Short information about encryption:
There are many encryption algorithm. They are divided into two as symetric and asymmetric. Symmetric encryption and asymmetric encryption are performed using different processes. Symmetric encryption is performed on streams and is therefore useful to encrypt large amounts of data. Asymmetric encryption is performed on a small number of bytes and is therefore useful only for small amounts of data.

For more information:
http://msdn.microsoft.com/en-us/library/as0w18af%28v=vs.110%29.aspx

This example shows how to encrypt a string or stream data in .NET Framework.
You can encrypt/decrypt all data types(string, file etc.) using sample class.

Usage:

            string plainText = "This is a plain text.";
            CryptoService cryptoService = new CryptoService();
            byte[] data = Encoding.Unicode.GetBytes(plainText);
            byte[] key = Encoding.Unicode.GetBytes("key");
            byte[] cipherData = cryptoService.Encrypt(data, key);

            byte[] decryptedData = cryptoService.Decrypt(cipherData, key);
            string againPlainText = Encoding.Unicode.GetString(decryptedData);

Crypto Code:

    public class CryptoService
    {
        private CipherMode _cipherMode = CipherMode.CFB;
        private PaddingMode _paddingMode = PaddingMode.PKCS7;

        private static readonly byte[] _salt = new byte[] { 0x25, 0xdc, 0xff, 0x00, 0xad, 0xed, 0x7a, 0xee, 0xc5, 0xfe, 0x07, 0xaf, 0x4d, 0x08, 0x12, 0x3c };

        private EncryptionAlgorithm _algorithm = EncryptionAlgorithm.AES;
        public EncryptionAlgorithm Algorithm
        {
            get { return _algorithm; }
            set { _algorithm = value; }
        }
        
        public Stream Encrypt(Stream streamToEncrypt, byte[] key)
        {
            SymmetricAlgorithm algorithm = null;
            if (_algorithm == EncryptionAlgorithm.TripleDES)
                algorithm = new TripleDESCryptoServiceProvider();
            else if (_algorithm == EncryptionAlgorithm.AES)
                algorithm = new RijndaelManaged();
            else
                throw new ApplicationException("Unexpected algorithm type!");

            algorithm.Mode = _cipherMode;
            algorithm.Padding = _paddingMode;

            Rfc2898DeriveBytes rfc = new Rfc2898DeriveBytes(key, _salt, 100);
            algorithm.Key = rfc.GetBytes(algorithm.KeySize / 8);
            algorithm.IV = rfc.GetBytes(algorithm.BlockSize / 8);

            try
            {
                ICryptoTransform encryptor = algorithm.CreateEncryptor();
                using (CryptoStream cryptoStream = new CryptoStream(streamToEncrypt, encryptor, CryptoStreamMode.Read))
                {
                    MemoryStream ms = new MemoryStream();
                    CopyStream(cryptoStream, ms);
                    ms.Seek(0, SeekOrigin.Begin);
                    return ms;
                }
            }
            finally
            {
                algorithm.Clear();
            }
        }

        public Stream Decrypt(Stream streamToDecrypt, byte[] key)
        {
            SymmetricAlgorithm algorithm = null;
            if (_algorithm == EncryptionAlgorithm.TripleDES)
                algorithm = new TripleDESCryptoServiceProvider();
            else if (_algorithm == EncryptionAlgorithm.AES)
                algorithm = new RijndaelManaged();
            else
                throw new ApplicationException("Unexpected algorithm type!");

            algorithm.Mode = _cipherMode;
            algorithm.Padding = _paddingMode;

            Rfc2898DeriveBytes rfc = new Rfc2898DeriveBytes(key, _salt, 100);
            algorithm.Key = rfc.GetBytes(algorithm.KeySize / 8);
            algorithm.IV = rfc.GetBytes(algorithm.BlockSize / 8);

            try
            {
                ICryptoTransform decryptor = algorithm.CreateDecryptor();
                using (CryptoStream cryptoStream = new CryptoStream(streamToDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    MemoryStream ms = new MemoryStream();
                    CopyStream(cryptoStream, ms);
                    ms.Seek(0, SeekOrigin.Begin);
                    return ms;
                }
            }
            finally
            {
                algorithm.Clear();
            }
        }

        public byte[] Encrypt(byte[] dataToEncrypt, byte[] key)
        {
            MemoryStream ms = new MemoryStream(dataToEncrypt);
            Stream res = Encrypt(ms, key);
            if (res != null)
            {
                byte[] tmp = new byte[res.Length];
                res.Read(tmp, 0, tmp.Length);
                return tmp;
            }

            return null;
        }

        public byte[] Decrypt(byte[] dataToDecrypt, byte[] key)
        {
            MemoryStream ms = new MemoryStream(dataToDecrypt);
            Stream res = Decrypt(ms, key);
            if (res != null)
            {
                byte[] tmp = new byte[res.Length];
                res.Read(tmp, 0, tmp.Length);
                return tmp;
            }

            return null;
        }

        public void CopyStream(Stream input, Stream output)
        {
            byte[] buffer = new byte[32768];
            int read;
            while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
            {
                output.Write(buffer, 0, read);
            }
        }
    }

    public enum EncryptionAlgorithm
    {
        TripleDES,
        AES,
    }