Patent ID: 8065531

Claim:
A method of determining a plaintext M based upon a cipher C in a chip card microcontroller and using a secret key d, wherein the secret key d is used in binary form containing bits d i , the method comprising: initializing a plaintext variable M, a second variable M n , and a third variable x, whereby x is initialized to the cipher C iteratively, for each bit d i of the secret key d depending on the value of the corresponding bit d i : determining whether d i is equal to 1: if d i is equal to 1, then calculating a new value of M from x and a previous value of M; if d i is not equal to 1, then calculating a new value of M n from x and a previous value of M n ; calculating a new value for x such that a final value of M is the plaintext M; comparing x to a value depending on final values of both M and M n ; and verifying, after a last iteration, whether calculation of the plaintext M was disrupted.