我正在尝试使用--recursive从github克隆一个示例应用程序以获取所需的子模块：
$ git clone --recursive https://github.com/opentok/OpenTok-iOS-Hello-World.git
Cloning into 'OpenTok-iOS-Hello-World'...
remote: Counting objects: 82, done.
remote: Compressing objects: 100% (62/62), done.
remote: Total 82 (delta 24), reused 71 (delta 13)
Unpacking objects: 100% (82/82), done.
Submodule 'opentok-ios-sdk' (git@github.com:opentok/opentok-ios-sdk.git) registered for path 'opentok-ios-sdk'
Cloning into 'opentok-ios-sdk'...
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
Clone of 'git@github.com:opentok/opentok-ios-sdk.git' into submodule path 'opentok-ios-sdk' failed
如何避免“权限被拒绝”错误？