Encryption and decryption in a .NET application using AES Rijndael with autogen IV. Symmetric Cipher Key accessed from config file. Recommend you encrypt key in config also.

EncryptionService.cs

public class EncryptionService : IEncryptionService
    {
        private const int Keysize = 256;

        public string Encrypt(string plainText)
        {
            var toEncryptBytes = Encoding.UTF8.GetBytes(plainText);
            using (var provider = CreateCipher())
            {
                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 Convert.ToBase64String(ms.ToArray());
                    }
                }
            }
        }

        private RijndaelManaged CreateCipher()
        {
            var cipher = new RijndaelManaged();
            string encryptionKey = ConfigurationManager.AppSettings["IbanCryptoKey"].ToString();

            cipher.KeySize = Keysize;
            cipher.Key = Convert.FromBase64String(encryptionKey);
            cipher.Mode = CipherMode.CBC;
            cipher.Padding = PaddingMode.PKCS7;
            return cipher;
        }

        public string Decrypt(string cipherText)
        {
            using (var provider = CreateCipher())
            {
                
                using (var ms = new MemoryStream(Convert.FromBase64String(cipherText)))
                {
                    byte[] buffer = new byte[16];
                    ms.Read(buffer, 0, 16);
                    provider.IV = buffer;
                    using (var decryptor = provider.CreateDecryptor())
                    {
                        using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                        {
                            using (var sr = new StreamReader(cs))
                            {
                                return sr.ReadToEnd();
                            }
                        }
                    }
                }
            }
        }
    }