sdk/communication/AzureCommunicationChat/Source/PushNotification/PushNotificationKeyStorage.swift

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import AzureCore
import Foundation

/*
 PushNotificationKeyStorage is used to manage encryption keys which are used to encrypt and decrypt the notification payload.
 If you want to implement the advanced version of Push Notification and customize the content in alert banner, please create a concrete class which conform to this protocol. Before you call chatClient.startPushNotifications(), please create a concrete key handler and assign it to chatClient.pushNotificationKeyStorage.
 Chat SDK provides a default implementation AppGroup PushNotificationKeyStorage for you to use. You can also create your own if you want.
 */
public protocol PushNotificationKeyStorage: AnyObject {
    /// Persist the encryption key in local persistent storage.
    /// - Parameters:
    ///   - encryptionKey: a string which contains two keys. The first half is the key for authorization and the second
    /// half is the key for encryption. It is generated by chat sdk.
    ///   - expiryTime: 45 minutes after the key is created. This time is generated by chat sdk.
    func onPersistKey(_ encryptionKey: String, expiryTime: Date) throws

    /// Retrieve the valid keys previously stored in local persistent storage.
    func onRetrieveKeys() throws -> [String]
}

extension PushNotificationKeyStorage {
    /// This method is used to decrypt notification payload in notification service extension.
    /// - Parameters:
    ///   - notification: The APNS push notification payload ( including "aps" and "data" )
    public func decryptPayload(
        notification: [AnyHashable: Any]
    ) throws -> PushNotificationEvent {
        // Retrieve the "data" part from the APNS push notification payload
        guard let dataPayload = notification["data"] as? [String: AnyObject] else {
            throw AzureError.client("Push notification does not contain data payload")
        }

        do {
            // 1.get "eventId"
            guard let eventId = dataPayload["eventId"] as? Int else {
                throw AzureError
                    .client("Push notification does not contain eventId or eventId can't be downcast to Int.")
            }

            let chatEventType = try PushNotificationChatEventType(forCode: eventId)

            // 2.get "e"
            guard let encryptedPayload = dataPayload["e"] as? String else {
                throw AzureError
                    .client(
                        "Push notification does not contain encryptedPayload or payload can't be downcast to String."
                    )
            }

            // 3.get keys for decryption
            let encryptionKeys = try onRetrieveKeys()

            guard encryptionKeys.count > 0 else {
                throw AzureError.client("Failed to retrieve valid encryption key from UserDefaults.")
            }

            // 4.Verify and decrypt the encrypted notification payload
            let decryptedPayload = try decryptPayload(
                encryptedStr: encryptedPayload,
                encryptionKeys: encryptionKeys
            )

            guard let data = decryptedPayload.data(using: .utf8) else {
                throw AzureError.client("Failed to create utf8 encoded Data from decrypted string.")
            }

            // 5. Create and return the PushNotificationEvent Model
            let pushNotificationEvent = try PushNotificationEvent(chatEventType: chatEventType, from: data)

            return pushNotificationEvent

        } catch {
            throw AzureError.client("Error in decrypting the notification payload: \(error)")
        }
    }

    internal func decryptPayload(
        encryptedStr: String,
        encryptionKeys: [String]
    ) throws -> String {
        do {
            // 1.Decode the Base64 input string into [UInt8].
            guard let decodedData = Data(base64Encoded: encryptedStr) else {
                throw AzureError
                    .client(
                        "Failed to initialize a data object with the given cryptoKey. Please ensure the encryptedStr is a Base64 encoded string."
                    )
            }
            let encryptedBytes = Array(decodedData)

            // 2.Split [UInt8] into different blocks.
            let iv: [UInt8] = CryptoUtils.extractInitializationVector(result: encryptedBytes)
            let cipherText: [UInt8] = CryptoUtils.extractCipherText(result: encryptedBytes)
            let hmac: [UInt8] = CryptoUtils.extractHmac(result: encryptedBytes)
            let cipherModeIVCipherText: [UInt8] = CryptoUtils.extractCipherModeIVCipherText(result: encryptedBytes)

            // 3.Loop over the key array and find the key used for encryption
            for encryptionKey in encryptionKeys {
                let encryptionKeys = try splitEncryptionKey(encryptionKey: encryptionKey)
                let aesKey = encryptionKeys[0]
                let authKey = encryptionKeys[1]

                // Each auth key can be computed into a unique HMAC signature. If the computed signature matches the
                // included signature, "verifyHMACResult" will be true.
                let verifyHMACResult = try verifyEncryptedPayload(
                    cipherModeIVCipherText: cipherModeIVCipherText,
                    authKey: authKey,
                    actualHmac: hmac
                )

                // If the "verifyHMACResult" is true, it indicates that the current keyPair is the one used by PNH for
                // encryption.
                if verifyHMACResult {
                    // Here we use aesKey to decrypt the payload.
                    let decryptedString = try decryptPushNotificationPayload(
                        cipherText: cipherText,
                        iv: iv,
                        cryptoKey: aesKey
                    )

                    return decryptedString
                }
            }

            // 4. Failed to decrypt the push notification if thre is no keyPair used for encryption.
            throw AzureError
                .client(
                    "Failed to decrypt the push notification."
                )

        } catch {
            throw AzureError.client("Error in decrypting the notification payload: \(error)")
        }
    }
}