Regardless of the person who has your iPhone, iPad, Mac, Apple Watch or Apple TV, Siri will understand anyone's voice. But, if a new patent filing is anything to go by, then that might soon change.
Apple Wants Siri to be as Personal as Ever - Wants Her to Recognize Your Voice Only
Any digital assistant is great to have as long as it does not turn into a security threat for the user. In the past, we have seen instances where Siri went off track and allowed access to a user's photos and contact details. In order to avoid that, a new security measure needs to be put in place which would allow Siri to recognize the voice of the user only and no one else. It seems as though Apple has figured this problem out, if its new patent applications is anything to go by.
Up until now when a digital assistant has been invoked with a voice command, the digital assistant is responsive to the speech itself, not to the speaker. Consequently, a user other than the owner of the electronic device is able to utilize the digital assistant, which may not be desirable in all circumstances […]
Apple’s invention provides a method to train Siri to know the voice of the owner of the iDevice before responding to a command.
Apple plans to boost security and identifiability by replacing the 'Hey Siri' keyword with something else that is absolutely unique for both the user and Siri. This is something which can be defined as a Siri access code. If Siri fails to verify the user's voice then it would revert to the second authentication method which happens to be a passcode or Touch ID. If all is well, then Siri will readily answer your questions or make a phone call or two.
Since this is nothing more than a patent for now, there's no telling whether this feature would actually end up being a part of iOS, macOS, watchOS or even tvOS. But given we are pretty close to the official unveiling of iOS 11, it won't be long till Apple's plans are revealed to the world in their entirety.