metatext/Notification Service Extension/NotificationService.swift

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// Copyright © 2020 Metabolist. All rights reserved.
import CryptoKit
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import Keychain
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import Mastodon
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import Secrets
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import UserNotifications
class NotificationService: UNNotificationServiceExtension {
var contentHandler: ((UNNotificationContent) -> Void)?
var bestAttemptContent: UNMutableNotificationContent?
override func didReceive(
_ request: UNNotificationRequest,
withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
self.contentHandler = contentHandler
bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)
guard let bestAttemptContent = bestAttemptContent else { return }
let pushNotification: PushNotification
do {
let decryptedJSON = try Self.extractAndDecrypt(userInfo: request.content.userInfo)
pushNotification = try MastodonDecoder().decode(PushNotification.self, from: decryptedJSON)
} catch {
contentHandler(bestAttemptContent)
return
}
bestAttemptContent.title = pushNotification.title
bestAttemptContent.body = pushNotification.body
let fileName = pushNotification.icon.lastPathComponent
let fileURL = URL(fileURLWithPath: NSTemporaryDirectory(), isDirectory: true).appendingPathComponent(fileName)
do {
let iconData = try Data(contentsOf: pushNotification.icon)
try iconData.write(to: fileURL)
bestAttemptContent.attachments = [try UNNotificationAttachment(identifier: fileName, url: fileURL)]
} catch {
// no-op
}
contentHandler(bestAttemptContent)
}
override func serviceExtensionTimeWillExpire() {
if let contentHandler = contentHandler, let bestAttemptContent = bestAttemptContent {
contentHandler(bestAttemptContent)
}
}
}
enum NotificationServiceError: Error {
case userInfoDataAbsent
case keychainDataAbsent
}
private extension NotificationService {
static let identityIDUserInfoKey = "i"
static let encryptedMessageUserInfoKey = "m"
static let saltUserInfoKey = "s"
static let serverPublicKeyUserInfoKey = "k"
static let keyLength = 16
static let nonceLength = 12
static let pseudoRandomKeyLength = 32
static let paddedByteCount = 2
static let curve = "P-256"
enum HKDFInfo: String {
case auth, aesgcm, nonce
var bytes: [UInt8] {
Array("Content-Encoding: \(self)\0".utf8)
}
}
static func extractAndDecrypt(userInfo: [AnyHashable: Any]) throws -> Data {
guard
let identityIDString = userInfo[identityIDUserInfoKey] as? String,
let identityID = UUID(uuidString: identityIDString),
let encryptedMessageBase64 = (userInfo[encryptedMessageUserInfoKey] as? String)?.URLSafeBase64ToBase64(),
let encryptedMessage = Data(base64Encoded: encryptedMessageBase64),
let saltBase64 = (userInfo[saltUserInfoKey] as? String)?.URLSafeBase64ToBase64(),
let salt = Data(base64Encoded: saltBase64),
let serverPublicKeyBase64 = (userInfo[serverPublicKeyUserInfoKey] as? String)?.URLSafeBase64ToBase64(),
let serverPublicKeyData = Data(base64Encoded: serverPublicKeyBase64)
else { throw NotificationServiceError.userInfoDataAbsent }
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let secretsService = Secrets(identityID: identityID, keychain: LiveKeychain.self)
guard
let auth = try secretsService.getPushAuth(),
let pushKey = try secretsService.getPushKey()
else { throw NotificationServiceError.keychainDataAbsent }
return try decrypt(encryptedMessage: encryptedMessage,
privateKeyData: pushKey,
serverPublicKeyData: serverPublicKeyData,
auth: auth,
salt: salt)
}
static func decrypt(encryptedMessage: Data,
privateKeyData: Data,
serverPublicKeyData: Data,
auth: Data,
salt: Data) throws -> Data {
let privateKey = try P256.KeyAgreement.PrivateKey(x963Representation: privateKeyData)
let serverPublicKey = try P256.KeyAgreement.PublicKey(x963Representation: serverPublicKeyData)
let sharedSecret = try privateKey.sharedSecretFromKeyAgreement(with: serverPublicKey)
var keyInfo = HKDFInfo.aesgcm.bytes
var nonceInfo = HKDFInfo.nonce.bytes
var context = Array(curve.utf8)
let publicKeyData = privateKey.publicKey.x963Representation
context.append(0)
context.append(0)
context.append(UInt8(publicKeyData.count))
context += Array(publicKeyData)
context.append(0)
context.append(UInt8(serverPublicKeyData.count))
context += Array(serverPublicKeyData)
keyInfo += context
nonceInfo += context
let pseudoRandomKey = sharedSecret.hkdfDerivedSymmetricKey(
using: SHA256.self,
salt: auth,
sharedInfo: HKDFInfo.auth.bytes,
outputByteCount: pseudoRandomKeyLength)
let key = HKDF<SHA256>.deriveKey(
inputKeyMaterial: pseudoRandomKey,
salt: salt,
info: keyInfo,
outputByteCount: keyLength)
let nonce = HKDF<SHA256>.deriveKey(
inputKeyMaterial: pseudoRandomKey,
salt: salt,
info: nonceInfo,
outputByteCount: nonceLength)
let sealedBox = try AES.GCM.SealedBox(combined: nonce.withUnsafeBytes(Array.init) + encryptedMessage)
let decrypted = try AES.GCM.open(sealedBox, using: key)
let unpadded = decrypted.suffix(from: paddedByteCount)
return Data(unpadded)
}
}
extension String {
func URLSafeBase64ToBase64() -> String {
var base64 = replacingOccurrences(of: "-", with: "+").replacingOccurrences(of: "_", with: "/")
let countMod4 = count % 4
if countMod4 != 0 {
base64.append(String(repeating: "=", count: 4 - countMod4))
}
return base64
}
}