CWE-1255: Comparison Logic is Vulnerable to Power Side-Channel Attacks

Description:
A device's real time power consumption may be monitored during security token evaluation and the information gleaned may be used to determine the value of the reference token.

Extended Description:
The power consumed by a device may be instrumented and monitored in real time. If the algorithm for evaluating security tokens is not sufficiently robust, the power consumption may vary by token entry comparison against the reference value. Further, if retries are unlimited, the power difference between a "good" entry and a "bad" entry may be observed and used to determine whether each entry itself is correct thereby allowing unauthorized parties to calculate the reference value.

Affected languages: Not Language-Specific
Affected technologies: Not Technology-Specific
Parent weaknesses: CWE-1300

Consequences:
  - Confidentiality: Modify Memory

Mitigations:
  - [Architecture and Design] The design phase must consider each check of a security token against a standard and the amount of power consumed during the check of a good token versus a bad token. The alternative is an all at once check where a retry counter is incremented PRIOR to the check.
  - [Architecture and Design] Another potential mitigation is to parallelize shifting of secret data (see example 2 below). Note that the wider the bus the more effective the result.
  - [Architecture and Design] An additional potential mitigation is to add random data to each crypto operation then subtract it out afterwards. This is highly effective but costly in performance, area, and power consumption. It also requires a random number generator.
  - [Implementation] If the architecture is unable to prevent the attack, using filtering components may reduce the ability to implement an attack, however, consideration must be given to the physical removal of the filter elements.

Demonstrative examples:
Consider an example hardware module that checks a user-provided password (or PIN) to grant access to a user. The user-provided password is compared against a stored value byte-by-byte.
Since the algorithm uses a different number of 1's and 0's for password validation, a different amount of power is consumed for the good byte versus the bad byte comparison. Using this information, an attacker may be able to guess the correct password for that byte-by-byte iteration with several repeated attempts by stopping the password evaluation before it completes.
static nonvolatile password_tries = NUM_RETRIES; 
           do 
	    
             while (password_tries == 0) ; // Hang here if no more password tries 
             password_ok = 0; 
             for (i = 0; i < NUM_PW_DIGITS; i++) 
	      


This code demonstrates the transfer of a secret key using Serial-In/Serial-Out shift. It's easy to extract the secret using simple power analysis as each shift gives data on a single bit of the key.
This code demonstrates the transfer of a secret key using a Parallel-In/Parallel-Out shift. In a parallel shift, data confounded by multiple bits of the key, not just one.
module siso(clk,rst,a,q); 
	     
              input a; 
              input clk,rst; 
              output q; 
              reg q; 
               
              always@(posedge clk,posedge rst) 
              begin 
	       
		if(rst==1'b1) 
		 
                  q<1'b0; 
		 
		else 
		 
                  q<a; 
		 
	       
              end 
	     
            endmodule
module pipo(clk,rst,a,q); 
	    
             input

Real-world examples:
  - CVE-2020-12788: CMAC verification vulnerable to timing and power attacks.