Tink for Obj-C HOW-TO

This document contains instructions and Obj-C code snippets for common tasks in Tink.

Setup instructions

Tink is released as a Cocoapod. It can be installed by using the pod command as described below, which is the recommended way to use Tink.

We also provide step-by-step instructions on building and using Tink from source.

Supported platforms

iOS 9.0 or newer
Xcode 9.2 or newer

Installing via Cocoapods

Change into the directory that contains your Xcode project.
```
cd /path/to/your/Xcode project/
```
Initialize Cocoapods.
```
pod init
```
This command creates a file called Podfile.
Edit the Podfile.
For the current stable release, add the following line:
```
pod 'Tink'
```
For a particular version, use a line like the following instead:
```
pod 'Tink', '1.2.0-rc2'
```
Note: Replace 1.2.0-rc2 with the pre-release version you want to install.
Install the pod.

$ pod install

Open the newly generated .xcworkspace and start using Tink.

You can import the umbrella header:

#import "Tink/Tink.h"

Or individual headers:

#import "Tink/TINKAeadConfig.h"
#import "Tink/TINKAeadKeyTemplate.h"
#import "Tink/TINKAead.h"

Installing from source

Prerequisites

To install Tink from the source code, the following prerequisites must be installed:

git - to download the source of Tink
Bazel v0.15.0 or newer - to build Tink

Step-by-step instructions

Clone Tink from GitHub.

git clone https://github.com/google/tink/

Build the library and generate a static iOS framework.
```
cd tink
export XCODE_VERSION=9.2
export IOS_SDK=11.2
bazel build -c opt --ios_multi_cpus=i386,x86_64,armv7,arm64 --xcode_version="${XCODE_VERSION}" --ios_sdk_version="${IOS_SDK}" //objc:Tink_framework
```
Adjust the following options according to your build environment:
- Set XCODE_VERSION to the Xcode version you are using to build your application.
- Set IOS_SDK to the version of the iOS SDK you are using in your application.
- The option ios_multi_cpus is used to generate a fat library that includes multiple architectures. Before submitting your application to the App Store you should generate a framework that includes only the ARM architectures and link it to your binary.

Unzip Tink_framework.zip into your Xcode project folder.

unzip bazel-bin/objc/Tink_framework.zip -d /path/to/your/project/folder/

Add the static framework to your Xcode project options:
- Open your Xcode project
- Navigate to your project‘s folder and drag Tink.framework into your Xcode’s left pane.
- In the following dialog select Copy items if needed and the target of your application that will use Tink. Click Finish.
- Select your project on the left pane and click on “Build Settings”
- Find Other Linker Flags and add -lc++
Start using Tink in your code.

You can import the umbrella header:

#import "Tink/Tink.h"

Or individual headers:

#import "Tink/TINKAeadConfig.h"
#import "Tink/TINKAeadKeyTemplate.h"
#import "Tink/TINKAead.h"

Initializing Tink

Tink provides customizable initialization, which allows for choosing specific implementations (identified by key types) of desired primitives. This initialization happens via registration of the implementations.

For example, if you want to use all implementations of all primitives in the current version of Tink, the initialization would look as follows:

   #import "Tink/TINKAllConfig.h"
   #import "Tink/TINKConfig.h"

   NSError *error = nil;
   TINKAllConfig *config = [[TINKAllConfig alloc] initWithError:&error];
   if (!config || error) {
     // handle error.
   }

   if (![TINKConfig registerConfig:config error:&error]) {
     // handle error.
   }

To use only implementations of the AEAD primitive:

    #import "Tink/TINKAeadConfig.h"
    #import "Tink/TINKConfig.h"

    NSError *error = nil;
    TINKAeadConfig *aeadConfig = [[TINKAeadConfig alloc] initWithError:&error];
    if (!aeadConfig || error) {
      // handle error.
    }

    if (![TINKConfig registerConfig:aeadConfig error:&error]) {
      // handle error.
    }

Generating new keys and keysets

To avoid accidental leakage of sensitive key material, you should avoid mixing keyset generation and usage in code. To support the separation between these activities the Tink package provides a command-line tool called Tinkey, which can be used for common key management tasks.

