In a letter posted online on Tuesday night, Cook responded to a federal order asking for Apple to help the FBI crack into an iPhone belonging to Syed Farook, one of the San Bernardino, California, attackers. It's just the latest development in the ongoing encryption debate between Silicon Valley and the government.
"We have great respect for the professionals at the FBI, and we believe their intentions are good. Up to this point, we have done everything that is both within our power and within the law to help them," Cook wrote. "But now the U.S. government has asked us for something we simply do not have, and something we consider too dangerous to create. They have asked us to build a backdoor to the iPhone."
What Could Happen If the Government Had a 'Master Key' to Your Phone
Cook said the FBI is essentially asking Apple to build a new operating system that could be installed on an iPhone recovered from an investigation. Such software does not exist today but Cook said if it did, there would be no way to guarantee it would only be used for investigations, putting the privacy of millions of Americans at risk.
"The government would have us remove security features and add new capabilities to the operating system, allowing a passcode to be input electronically. This would make it easier to unlock an iPhone by 'brute force' trying thousands or millions of combinations with the speed of a modern computer," Cook wrote.
The 'Chilling' Implications
The federal government's request was made under the All Writs Act, a colonial law which allows the government to use its authority to issue orders that are not covered by a statute.
"The implications of the government’s demands are chilling," Cook wrote. "If the government can use the All Writs Act to make it easier to unlock your iPhone, it would have the power to reach into anyone’s device to capture their data."
He suggested it could be taken further with the government demanding Apple create surveillance software to "intercept your messages, access your health records or financial data, track your location, or even access your phone’s microphone or camera without your knowledge."
How Apple's Security Features Work
Apple turns on encryption by default, meaning law enforcement would have to have a person's passcode to access any data on an iPhone.
"For many years, we have used encryption to protect our customers’ personal data because we believe it’s the only way to keep their information safe," Cook said. "We have even put that data out of our own reach, because we believe the contents of your iPhone are none of our business."
The 4-digit code you enter into your phone initiates a complex calculation which generates a unique key to unlock the data on the phone. No key, no data. The auto erase function, if triggered, will wipe out all the encryption keys rendering the data on the iPhone useless.
The iPhone has another feature to frustrate automated attempts to unlock a phone. A 4-digit code would produce 9,999 unique possibilities. Not a particularly big challenge by itself, but the code must be punched in manually. This would be time consuming enough, but after five failed attempts, the iPhone will require the the user to wait one minute before another attempt. After attempt six the wait is five minutes. Attempt seven and eight, 15 minutes and an hour after the ninth try. More time can be added in the software.
ABC News' Jack Date contributed to this report.