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.

password_ok |= 1; // Power consumption is different here

password_ok |= 0; // than from hereif (GetPasswordByte() == stored_password([i])else

password_tries = NUM_RETRIES;break_to_Ok_to_proceed
while (password_tries == 0) ; // Hang here if no more password triespassword_ok = 0;for (i = 0; i < NUM_PW_DIGITS; i++)endif (password_ok > 0)password_tries--;
static nonvolatile password_tries = NUM_RETRIES;dowhile (true)// 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.

password_ok |= 0x10; // Power consumption here

password_ok |= 0x01; // is now the same hereif (GetPasswordByte() == stored_password([i])else

password_tries = NUM_RETRIES;break_to_Ok_to_proceedwhile (password_tries == 0) ; // Hang here if no more password triespassword_tries--;  // Put retry code here to catch partial retriespassword_ok = 0;for (i = 0; i < NUM_PW_DIGITS; i++)endif ((password_ok & 1) == 0)
static nonvolatile password_tries = NUM_RETRIES;dowhile (true)// 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.

An alternative to the previous example is simply comparing the whole password simultaneously.

stored_password([i]) = GetPasswordByte();

password_tries = NUM_RETRIES;break_to_Ok_to_proceedwhile (password_tries == 0) ; // Hang here if no more password triespassword_tries--;  // Put retry code here to catch partial retriesfor (i = 0; i < NUM_PW_DIGITS; i++)endif (stored_password == saved_password)
static nonvolatile password_tries = NUM_RETRIES;dowhile (true)// Password OK

Since comparison is done atomically, there is no indication which bytes fail forcing the attacker to brute force the whole password at once. Note that other mitigations may exist such as masking - causing a large current draw to mask individual bit flips.

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.

q<1'b0;

q<a;if(rst==1'b1)else
input a;input clk,rst;output q;reg q;always@(posedge clk,posedge rst)beginend
module siso(clk,rst,a,q);endmodule

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.

q<4'b0000;

q<a;if (rst==1'b1)else
input clk,rst;input[3:0]a;output[3:0]q;reg[3:0]q;always@(posedge clk,posedge rst)beginend
module pipo(clk,rst,a,q);endmodule