A recent government filing in connection with the continuing fight between Apple and the FBI over data encryption issues indicated that the U.S. Justice Department has a plan B that will demand the electronic signature of the tech giant.

As implied by the Department of Justice, if the Cupertino, California-based company does not comply with the court order, it may be compelled to surrender the source code of the whole operating system.

The legal rebuttal of the Justice Department addressed one of the main legal arguments made by Apple. According to the tech giant, forcing it to write the code will take away key security features from the iPhone of the terrorist Syed Farook.

In its filing, the department wrote a footnote indicating that "The FBI cannot itself modify the software on Farook's iPhone without access to the source code and Apple's private electronic signature."

"The government did not seek to compel Apple to turn those over because it believed such a request would be less palatable to Apple," continued the filing.

"If Apple would prefer that course, however, that may provide an alternative that requires less labor by Apple programmers," the filing added.

Could this latest legal strategy of the FBI be the Achilles heel of Apple Inc.? If successful, the government agency could demand that the tech giant surrender its iOS source code and its encryption key that it uses to sign updates.

The Justice Department's footnote in its legal filing said the agency would be happy to have the source code and the digital signing key of the tech giant and not require the company to create a tool that will unlock the terrorist's iPhone any more.

There is a precedent for a court order such as this. It was done in August 2013 during the NSA-Edward Snowden scandal when it later turned out that the FBI attempted to shut down Lavabit, an NSA secure email service.

Ladar Levison, the site owner was being forced then by the FBI to turn over his encryption keys to enable the agency to monitor Snowden's email messages.