Helper to use managed AES encryption and easily encrypt an decrypt data. To see how the stream based methods work, have a look at `EncryptString()`.

CryptoHelpers.cs

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

namespace CryptoDemo
{
	/// <summary>
	/// Helpers for cryptography.
	/// Cryptography is unavailable in PCL but on most platforms.
	/// </summary>
	public static class SharedCryptoHelpers
	{
		/// <summary>
		/// Generates a random encryption key and returns it Base64 encoded.
		/// </summary>
		/// <returns>The random key as Base64 encoded string.</returns>
		public static string GenerateRandomKeyAsBase64()
		{
			byte[] key = null;
			
			// Aes.Create() will use hardware accelerated encryption if used on Xamarin.iOS!
			using (var aesAlgo = Aes.Create())
			{
				// Must be 128 for Rijndael.
				aesAlgo.BlockSize = 128;

				// Maximum for Rijndael is 256. This means 14 cycles of repetion when turning plaintext into the final output.
				aesAlgo.KeySize = 256;

				// Ciphe block chaining mode. This means each encrypted block depends on all previous blocks.
				// Hence, an IV (Initialization Vector) is required for the first block.
				aesAlgo.Mode = CipherMode.CBC;

				// Rijndael works on fixed size blocks but messages come in various lengths. Hence padding is required. 
				// PKCS7 padding is the most commonly used padding method (see: http://en.wikipedia.org/wiki/Padding_(cryptography)#PKCS7).
				aesAlgo.Padding = PaddingMode.PKCS7;

				aesAlgo.GenerateKey();
				key = aesAlgo.Key;
			}

			return Convert.ToBase64String(key);
		}


		/// <summary>
		/// Encrypts an incoming stream into an outgoing stream.
		/// </summary>
		/// <param name="streamToWrap">the stream to wrap into a crypto stream</param>
		/// <param name="password">the password to use for encryption</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		public static CryptoStream GetEncryptionStream(Stream streamToWrap, string password, int iterations)
		{
			if (streamToWrap == null)
			{
				throw new ArgumentException("Cannot encrypt NULL stream!");
			}

			if (!streamToWrap.CanWrite)
			{
				throw new ArgumentException("Stream to wrap cannot be written to.");
			}

			if (String.IsNullOrWhiteSpace(password))
			{
				throw new ArgumentException("Password must be defined");
			}

			CryptoStream cryptoStream = null;

			// Create a Rijndael algorithm. Note: this way of construction instead of using AesCryptoServiceProvider() is 
			// better portable across platforms and will call Apple's hardware accelrated CommonCrypto on iOS.
			// For details about Rijndael, see Wikipedia: http://en.wikipedia.org/wiki/Advanced_Encryption_Standard
			using (var aesAlgo = Aes.Create())
			{
				// Must be 128 for Rijndael.
				aesAlgo.BlockSize = 128;

				// Maximum for Rijndael is 256. This means 14 cycles of repetion when turning plaintext into the final output.
				aesAlgo.KeySize = 256;

				// Cipher block chaining mode. This means each encrypted block depends on all previous blocks.
				// Hence, an IV (Initialization Vector) is required for the first block.
				aesAlgo.Mode = CipherMode.CBC;

				// Rijndael works on fixed size blocks but messages come in various lengths. Hence padding is required. 
				// PKCS7 padding is the most commonly used padding method (see: http://en.wikipedia.org/wiki/Padding_(cryptography)#PKCS7).
				aesAlgo.Padding = PaddingMode.PKCS7;

				// Set position to start of stream.
				streamToWrap.Seek(0, SeekOrigin.Begin);

				// Store the salt in the output stream. The salt is not a secret. Salt is used to generate different keys for identical passwords.
				var keyInfo = DeriveKey(password, iterations);
				streamToWrap.Write(keyInfo.Salt, 0, keyInfo.Salt.Length);

				// Store the IV in the output stream. The IV is randomly generated if not set explicitly. It is not a secret and used to create 
				// different encrypted output for identical plaintext input when using CBC cipher mode.
				streamToWrap.Write(aesAlgo.IV, 0, aesAlgo.IV.Length);

				// Let the algorithm know our key.
				aesAlgo.Key = keyInfo.Key;

				// Get an encrypting ICryptoTransform interface from the algorithm.
				// Wrap a crypto stream around the plain text stream.
				var cryptoTransform = aesAlgo.CreateEncryptor();
				cryptoStream = new CryptoStream(streamToWrap, cryptoTransform, CryptoStreamMode.Write);
			}

			return cryptoStream;
		}

		/// <summary>
		/// Encrypts an incoming stream into an outgoing stream.
		/// </summary>
		/// <param name="inStream">Unencrypted input stream. Stream will be read from its current position to the end. Be sure to position the stream correctly.</param>
		/// <param name="outStream">out stream. Stream has to be writable and seekable. Will be positioned to offset zero.</param>
		/// <param name="password">The password to use for encryption. The password is not used directly. A cryptographic strong key will be derived from it.</param> 
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		public static void Encrypt(Stream inStream, Stream outStream, string password, int iterations)
		{
			if (inStream == null)
			{
				throw new ArgumentException("Cannot encrypt NULL stream!");
			}

			if (!inStream.CanRead)
			{
				throw new ArgumentException("Input stream cannot be read from.");
			}

			if (outStream == null)
			{
				throw new ArgumentException("Output stream may not be NULL!");
			}

			if (!outStream.CanWrite)
			{
				throw new ArgumentException("Output stream cannot be written to.");
			}

			if (!outStream.CanSeek)
			{
				throw new ArgumentException("Cannot seek in output stream!");
			}

			if (String.IsNullOrWhiteSpace(password))
			{
				throw new ArgumentException("Password must be defined");
			}

