Oblivious HTTP client for Swift

Description

This project provides a way to use Oblivious HTTP in iOS client applications via the provided URLProtocol. Implementation corresponds to version 8 of IETF Oblivious HTTP draft, and is confirmed to be working with the relay and the gateway implementations from Cloudflare.

Installation

Swift Package Manager

You can use The Swift Package Manager to install OHTTPClientSwift by adding the dependency to your Package.swift file:

// swift-tools-version:4.0
import PackageDescription


let package = Package(
    name: "YOUR_PROJECT_NAME",
    dependencies: [
        .package(url: "git@github.com:flohealth/ohttp-client-swift.git", from: "0.1.0"),
    ]
)

Usage

1. Import OHTTPSwift
2. Provide your OHTTP configuration by calling OHTTPSwift.configure. If not configured, OHTTP will be disabled and throw an assertion in debug if any requests pass. Configuration consists of:
   1. Custom value of the User-Agent header for OHTTP requests. Default iOS implementation may give too much information about the client, increasing the chances of identifying a user. Hence, the recommendation is to use something more opaque, such as a plain application name.
   2. URL to gateway endpoint with OHTTP keys configuration
   3. URL to OHTTP relay
3. Insert OHTTPSwift.urlProtocol into your URLSession configuration. You can do this at any point in time, even after steps 4 and 5.
4. Set OHTTPSwift.isEnabled to true (it is false by default) when you are ready to turn on OHTTP for all URLSessions configured in step 3.
5. At last, the recommendation is to call OHTTPSwift.fetchOHTTPKeyConfig() at certain times (for example, once at the app launch) for all app users - even if some of them have OHTTP turned off. The same behavior for all users makes it harder to identify which ones use OHTTP and which don't.

// Import the Oblivious HTTP library
import OHTTPSwift

// Configure it
OHTTPSwift.configure(
 userAgent: "The Application (iOS)",
 configURL: URL(string: "https://www.theapp-gateway.com/ohttp-keys")!,
 relayURL: URL(string: "https://www.theapp.relay.cloudflare.com/relay")!
)

// Configure URLSession
let defaultConfig = URLSessionConfiguration.default
if var currentProtocols = defaultConfig.protocolClasses {
 currentProtocols.insert(OHTTPSwift.urlProtocol, at: 0)
 defaultConfig.protocolClasses = currentProtocols
} else {
 defaultConfig.protocolClasses = [OHTTPSwift.urlProtocol]
}
let urlSession = URLSession(configuration: defaultConfig, delegate: nil, delegateQueue: nil)

// Turn on OHTTP
OHTTPSwift.isEnabled = true

// Initiate config fetch
OHTTPSwift.fetchOHTTPKeyConfig()

How this library manages OHTTP keys configuration

This library has a built-in key caching and invalidation mechanism. It relies on URLSession default caching logic and thus requires standard HTTP caching headers (like 'Cache-Control') to be in place in the key config response. Nothing will be cached without them, and keys will be fetched for each request. If you wish to have custom keys fetching and caching logic, use an alternative version of OHTTPSwift.configure: the one that takes OHTTPKeyConfigClient parameter instead of the plain url - you can pass your own implementation this way. This interface has a single function, fetchKeyConfiguration, with a Bool parameter indicating whether the cache needs to be invalidated.

If the relay answers with HTTP code 401 to any request, this library will treat it as 'key config is invalid' and will try to invalidate current key cache and fetch new key from the remote, then automatically retry the request.

Limitations

OHTTP cannot be enabled globally for the whole app - each URLSession should be configured explicitly to use the provided URLProtocol. It means that URLSessions you don't control, such as from other 3rd-party libraries, will still work without OHTTP. Also, there is currently no way to set up WKWebViews to work with URLProtocol, so all traffic from them will also be unaffected.

As OHTTP URLProtocol significantly changes the client-server interaction, we can't prove that every feature of the HTTP protocol will correctly work with OHTTP enabled. Please perform proper testing of your use cases before use.

All limitations of bhttp-swift are also applied.

License

Released under MIT License.