A generic framework to build secured QR code for contact tracing. (ie. to fight COVID-19)

Disclamer: not production ready! Not validated by a security audit. Don't integrate it in a product. It's only a demonstration.

Simply add https://github.com/florentmorin/QRContactTracingCore to Swift Package via Xcode.

This will describe a generic workflow.

First, implement Code to describe the code your users will scan or will enter manually.

import QRContactTracingCore

struct MyCode: Code {

    enum Option: String, Hashable {
        case type       = "t"
        case capacity   = "c"
    }

    enum CodeType: String, Hashable {
       case person      = "p"
       case house       = "h"
       case enterprise  = "e"
       case shop        = "s"
    }
    
    let id: UUID
    let options: [Option: Any]

    var type: CodeType? {
        options[.type] as? CodeType
    }

    var capacity: Int? {
        options[.capacity] as? Int
    }
    
    init?(url: URL) {
        <# set `id` and `options` from url #>
    }
    
    init(id: UUID, options: [Option : Any] = [:]) {
        self.id = id
        self.options = options
    }
    
    func buildURL() -> URL? {
        <# build URL from `id` and `options` #>
    }
}

Then, you can create a code from URL after a scan:

let code = MyCode(url: <# received URL #>)

Or you can create a code from raw data after a manual input:

let code = MyCode(id: <# UUID from text #>)

This can also be used to generate a code. The URL will be built with:

let code = MyCode(id: UUID())
let url = myCode.buildURL()

You can generate an image of your QR code for your favorite platform:

struct MyCode: Code {
    // ....
}

extension MyCode: UIKitCode { }
extension MyCode: AppKitCode { }
extension MyCode: CoreGraphicsCode { }
extension MyCode: CoreImageCode { }
extension MyCode: SwiftUICode { }

// Raw PNG data (to store in file)
let imageData = code.pngData()
let imageData = code.pngData(length: 200)

// SwiftUI Image (macOS, iOS, macOS Catalyst, tvOS)
let image = code.image()
let image = code.image(length: 200)

// UIImage (iOS, macOS Catalyst, tvOS)
let image = code.uiImage()
let image = code.uiImage(length: 200)

// NSImage (macOS)
let image = code.nsImage()
let image = code.nsImage(length: 200)

// CGImage (macOS, iOS, macOS Catalyst, tvOS)
let image = code.cgImage()
let image = code.cgImage(length: 200)

// CIImage (macOS, iOS, macOS Catalyst, tvOS)
let image = code.ciImage()
let image = code.ciImage(length: 200)

You can easily transform AVMetadataObject to Code.

struct MyCode: Code {
    // ....
}

extension MyCode: AVMetadataCode { }

func setup() {
    captureSession = AVCaptureSession()
    // ...
    let metadataOutput = AVCaptureMetadataOutput()
    // ...
    metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
    metadataOutput.metadataObjectTypes = [.qr]
}

// AVCaptureMetadataOutputObjectsDelegate

func metadataOutput(_ output: AVCaptureMetadataOutput,
                    didOutput metadataObjects: [AVMetadataObject],
                    from connection: AVCaptureConnection) {
   // `MyCode` conforming to `AVMetadataCode`
   if let code = MyCode.create(from: metadataObjects)?.first {
       // code found
   }
}

You can store a code with your preferred method: Core Data, Realm, JSON file, raw data.

You need to store at least 3 informations:

• Code identifier
• Date of scan / manual input (calculation is based on day + month + year)
• A 128 bits key generated from code identifier, date offset (if present) and date (called transportableKey)

So, once you received the code, you need to generate local content like this:

let content = LocalContent(codeId: <# The ID from code #>, date: <# Date of scan #>)

// Then, you can use `content.codeId`, `content.date` and `content.transportableKey`

<# Store your data from content #>

If you want to add an offset for each date (ie 1 = morning, 2 = afternoon), it's possible.

You just need to store date offset. Date offset will be used in key calculation.

let content = LocalContent(codeId: <# The ID from code #>, date: <# Date of scan #>, dateOffset: <# Offset #>)

// Then, you can use `content.codeId`, `content.date`, `content.dateOffset` and `content.transportableKey`

<# Store your data from content #>

Your data are stored locally and you want to share them with other users.

First, rebuild your content:

let localContent = LocalContent(codeId: <# UUID #>, date: <# Date #>, transportableKey: <# Data #>)

Then, create transportable content with data required for diagnosis:

let transportableContent = TransportableContent(localContent: <# LocalContent #>, clearData: <# Data #>)

Now, you can export diagnosis data, now encrypted:

let key = transportableContent.key
let encryptedData = transportableContent.encryptedData

// Here, you can package key + encrypted data

Your users will receive key and, now or later, encrypted data.

How to find LocalContent from TransportableContent?

Simply search local content in your local storage with LocalContent.transportableKey which is the same than TransportableContent.key.

You've got LocalContent with corresponding transportableKey? Retrieve all the encrypted data corresponding to this key.

Now, you can rebuild transportable data:

let content = TransportableContent(key: <# Data #>, encryptedData: <# Data #>)

It's so easy:

let localContent: LocalContent
let receivedContent: TransportableContent

let diagnosisData = receivedContent.decryptData(localContent: localContent)

Date offset can depend on time spent in a specific place:

• if you are in a restaurant, time offset will represent a meal time
• if you are in an enterprise, time offset will represent a team turning time
• if you are in a school classroom, it will represent an hour of time.

You can adjust it by passing information using code.

In example, you can consider default date offset depending of place type. Place type is sent by code.

But, if you want to specify current date offset, you can use a specific code (ie. "do=15" in URL means that date offset is 15).

Or you can specify all date offsets in code and current date offset is calculated. In exemple, you can use these informations:

• do1=00000430 means date offset 1 represents period from 00:00 to 04:30
• do2=04301230 means date offset 2 represents period from 04:30 to 12:30
• do3=12301900 means date offset 3 represents period from 12:30 to 19:00
• do4=19002359 means date offset 4 represents period from 19:00 to 23:59

First, local content generate a local identifier using UUIDv4, which is a random 16 bytes content. Perfect 128-bits key. Stored locally, exclusively.

An other 128-bits key is generated using PBKDF2, which is slow to avoid brute-force attacks. This key is generated using date as password and local identifier as salt. This new key is stored on every device scanning the QR code at the same date (not date and time, date only).

When user need to share its keys and diagnosis informations, these informations are encrypted using AES-128 algorithm with a PKCS7 padding. Local key is used as encryption key and transportable key as initialization vector.

If you scan a QR code for multiple times a day, you will have a 128 bits transportable key associated to multiple diagnosis informations. (date & time, sickness level)

So, encrypted data will be shared across network. Then distributed by a server. And received by every users devices.

Once receiving transportable content, device will first check the transportable key then download associated encrypted diagnosis information. With its local content, it will have a stored local key slowly generated by scan. The local key will be used to decrypt content.

It seems to be OK for privacy and security because sensitive key is generated locally and don't move outside except if user want to share securely its diagnosis.

QR code can be generated locally, without internet connection.

Every information leaving the device is encrypted.

A shop can have a QR code. An enterprise can also have a QR code. Or simply your house.

Everyone can have a QR code generated by device. You can scan QR code like checking your hands to say "hello!".

And it's also inclusive: a QR code can be printed for disabled or old persons. The 16 bytes code can be manually written on a paper or simply distributed as printed piece of paper when you enter in a shop.

Complementary diagnosis informations can be shared here.