			// Create a crypto stream around the output stream and copy input stream to (encrypted) output.
			using(var cryptoStream = SharedCryptoHelpers.GetEncryptionStream(outStream, password, iterations))
			{
				inStream.CopyTo(cryptoStream);
			}
		}

		/// <summary>
		/// Gets a stream that decrypts data.
		/// The input stream needs to start with 8 bytes of salt, followed by 16 bytes of IV.
		/// </summary>
		/// <param name="encryptedInStream">the encrypted stream</param>
		/// <param name="password">the password for decryption</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		/// <returns>the decrypted stream</returns>
		public static CryptoStream GetDecryptionStream(Stream encryptedInStream, string password, int iterations)
		{
			if (encryptedInStream == null)
			{
				throw new ArgumentException("Cannot decrypt NULL stream!");
			}

			if (!encryptedInStream.CanRead)
			{
				throw new ArgumentException("Input stream cannot be read from.");
			}

			if (!encryptedInStream.CanSeek)
			{
				throw new ArgumentException("Cannot seek in input stream!");
			}

			CryptoStream cryptoStream = null;

			// Create a Rijndael algorithm. Note: this way of construction instead of using AesCryptoServiceProvider() is 
			// better portable across platforms and will call Apple's hardware accelrated CommonCrypto on iOS.
			// For details about Rijndael, see Wikipedia: http://en.wikipedia.org/wiki/Advanced_Encryption_Standard
			using (var aesAlgo = Aes.Create())
			{
				// Must be 128 for Rijndael.
				aesAlgo.BlockSize = 128;

				// Maximum for Rijndael is 256. This means 14 cycles of repetion when turning plaintext into the final output.
				aesAlgo.KeySize = 256;

				// Ciphe block chaining mode. This means each encrypted block depends on all previous blocks.
				// Hence, an IV (Initialization Vector) is required for the first block.
				aesAlgo.Mode = CipherMode.CBC;

				// Rijndael works on fixed size blocks but messages come in various lengths. Hence padding is required. 
				// PKCS7 padding is the most commonly used padding method (see: http://en.wikipedia.org/wiki/Padding_(cryptography)#PKCS7).
				aesAlgo.Padding = PaddingMode.PKCS7;

				// Set position to start of stream to read the IV.
				encryptedInStream.Seek(0, SeekOrigin.Begin);

				// Read the salt.
				byte[] salt = new byte[8];
				encryptedInStream.Read(salt, 0, 8);

				// Read the IV.
				byte[] iv = new byte[16];
				encryptedInStream.Read(iv, 0, 16);
				aesAlgo.IV = iv;

				// Generate the key from the password and the salt.
				var keyInfo = DeriveKey(password, salt, iterations);
				aesAlgo.Key = keyInfo.Key;

				// Get a decrypting ICryptoTransform interface from the algorithm.
				var cryptoTransform = aesAlgo.CreateDecryptor();
				cryptoStream = new CryptoStream(encryptedInStream, cryptoTransform, CryptoStreamMode.Read);
			}
			return cryptoStream;
		}

		/// <summary>
		/// Decrypts a stream.
		/// The stream needs to start with 8 bytes of salt, followed by 16 bytes of IV.
		/// </summary>
		/// <param name="encryptedInStream">the encrypted stream</param>
		/// <param name="decryptedOutStream">the stream the decrypted output will be written to</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		/// <param name="password">the password for decryption</param>
		public static void Decrypt(Stream encryptedInStream, Stream decryptedOutStream, string password, int iterations)
		{
			if (encryptedInStream == null)
			{
				throw new ArgumentException("Cannot decrypt NULL stream!");
			}

			if (!encryptedInStream.CanRead)
			{
				throw new ArgumentException("Input stream cannot be read from.");
			}

			if (!encryptedInStream.CanSeek)
			{
				throw new ArgumentException("Cannot seek in input stream!");
			}


			if (decryptedOutStream == null)
			{
				throw new ArgumentException("Output stream may not be NULL!");
			}

			if (!decryptedOutStream.CanWrite)
			{
				throw new ArgumentException("Output stream cannot be written to.");
			}

			using(var cryptoStream = GetDecryptionStream(encryptedInStream, password, iterations))
			{
				cryptoStream.CopyTo(decryptedOutStream);
			}
		}

		/// <summary>
		/// Contains information about an encryption key and the salt value.
		/// </summary>
		public struct CryptoKeyInfo
		{
			/// <summary>
			/// The key.
			/// </summary>
			public byte[] Key;
			/// <summary>
			/// The salt value.
			/// </summary>
			public byte[] Salt;
		}


