrypt = new BinaryWriter(csEncrypt))
                    {
                        //Write all data to the stream.
                        swEncrypt.Write(plainTextInBytes);
                    }
                    encrypted = msEncrypt.ToArray();
                }
            }
            // Prepend the IV to the ciphertext so it can be used in the decryption process.
            using (MemoryStream ivPlusCipher = new MemoryStream())
            {
                using (BinaryWriter tBinaryWriter = new BinaryWriter(ivPlusCipher))
                {
                    tBinaryWriter.Write(IV);
                    tBinaryWriter.Write(encrypted);
                    tBinaryWriter.Flush();
                }
                return ivPlusCipher.ToArray();
            }
        }
    }
    public byte[] DecryptStringFromBytes_Aes(byte[] cipherText, string Key)
    {
        // Check arguments. 
        if (cipherText == null || cipherText.Length <= 0)
            throw new ArgumentNullException("cipherText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        // Declare the string used to hold 
        // the decrypted text. 
        byte[] decrypted;
        // Create an Aes object 
        // with the specified key and IV. 
        // Create the streams used for decryption. 
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.Mode = CipherMode.CBC;
            aesAlg.Padding = PaddingMode.PKCS7;
            //Grab IV from ciphertext
            byte[] IV = new byte[16];
            Array.Copy(cipherText,0,IV,0,16);
            //Use the IV for the Salt
            byte[] theSalt = new byte[8];
            Array.Copy(IV,theSalt,8);
            Rfc2898DeriveBytes keyGen = new Rfc2898DeriveBytes(Key,theSalt);
            byte[] aesKey = keyGen.GetBytes(16);
            aesAlg.Key = aesKey;
            // Create a decrytor to perform the stream transform.
            ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, IV);
            using (MemoryStream msDecrypt = new MemoryStream())
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Write))
                {
                    using (BinaryWriter srDecrypt = new BinaryWriter(csDecrypt))
                    {
                        //Decrypt the ciphertext
                        srDecrypt.Write(cipherText, IV.Length, (cipherText.Length - IV.Length));
                    }
                    decrypted = msDecrypt.ToArray();
                    return decrypted;
                }
            }   
        }
    }
}