Improper Protection of Physical Side Channels

Description

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

Consider a device that checks a passcode to unlock the screen.

As each character of
		    the PIN number is entered, a correct character
		    exhibits one current pulse shape while an
		    incorrect character exhibits a different current
		    pulse shape.

PIN numbers used to unlock a cell phone should not exhibit any characteristics about themselves. This creates a side channel. An attacker could monitor the pulses using an oscilloscope or other method. Once the first character is correctly guessed (based on the oscilloscope readings), they can then move to the next character, which is much more efficient than the brute force method of guessing every possible sequence of characters.

Rather than comparing
		    each character to the correct PIN value as it is
		    entered, the device could accumulate the PIN in a
		    register, and do the comparison all at once at the
		    end. Alternatively, the components for the
		    comparison could be modified so that the current
		    pulse shape is the same regardless of the
		    correctness of the entered
		    character.

Example Two

Consider the device vulnerability CVE-2021-3011, which affects certain microcontrollers [REF-1221]. The Google Titan Security Key is used for two-factor authentication using cryptographic algorithms. The device uses an internal secret key for this purpose and exchanges information based on this key for the authentication. If this internal secret key and the encryption algorithm were known to an adversary, the key function could be duplicated, allowing the adversary to masquerade as the legitimate user.

The local method of extracting the secret key consists of plugging the key into a USB port and using electromagnetic (EM) sniffing tools and computers.

Several solutions could have been considered by the manufacturer. For example, the manufacturer could shield the circuitry in the key or add randomized delays, indirect calculations with random values involved, or randomly ordered calculations to make extraction much more difficult or a combination of these techniques.

See Also