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

/** @var kReceiptKey
    @brief The key used to encode the receipt property for NSSecureCoding.
 */
private let kReceiptKey = "receipt"

/** @var kSecretKey
    @brief The key used to encode the secret property for NSSecureCoding.
 */
private let kSecretKey = "secret"

private let kFiveMinutes = 5 * 60.0

/** @class FIRAuthAppCredential
    @brief A class represents a credential that proves the identity of the app.
 */
@objc(FIRSecureTokenService) public class SecureTokenService: NSObject, NSSecureCoding {
  /** @property requestConfiguration
      @brief The configuration for making requests to server.
   */
  @objc public var requestConfiguration: AuthRequestConfiguration?

  /** @property accessToken
      @brief The cached access token.
      @remarks 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.
   */
  @objc public var accessToken: String

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

  /** @property accessTokenExpirationDate
      @brief The expiration date of the cached access token.
   */
  @objc public var accessTokenExpirationDate: Date?

  /** @fn initWithRequestConfiguration:accessToken:accessTokenExpirationDate:refreshToken
      @brief Creates a @c FIRSecureTokenService with access and refresh tokens.
      @param requestConfiguration The configuration for making requests to server.
      @param accessToken The STS access token.
      @param accessTokenExpirationDate The approximate expiration date of the access token.
      @param refreshToken The STS refresh token.
   */
  @objc public init(withRequestConfiguration requestConfiguration: AuthRequestConfiguration?,
                    accessToken: String,
                    accessTokenExpirationDate: Date?,
                    refreshToken: String) {
    self.requestConfiguration = requestConfiguration
    self.accessToken = accessToken
    self.refreshToken = refreshToken
    self.accessTokenExpirationDate = accessTokenExpirationDate
    taskQueue = AuthSerialTaskQueue()
  }

  /** @fn fetchAccessTokenForcingRefresh:callback:
      @brief 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.
      @param forceRefresh Forces the token to be refreshed.
      @param callback Callback block that will be called to return either the token or an error.
          Invoked asyncronously on the auth global work queue in the future.
   */
  @objc public 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.")
        self.requestAccessToken(retryIfExpired: true) { token, error, tokenUpdated in
          complete()
          callback(token, error, tokenUpdated)
        }
      }
    }
  }

  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"

  public static var supportsSecureCoding: Bool {
    true
  }

  public 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
    // TODO: the nil matches the ObjC implementation, but doesn't seem right.
    self.init(withRequestConfiguration: nil,
              accessToken: accessToken,
              accessTokenExpirationDate: accessTokenExpirationDate,
              refreshToken: refreshToken)
  }

  public 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

  /** @fn requestAccessToken:
      @brief Makes a request to STS for an access token.
      @details 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 @c _taskQueue task.
      @param callback Called when the fetch is complete. Invoked asynchronously on the main thread in
          the future.
      @remarks Because this method is guaranteed to only be called from tasks enqueued in
          @c _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.
   */
  private func requestAccessToken(retryIfExpired: Bool,
                                  callback: @escaping (String?, Error?, Bool) -> Void) {
    // 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)
    AuthBackend.post(withRequest: request) { rawResponse, error in
      var tokenUpdated = false
      if let response = rawResponse as? SecureTokenResponse {
        if let newAccessToken = response.accessToken,
           newAccessToken.count > 0,
           newAccessToken != self.accessToken {
          let tokenResult = 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,
             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.
            self.requestAccessToken(retryIfExpired: false, callback: callback)
            return
          }
          self.accessToken = newAccessToken
          self.accessTokenExpirationDate = response.approximateExpirationDate
          tokenUpdated = true
          AuthLog.logDebug(
            code: "I-AUT000017",
            message: "Updated access token. Estimated expiration date: " +
              "\(String(describing: self.accessTokenExpirationDate)), current date: \(Date())"
          )
        }
        if let newRefreshToken = response.refreshToken,
           newRefreshToken != self.refreshToken {
          self.refreshToken = newRefreshToken
          tokenUpdated = true
        }
        callback(response.accessToken, error, tokenUpdated)
        return
      }
      // Not clear this fall through case was considered in original ObjC implementation.
      callback(nil, error, false)
    }
  }

  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
  }
}