Calculating unit and method for performing an arithmetic operation with encrypted operands

Abstract

A calculating unit for performing an arithmetic operation with at least two operands, the at least two operands being encrypted, includes an arithmetic-logic unit with a first input for the first encrypted operand, a second input for the second encrypted operand, a third input for an encryption parameter and an output for an encrypted result of the operation, the arithmetic-logic unit being formed so as to operate on the first input, the second input and the third input by means of arithmetic sub-operations, while considering the type of encryption of the operands, such that at the output, an encrypted result is obtained which equals a value that would be obtained if the first operand was subjected to the arithmetic operation in a non-encrypted state and if the second operand would be subjected to the arithmetic operation in a non-encrypted state, and a result obtained was subsequently encrypted, no decryption of the operands being performed in the arithmetic-logic unit. In this manner, a processor system may be obtained in which no data whatsoever occurs in clear text, i.e. in a non-encrypted form, since no decryption upstream of an arithmetic-logic unit and no encryption downstream of the arithmetic-logic unit are required, as the arithmetic-logic unit operates with encrypted input operands to obtain an encrypted result. Interception attacks on transmission lines of the calculating unit are thus ruled out.