		/// <summary>
		/// Uses Rfc2898DeriveBytes (and that corresponds to PBKDF2) to derive a crypto key and a salt from a password.
		/// Use this if you want to create a key for first time encryption of data.
		/// </summary>
		/// <returns>The key information.</returns>
		/// <param name="password">Password to use.</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		public static CryptoKeyInfo DeriveKey(string password, int iterations)
		{
			if (password == null)
			{
				throw new ArgumentNullException("password", "Password is required!");
			}

			var key = new CryptoKeyInfo();

			// Derive a key together with an 8 byte salt value.
			var deriveBytes = new Rfc2898DeriveBytes(password, 8, iterations);

			// Get exactly 32 bytes (which maps to 256 bit and is the maximum key size for AES/Rijndael).
			key.Key = deriveBytes.GetBytes(32);
			key.Salt = deriveBytes.Salt;

			return key;
		}

		/// <summary>
		/// Uses Rfc2898DeriveBytes (and that corresponds to PBKDF2) to derive a crypto key from a password and a given salt value.
		/// Use this if you want to decrypt data where the salt value is stored as part of the data.
		/// </summary>
		/// <returns>The key information.</returns>
		/// <param name="password">Password to use.</param>
		/// <param name="salt">Salt to use to derive the key.</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		public static CryptoKeyInfo DeriveKey(string password, byte[] salt, int iterations)
		{
			if (password == null)
			{
				throw new ArgumentNullException("password","Password is required!");
			}

			if (salt == null || salt.Length != 8)
			{
				throw new ArgumentException("Salt has to be exactly 8 bytes!");
			}

			var key = new CryptoKeyInfo();

			// Derive a key together with an 8 byte salt value.
			var deriveBytes = new Rfc2898DeriveBytes(password, salt, iterations);

			// Get exactly 32 bytes (which maps to 256 bit and is the maximum key size for AES/Rijndael).
			key.Key = deriveBytes.GetBytes(32);
			// The derived salt is the same as the one we pass in.
			key.Salt = salt;

			return key;
		}

		/// <summary>
		/// Creates the hash of a PIN and returns it as Base64 encoded string.
		/// </summary>
		/// <param name="pin">PIN to create hash for</param>
		public static string CalculatePinHashAsBase64(string pin)
		{
			string base64Hash = string.Empty;
			if (pin == null)
			{
				pin = string.Empty;
			}
			string spicedPin = string.Format("Start_SDSClient_{0}_End", pin);
			using (var hashAlgo = HashAlgorithm.Create("SHA256"))
			{
				Debug.Assert(hashAlgo != null, "No hash algorithm returned!");
				byte[] hashValue = hashAlgo.ComputeHash(Encoding.UTF8.GetBytes(spicedPin));
				base64Hash = Convert.ToBase64String(hashValue);
			}
			return base64Hash;
		}

		/// <summary>
		/// Encrypts a string and returns it Base64 encoded.
		/// </summary>
		/// <param name="s">the string to encode</param>
		/// <param name="password">The password to use for encryption. The password is not used directly. A cryptographic strong key will be derived from it.</param> 
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		/// <returns>the encrypted string</returns>
		public static string EncryptString(string s, string password, int iterations)
		{
			if (s == null)
			{
				throw new ArgumentException("Cannot encrypt NULL string!");
			}

			if (string.IsNullOrWhiteSpace(password))
			{
				throw new ArgumentException("Password is required for encryption!");
			}

			string encryptedString = null;

			using (var encryptedOutStream = new MemoryStream())
			using (var plainInStream = new MemoryStream(Encoding.UTF8.GetBytes(s)))
			{
				Encrypt(plainInStream, encryptedOutStream, password, iterations);
				encryptedString = Convert.ToBase64String(encryptedOutStream.ToArray());
			}

			return encryptedString;
		}

		/// <summary>
		/// Decrypts an encrypted string.
		/// </summary>
		/// <param name="s">the string to decrypt</param>
		/// <param name="password">the password to use</param>
		/// <param name="iterations">Iterations for hashing algorithm. 10000 is a reasonable value.</param>
		/// <returns>the decrypted string. NULL if decryption fails.</returns>
		public static string DecryptString(string s, string password, int iterations)
		{
			if (s == null)
			{
				throw new ArgumentException("Cannot encrypt NULL string!");
			}

			if (string.IsNullOrWhiteSpace(password))
			{
				throw new ArgumentException("Password is required for encryption!");
			}

			string decryptedString = null;

			try
			{
				using (var decryptedOutStream = new MemoryStream())
				using (var encryptedInStream = new MemoryStream(Convert.FromBase64String(s)))
				{
					Decrypt(encryptedInStream, decryptedOutStream, password, iterations);
					decryptedString = Encoding.UTF8.GetString(decryptedOutStream.ToArray());
				}
			}
			catch (Exception ex)
			{
				Tracking.Report (ex, true);
				// Fail gracefully if a crypto exception occurs.
				return null;
			}

			return decryptedString;
		}
	}
}