A method for transforming data with a secret parameter in an elliptic curve cryptosystem based on an elliptic curve defined over an underlying binary polynomial field, includes multiplying a point of the elliptic curve, and representing the data to be transformed by a scalar representing the secret parameter, wherein the multiplying includes performing at least one point addition operation and at least one point doubling operation on points of the elliptic curve. The point addition operation includes a first sequence of elementary field operations, and the point doubling operation includes a second sequence of elementary field operations, both the first and the second sequences of elementary field operations including a field inversion of coordinates of the elliptic curve points. A representation of the elliptic curve points in affine coordinates is provided and the first and second sequences of elementary field operations are balanced. The field inversion of coordinates is performed by the extended Euclidean algorithm and the balancing includes balancing the extended Euclidean algorithm by adding at least one dummy operation.
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