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.
The code snippet provided here is part of the modular exponentiation module found in the HACK@DAC'21 Openpiton System-on-Chip (SoC), specifically within the RSA peripheral [REF-1368]. Modular exponentiation, denoted as "a^b mod n," is a crucial operation in the RSA public/private key encryption. In RSA encryption, where 'c' represents ciphertext, 'm' stands for a message, and 'd' corresponds to the private key, the decryption process is carried out using this modular exponentiation as follows: m = c^d mod n, where 'n' is the result of multiplying two large prime numbers.
(bad code)
...