CWE-1255: Comparison Logic is Vulnerable to Power Side-Channel Attacks
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Edit Custom FilterA 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.
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. ![]()
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![]() Languages Class: Not Language-Specific (Undetermined Prevalence) Operating Systems Class: Not OS-Specific (Undetermined Prevalence) Architectures Class: Not Architecture-Specific (Undetermined Prevalence) Technologies Class: Not Technology-Specific (Undetermined Prevalence) Example 1 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. (bad code)
Example Language: C
static nonvolatile password_tries = NUM_RETRIES;
do
while (password_tries == 0) ; // Hang here if no more password tries
while (true)password_ok = 0; for (i = 0; i < NUM_PW_DIGITS; i++)
if (GetPasswordByte() == stored_password([i])
end
password_ok |= 1; // Power consumption is different here
else
password_ok |= 0; // than from here
if (password_ok > 0)
password_tries = NUM_RETRIES;
password_tries--;break_to_Ok_to_proceed // Password OK 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. Among various options for mitigating the string comparison is obscuring the power consumption by having opposing bit flips during bit operations. Note that in this example, the initial change of the bit values could still provide power indication depending upon the hardware itself. This possibility needs to be measured for verification. (good code)
Example Language: C
static nonvolatile password_tries = NUM_RETRIES;
do
while (password_tries == 0) ; // Hang here if no more password tries
while (true)password_tries--; // Put retry code here to catch partial retries password_ok = 0; for (i = 0; i < NUM_PW_DIGITS; i++)
if (GetPasswordByte() == stored_password([i])
end
password_ok |= 0x10; // Power consumption here
else
password_ok |= 0x01; // is now the same here
if ((password_ok & 1) == 0)
password_tries = NUM_RETRIES;
break_to_Ok_to_proceed // Password OK Example 2 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. (bad code)
Example Language: Verilog
module siso(clk,rst,a,q);
input a;
endmoduleinput clk,rst; output q; reg q; always@(posedge clk,posedge rst) begin
if(rst==1'b1)
end
q<1'b0;
else
q<a;
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. (good code)
Example Language: Verilog
module pipo(clk,rst,a,q);
input clk,rst;
endmoduleinput[3:0]a; output[3:0]q; reg[3:0]q; always@(posedge clk,posedge rst) begin
if (rst==1'b1)
end
q<4'b0000;
else
q<a;
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