Apple thinks it has a solution, using a combination of a special phrase and authenticating your unique voice to ensure that Siri responds to you and only to you …
Patently Apple spotted that Apple has applied for a patent for this solution.
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.
The approach Apple describes would ask you to choose a word or phrase as your Siri access code, which could then be used in place of ‘Hey Siri.’ Your iOS device would store both the phrase itself and characteristics of your voice, and would only allow Siri to be activated if this combination was detected.
If your voice doesn’t appear to match the stored pattern, your iPhone or iPad would ask you to use Touch ID or enter your passcode to verify your identity.
Distinguishing an individual voice is exceedingly difficult to do reliably, hence the requirement for a specific phrase, vastly reducing the complexity of the matching process. This is the approach taken by many commercial voice verification security systems like telephone banking services. It’s the fact that you are speaking a known phrase – such as your place of birth – that allows the system to reliably identify your voice.
As ever with Apple patents, there’s no telling whether or when Apple may choose to implement it.