Apparatus and method for converting input bit sequences

A cryptographical apparatus for converting input bit sequences, whose overflow-free arithmetic addition results in a secret, into output bit sequences whose logic XORing results in the secret. The apparatus comprises a data interface for providing a first input bit sequence and a second input bit sequence and a processing circuit configured to a) gate the first input bit sequence and the second input bit sequence to obtain a logic result indicating overflow bit positions at which both the first input bit sequence and the second input bit sequence have a value of one; and to b) change the first and/or second input bit sequence at at least one overflow bit position. The processing circuit is configured to repeatedly perform steps a) and b) by using the respectively changed input bit sequences, until the logic result indicates no further overflow bit position and the output bit sequences are obtained.