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Asymmetric Encryption Without User Input/Hardcoded Values for Xamarin iOS

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Asymmetric Encryption Without User Input/Hardcoded Values for Xamarin iOS

This post is all about asymmetric encryption on iOS, utilizing the built-in KeyChain API of iOS without any user input or any hardcoded values.

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iOS KeyChain

The KeyChain API is the most important security element on iOS (and MacOS). The interaction with it is not as difficult as one thinks, after getting the concept for the different use case scenarios it supports. In our case, we want to create a public/private key pair for use within our app. Pretty much like on Android, we want no user input and no hardcoded values to get this pair.

Preparing Key (Pair) Creation

On iOS, things are traditionally a little bit more complex than on other platforms. This is also true for things like encryption. The first step would be to prepare the RSA parameters we want to use for encryption. However, that turned out to be a bit challenging because we need to pass in some keys and values that live in the native iOS security library and Xamarin does not fully expose them. Luckily, there is at least a Xamarin API that helps us to extract those values. I found this SO post helpful to understand what is needed for the creation of the key pair. I adapted some of the snippets into my own helper class, this is also true for the IosConstants class:

    internal class IosConstants
    {
        private static IosConstants _instance;

        public static IosConstants Instance => _instance ?? (_instance = new IosConstants());

        public readonly NSString KSecAttrKeyType;
        public readonly NSString KSecAttrKeySize;
        public readonly NSString KSecAttrKeyTypeRSA;
        public readonly NSString KSecAttrIsPermanent;
        public readonly NSString KSecAttrApplicationTag;
        public readonly NSString KSecPrivateKeyAttrs;
        public readonly NSString KSecClass;
        public readonly NSString KSecClassKey;
        public readonly NSString KSecPaddingPKCS1;
        public readonly NSString KSecAccessibleWhenUnlocked;
        public readonly NSString KSecAttrAccessible;

        public IosConstants()
        {
            var handle = Dlfcn.dlopen(Constants.SecurityLibrary, 0);

            try
            {
                KSecAttrApplicationTag = Dlfcn.GetStringConstant(handle, "kSecAttrApplicationTag");
                KSecAttrKeyType = Dlfcn.GetStringConstant(handle, "kSecAttrKeyType");
                KSecAttrKeyTypeRSA = Dlfcn.GetStringConstant(handle, "kSecAttrKeyTypeRSA");
                KSecAttrKeySize = Dlfcn.GetStringConstant(handle, "kSecAttrKeySizeInBits");
                KSecAttrIsPermanent = Dlfcn.GetStringConstant(handle, "kSecAttrIsPermanent");
                KSecPrivateKeyAttrs = Dlfcn.GetStringConstant(handle, "kSecPrivateKeyAttrs");
                KSecClass = Dlfcn.GetStringConstant(handle, "kSecClass");
                KSecClassKey = Dlfcn.GetStringConstant(handle, "kSecClassKey");
                KSecPaddingPKCS1 = Dlfcn.GetStringConstant(handle, "kSecPaddingPKCS1");
                KSecAccessibleWhenUnlocked = Dlfcn.GetStringConstant(handle, "kSecAttrAccessibleWhenUnlocked");
                KSecAttrAccessible = Dlfcn.GetStringConstant(handle, "kSecAttrAccessible");

            }
            finally
            {
                Dlfcn.dlclose(handle);
            }
        }
    }

This class picks out the values we need to create the RSA parameters that will be passed to the KeyChain API later. Now, we have everything in place to create key paris with this helper method:

private NSDictionary CreateRsaParams()
{
    IList<object> keys = new List<object>();
    IList<object> values = new List<object>();

    //creating the private key params
    keys.Add(IosConstants.Instance.KSecAttrApplicationTag);
    keys.Add(IosConstants.Instance.KSecAttrIsPermanent);
    keys.Add(IosConstants.Instance.KSecAttrAccessible);

    values.Add(NSData.FromString(_keyName, NSStringEncoding.UTF8));
    values.Add(NSNumber.FromBoolean(true));
    values.Add(IosConstants.Instance.KSecAccessibleWhenUnlocked);

    NSDictionary privateKeyAttributes = NSDictionary.FromObjectsAndKeys(values.ToArray(), keys.ToArray());

    keys.Clear();
    values.Clear();

    //creating the keychain entry params
    //no need for public key params, as it will be created from the private key once it is needed
    keys.Add(IosConstants.Instance.KSecAttrKeyType);
    keys.Add(IosConstants.Instance.KSecAttrKeySize);
    keys.Add(IosConstants.Instance.KSecPrivateKeyAttrs);

    values.Add(IosConstants.Instance.KSecAttrKeyTypeRSA);
    values.Add(NSNumber.FromInt32(this.KeySize));
    values.Add(privateKeyAttributes);

    return NSDictionary.FromObjectsAndKeys(values.ToArray(), keys.ToArray());
}

In order to use the SecKeyAPI to create a random key for us, we need to pass in a NSDictionarythat holds a list of private key attributes and is attached to a parent NSDictionary that holds it together with some other configuration values for the KeyChain API. If you want, you could also create a NSDictionary for the public key, but that is not needed for my implementation as I request it later from the private key (we'll have a look on that as well).

