PCLCrypto

public override void ViewDidLoad ()
{
		base.ViewDidLoad ();
		// Perform any additional setup after loading the view, typically from a nib.

		var password = "9a55W0rd!!";
		var salt = Encoding.Unicode.GetBytes("5a1t");

		byte[] keymaterial = NetFxCrypto.DeriveBytes.GetBytes(password, salt, 1000, 32);    

		byte[] data = Encoding.UTF8.GetBytes ("Some very important text");//because you should

		var provider = WinRTCrypto.SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithm.AesCbcPkcs7);
		var key = provider.CreateSymmetricKey(keymaterial);

		byte[] iv = null; // this is optional, but must be the same for both encrypting and decrypting
		byte[] cipherText = WinRTCrypto.CryptographicEngine.Encrypt(key, data, iv);
		byte[] plainText = WinRTCrypto.CryptographicEngine.Decrypt(key, cipherText, iv);

		var decoded = Encoding.UTF8.GetString(plainText, 0, plainText.Length); 

		Debug.WriteLine (String.Format("decoded: {0}", decoded));
}

Quick encryption/decryption example using PCLCrypto (tested on Xamarin.iOS)