Abstract. In the last few years, several studies addressed the problem of data
security in Mifare Classic. One of its weaknesses is the low random number
quality. This causes SAT solver attacks to have lower complexity. In order to
strengthen Crypto-1 against SAT solver attacks, a modification of the feedback
function with better cryptographic properties is proposed. It applies a primitive
polynomial companion matrix. SAT solvers cannot directly attack the feedback
shift register that uses the modified Boolean feedback function, the register has
to be split into smaller groups. Experimental testing showed that the amount of
memory and CPU time needed were highest when attacking the modified
Crypto-1 using the modified feedback function and the original filter function. In
addition, another modified Crypto-1, using the modified feedback function and a
modified filter function, had the lowest percentage of revealed variables. It can
be concluded that the security strength and performance of the modified Crypto-1 using the modified feedback function and the modified filter function are better than those of the original Crypto-1.