firebase-ios-sdk/FirebaseAuth/Sources/Swift/SystemService/SecureTokenService.swift

// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import Foundation

private let kFiveMinutes = 5 * 60.0

/// A class represents a credential that proves the identity of the app.
@available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
@objc(FIRSecureTokenService) // objc Needed for decoding old versions
class SecureTokenService: NSObject, NSSecureCoding {
  /// The configuration for making requests to server.
  var requestConfiguration: AuthRequestConfiguration?

  /// The cached access token.
  ///
  ///  This method is specifically for providing the access token to internal clients during
  /// deserialization and sign-in events, and should not be used to retrieve the access token by
  ///     anyone else.
  var accessToken: String

  /// The refresh token for the user, or `nil` if the user has yet completed sign-in flow.
  ///
  /// This property needs to be set manually after the instance is decoded from archive.
  var refreshToken: String?

  /// The expiration date of the cached access token.
  var accessTokenExpirationDate: Date?

  /// Creates a `SecureTokenService` with access and refresh tokens.
  /// - Parameter requestConfiguration: The configuration for making requests to server.
  /// - Parameter accessToken: The STS access token.
  /// - Parameter accessTokenExpirationDate: The approximate expiration date of the access token.
  /// - Parameter refreshToken: The STS refresh token.
  init(withRequestConfiguration requestConfiguration: AuthRequestConfiguration?,
       accessToken: String,
       accessTokenExpirationDate: Date?,
       refreshToken: String) {
    self.requestConfiguration = requestConfiguration
    self.accessToken = accessToken
    self.refreshToken = refreshToken
    self.accessTokenExpirationDate = accessTokenExpirationDate
    taskQueue = AuthSerialTaskQueue()
  }

  /// Fetch a fresh ephemeral access token for the ID associated with this instance. The token
  ///   received in the callback should be considered short lived and not cached.
  ///
  ///    Invoked asyncronously on the auth global work queue in the future.
  /// - Parameter forceRefresh: Forces the token to be refreshed.
  /// - Parameter callback: Callback block that will be called to return either the token or an
  /// error.
  func fetchAccessToken(forcingRefresh forceRefresh: Bool,
                        callback: @escaping (String?, Error?, Bool) -> Void) {
    taskQueue.enqueueTask { complete in
      if !forceRefresh, self.hasValidAccessToken() {
        complete()
        callback(self.accessToken, nil, false)
      } else {
        AuthLog.logDebug(code: "I-AUT000017", message: "Fetching new token from backend.")
        Task {
          do {
            let (token, tokenUpdated) = try await self.requestAccessToken(retryIfExpired: true)
            complete()
            callback(token, nil, tokenUpdated)
          } catch {
            complete()
            callback(nil, error, false)
          }
        }
      }
    }
  }

  private let taskQueue: AuthSerialTaskQueue

  // MARK: NSSecureCoding

  // Secure coding keys
  private let kAPIKeyCodingKey = "APIKey"
  private static let kRefreshTokenKey = "refreshToken"
  private static let kAccessTokenKey = "accessToken"
  private static let kAccessTokenExpirationDateKey = "accessTokenExpirationDate"

  static var supportsSecureCoding: Bool {
    true
  }

  required convenience init?(coder: NSCoder) {
    guard let refreshToken = coder.decodeObject(of: [NSString.self],
                                                forKey: Self.kRefreshTokenKey) as? String,
      let accessToken = coder.decodeObject(of: [NSString.self],
                                           forKey: Self.kAccessTokenKey) as? String else {
      return nil
    }
    let accessTokenExpirationDate = coder.decodeObject(
      of: [NSDate.self], forKey: Self.kAccessTokenExpirationDateKey
    ) as? Date
    // requestConfiguration is filled in after User is set by Auth.protectedDataInitialization.
    self.init(withRequestConfiguration: nil,
              accessToken: accessToken,
              accessTokenExpirationDate: accessTokenExpirationDate,
              refreshToken: refreshToken)
  }