Still, if there is a need to generate a KeysetHandle with fresh key material directly in Obj-C code, one can use TINKKeysetHandle with one of the available KeyTemplates (AeadKeyTemplate, HybridKeyTemplate, etc.):

    #import "Tink/TINKAeadKeyTemplate.h"
    #import "Tink/TINKKeysetHandle.h"

    NSError *error = nil;
    TINKAeadKeyTemplate *tpl = [[TINKAeadKeyTemplate alloc] initWithKeyTemplate:TINKAes128Gcm error:&error];
    if (!tpl || error) {
      // handle error.
    }

    TINKKeysetHandle *handle = [[TINKKeysetHandle alloc] initWithKeyTemplate:tpl error:&error];
    if (!handle || error) {
      // handle error.
    }

Storing keysets

After generating key material, you might want to persist it to a storage system. Tink supports storing keysets to the iOS keychain where they remain encrypted:

    #import "Tink/TINKKeysetHandle.h"

    NSError *error = nil;
    TINKKeysetHandle *handle = [[TINKKeysetHandle alloc] initWithKeyTemplate:tpl error:&error];
    if (!handle || error) {
      // handle error.
    }

    NSString *keysetName = @"com.yourcompany.yourapp.uniqueKeysetName";
    if (![handle writeToKeychainWithName:keysetName overwrite:NO error:&error]) {
      // handle error.
    }

The keysets are stored in the keychain with the following options set:

kSecAttrAccessible = kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly
kSecAttrSynchronizable = False

These settings prevent the keysets from leaving the device and keep them encrypted until the device is unlocked once.

Loading existing keysets

To load keysets from the iOS keychain:

    #import "Tink/TINKKeysetHandle.h"

    NSError *error = nil;
    NSString *keysetName = @"com.yourcompany.yourapp.uniqueKeysetName";
    TINKKeysetHandle *handle = [[TINKKeysetHandle alloc] initFromKeychainWithName:keysetName error:&error];
    if (!handle || error) {
      // handle error.
    }

To load cleartext keysets, use TINKKeysetHandle+Cleartext and an appropriate KeysetReader, depending on the wire format of the stored keyset, for example a TINKBinaryKeysetReader.

Note: We don't recommend storing keysets in cleartext in the filesystem. Instead, use the iOS keychain as demonstrated above.

    #import "Tink/TINKBinaryKeysetReader.h"
    #import "Tink/TINKKeysetHandle+Cleartext.h"

    NSError *error = nil;
    NSData *binaryKeyset = ...;
    TINKBinaryKeysetReader *reader = [[TINKBinaryKeysetReader alloc] initWithSerializedKeyset:binaryKeyset
                                                                                        error:&error];
    if (!reader || error) {
      // handle error.
    }

    TINKKeysetHandle *handle = [[TINKKeysetHandle alloc] initCleartextKeysetHandleWithKeysetReader:reader
                                                                                             error:&error];
    if (!handle || error) {
      // handle error.
    }

Obtaining and using primitives

Primitives represent cryptographic operations offered by Tink, hence they form the core of Tink API. A primitive is just an interface that specifies what operations are offered by the primitive. A primitive can have multiple implementations, and you choose a desired implementation by using a key of corresponding type (see the this section for details). A list of primitives and their implementations currently supported by Tink in Objective-C can be found here.

You access implementations of a primitive via a factory that corresponds to the primitive which offers corresponding primitiveWithKeysetHandle:error: methods.

AEAD via TINKAeadFactory
MAC via TINKMacFactory
etc.

Symmetric key encryption

You can use an AEAD (Authenticated Encryption with Associated Data) primitive to encrypt or decrypt data:

    #import "Tink/TINKAead.h"
    #import "Tink/TINKKeysetHandle.h"
    #import "Tink/TINKAeadFactory.h"

    // 1. Get a handle to the key material.
    TINKKeysetHandle *keysetHandle = ...;

    // 2. Get the primitive.
    NSError *error = nil;
    id<TINKAead> aead = [TINKAeadFactory primitiveWithKeysetHandle:keysetHandle error:&error];
    if (!aead || error) {
      // handle error.
    }

    // 3. Use the primitive.
    NSData *ciphertext = [aead encrypt:plaintext withAdditionalData:aad error:&error];
    if (!ciphertext || error) {
      // handle error.
    }

Hybrid encryption

To decrypt using a combination of public key encryption and symmetric key encryption:

    #import "Tink/TINKHybridDecrypt.h"
    #import "Tink/TINKKeysetHandle.h"
    #import "Tink/TINKHybridDecryptFactory.h"

    // 1. Get a handle to the key material.
    TINKKeysetHandle *keysetHandle = ...;

    // 2. Get the primitive.
    NSError *error = nil;
    id<TINKHybridDecrypt> hybridDecrypt = [TINKHybridDecryptFactory primitiveWithKeysetHandle:keysetHandle
                                                                                        error:&error];
    if (!hybridDecrypt || error) {
      // handle error.
    }

    // 3. Use the primitive.
    NSData *plaintext = [hybridDecrypt decrypt:ciphertext withContextInfo:contextInfo error:&error];
    if (!plaintext || error) {
      // handle error.
    }