Finally, Let the OS Create a Private Key

Now we have all our parameters in place, we are able to create a new private key by calling the SecKey.CreateRandomKey() method:

public bool CreatePrivateKey()
{
    Delete();
    var keyParams = CreateRsaParams();

    SecKey.CreateRandomKey(keyParams, out var keyCreationError);

    if (keyCreationError != null)
    {
        Debug.WriteLine($"{keyCreationError.LocalizedFailureReason}\n{keyCreationError.LocalizedDescription}");
    }

    return keyCreationError == null;
}

Like on Android, it is a good idea to call into the Delete() method before creating a new key (I'll show you that method later). This makes sure your app uses just one key with the specified name. After that, we create a new random key with the help of the OS. Because we specified it to be a private key for RSA before, it will be exactly that. If there is an error, we will return false and print it in the Debug console.

Retrieving the Private Key

Now we have created the new private key, we are able to retrieve it like this:

public SecKey GetPrivateKey()
{
    var privateKey = SecKeyChain.QueryAsConcreteType(
        new SecRecord(SecKind.Key)
        {
            ApplicationTag = NSData.FromString(_keyName, NSStringEncoding.UTF8),
            KeyType = SecKeyType.RSA,
            Synchronizable = shouldSyncAcrossDevices
        },
        out var code);

    return code == SecStatusCode.Success ? privateKey as SecKey : null;
}

We are using the QueryAsConcreteType method to find our existing key in the Keychain. If the OS does not find the key, we are returning null. In this case, we would need to create a new key.

Retrieving the Public Key for Encryption

Of course, we need a public key if we want to encrypt our data. Here is how to get this public key from the private key:

public SecKey GetPublicKey()
{
    return GetPrivateKey()?.GetPublicKey();
}

Really, that's it. Even if we are not creating a public key explicitly when we are creating our private key, we are getting a valid public key for encryption from the GetPublicKeymethod, called on the private key instance.

Deleting the Key Pair

Like I said earlier, sometimes we need to delete our encryption key(s). This little helper method does the job for us:

public bool Delete()
{
    var findExisting = new SecRecord(SecKind.Key)
    {
        ApplicationTag = NSData.FromString(_keyName, NSStringEncoding.UTF8),
        KeyType = SecKeyType.RSA,
        Synchronizable = _shouldSyncAcrossDevices
    };

    SecStatusCode code = SecKeyChain.Remove(findExisting);

    return code == SecStatusCode.Success;
}

This time, we are searching for a SecRecord with the kind key, and calling the Removemethod of the SecKeyChain API. Based on the status code, we finally return a bool that indicates if we were successful. Note: When we create a new key, (actually) I do not care about the status and just create a new one in my helper class. If we delete the key from another place, we are probably going to work with that status code.

As I did with the Android version, I did not create a demo project, but you can have a look at the full class in this Gist on GitHub.

Usage

Now that we have our helper in place, we are able to encrypt and decrypt data in a very easy way. First, we need to obtain a private and a public key:

var helper = new PlatformEncryptionKeyHelper("testKeyHelper");

if (!helper.KeysExist())
{
    helper.CreateKeyPair();
}

var privKey = helper.GetPrivateKey();
var pubKey = helper.GetPublicKey();

The encryption method needs to be called directly on the public key instance:

var textToCrypt = "this is just a plain test text that will be encrypted and decrypted";
pubKey.Encrypt(SecPadding.PKCS1, Encoding.UTF8.GetBytes(textToCrypt), out var encBytes);

For getting the plain value back, we need to call the decryption method on the private key instance:

privKey.Decrypt(SecPadding.PKCS1, encBytes, out var decBytes);
var decrypted = Encoding.UTF8.GetString(decBytes);

It may make sense to wrap these calls into helper methods, but you could also just use it like I did for demoing purposes. Just remember to use always the same padding method, otherwise, you will not get any value back from the encrypted byte array.

Once again, if you need encryption of data in a Xamarin.Forms project, just extract an interface from the class or match it with the interface you may already have extracted from the Android version. As I stated before, every developer should use the right tools to encrypt data really securely in their apps. With that post, you now have also a starting point for your own Xamarin iOS implementation.

Like always, I hope this will be helpful for some of you. In my next post, we will have a look into the OS provided options for encryption and decryption on Windows 10 (UWP).

Until then, happy coding, everyone!

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Topics:
asymmetric encryption ,ios security ,security ,mobile application security ,appsec

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