  func encode(with coder: NSCoder) {
    // The API key is encoded even it is not used in decoding to be compatible with previous
    // versions of the library.
    coder.encode(requestConfiguration?.apiKey, forKey: kAPIKeyCodingKey)
    // Authorization code is not encoded because it is not long-lived.
    coder.encode(refreshToken, forKey: SecureTokenService.kRefreshTokenKey)
    coder.encode(accessToken, forKey: SecureTokenService.kAccessTokenKey)
    coder.encode(
      accessTokenExpirationDate,
      forKey: SecureTokenService.kAccessTokenExpirationDateKey
    )
  }

  // MARK: Private methods

  /// Makes a request to STS for an access token.
  ///
  /// This handles both the case that the token has not been granted yet and that it just
  /// needs to be refreshed. The caller is responsible for making sure that this is occurring in
  /// a  `_taskQueue` task.
  ///
  /// Because this method is guaranteed to only be called from tasks enqueued in
  /// `_taskQueue`, we do not need any @synchronized guards around access to _accessToken/etc.
  /// since only one of those tasks is ever running at a time, and those tasks are the only
  /// access to and mutation of these instance variables.
  /// - Returns: Token and Bool indicating if update occurred.
  private func requestAccessToken(retryIfExpired: Bool) async throws -> (String?, Bool) {
    // TODO: This was a crash in ObjC SDK, should it callback with an error?
    guard let refreshToken, let requestConfiguration else {
      fatalError("refreshToken and requestConfiguration should not be nil")
    }

    let request = SecureTokenRequest.refreshRequest(refreshToken: refreshToken,
                                                    requestConfiguration: requestConfiguration)
    let response = try await AuthBackend.call(with: request)
    var tokenUpdated = false
    if let newAccessToken = response.accessToken,
       newAccessToken.count > 0,
       newAccessToken != accessToken {
      if let tokenResult = try? AuthTokenResult.tokenResult(token: newAccessToken) {
        // There is an edge case where the request for a new access token may be made right
        // before the app goes inactive, resulting in the callback being invoked much later
        // with an expired access token. This does not fully solve the issue, as if the
        // callback is invoked less than an hour after the request is made, a token is not
        // re-requested here but the approximateExpirationDate will still be off since that
        // is computed at the time the token is received.
        if retryIfExpired {
          let expirationDate = tokenResult.expirationDate
          if expirationDate.timeIntervalSinceNow <= kFiveMinutes {
            // We only retry once, to avoid an infinite loop in the case that an end-user has
            // their local time skewed by over an hour.
            return try await requestAccessToken(retryIfExpired: false)
          }
        }
      }
      accessToken = newAccessToken
      accessTokenExpirationDate = response.approximateExpirationDate
      tokenUpdated = true
      AuthLog.logDebug(
        code: "I-AUT000017",
        message: "Updated access token. Estimated expiration date: " +
          "\(String(describing: accessTokenExpirationDate)), current date: \(Date())"
      )
    }
    if let newRefreshToken = response.refreshToken,
       newRefreshToken != self.refreshToken {
      self.refreshToken = newRefreshToken
      tokenUpdated = true
    }
    return (response.accessToken, tokenUpdated)
  }

  private func hasValidAccessToken() -> Bool {
    if let accessTokenExpirationDate,
       accessTokenExpirationDate.timeIntervalSinceNow > kFiveMinutes {
      AuthLog.logDebug(code: "I-AUT000017",
                       message: "Has valid access token. Estimated expiration date:" +
                         "\(accessTokenExpirationDate), current date: \(Date())")
      return true
    }
    AuthLog.logDebug(code: "I-AUT000017",
                     message: "Does not have valid access token. Estimated expiration date:" +
                       "\(String(describing: accessTokenExpirationDate)), current date: \(Date())")
    return false
  }
}