Xamarin.iOS Touch ID Using Keychain

Example

Working Source - https://github.com/benhysell/V.TouchIdExample

Long form description - http://benjaminhysell.com/archive/2014/11/authentication-in-xamarin-ios-with-touch-id-or-passcode/

//Simple View with a switch to enable / disable Touch ID and 
//a button to invoke authentication 

/// <summary>
/// Enable/Disable Touch ID
/// </summary>
/// <param name="sender">Sender.</param>
partial void TouchIdEnableDisable(UISwitch sender)
{
    if (sender.On)
    {
        //enable Touch ID          
        //set our record
        //note what you fill in here doesn't matter, just needs to be 
        //consistent across all uses of the record
        var secRecord = new SecRecord(SecKind.GenericPassword)
        {
            Label = "Keychain Item",
            Description = "fake item for keychain access",
            Account = "Account",
            Service = "com.yourcompany.touchIdExample",
            Comment = "Your comment here",
            ValueData = NSData.FromString("my-secret-password"),
            Generic = NSData.FromString("foo")
        };

        secRecord.AccessControl = new SecAccessControl(SecAccessible.WhenPasscodeSetThisDeviceOnly, SecAccessControlCreateFlags.UserPresence);
        SecKeyChain.Add(secRecord);
                                
        authenticateButton.Enabled = true;
    }
    else
    {
        //disable Touch ID
        var record = new SecRecord(SecKind.GenericPassword)
        {
            Service = "com.yourcompany.touchIdExample",
            UseOperationPrompt = "Authenticate to Remove Touch ID / Passcode from Test App"
        };

        SecStatusCode result;

        //query one last time to ensure they can remove it
        SecKeyChain.QueryAsRecord(record, out result);
        if (SecStatusCode.Success == result || SecStatusCode.ItemNotFound == result)
        {
            //remove the record
            SecKeyChain.Remove(record);
            authenticateButton.Enabled = false;
        }
        else
        {
            //could not authenticate, leave switch on
            sender.On = true;
         }                
    }
}

/// <summary>
/// Show Touch ID to user and evaluate authentication
/// </summary>
/// <param name="sender">Sender.</param>
partial void AuthenticateUser(UIButton sender)
{
    var rec = new SecRecord(SecKind.GenericPassword)
    {
        Service = "com.yourcompany.touchIdExample",
        UseOperationPrompt = "Authenticate to access Test App"
    };
    SecStatusCode res;
    SecKeyChain.QueryAsRecord(rec, out res);
    if (SecStatusCode.Success == res || SecStatusCode.ItemNotFound == res)
    {
        //Success!!  
        //add your code here to continue into your application
        AuthenticatedLabel.Hidden = false;
    }
    else
    {
        //Failure
        AuthenticatedLabel.Hidden = true;
    }
}