A whitebox descrambler for descrambling a ciphertext to obtain a cleartext message, wherein the descrambler is configured to: receive a personalized transformed key (T i) from an external preprocessing module, the personalized transformed key (T i) being personalized for the descrambler; receive the input ciphertext; generate an output data by applying a second part of a descrambling operation to the input ciphertext using the personalized transformed key (T i) as an input to the second part of the descrambling operation; and apply an inverse transformation to the personalized transformed key (T i) before generating the output data, wherein the personalized transformed key (T i) comprises preprocessed data as a result of applying a first part of the descrambling operation in the external preprocessing module; wherein the descrambler is a whitebox block cipher based descrambler, the descrambler comprising: a key partitioning module configured to split the personalized transformed key (T i) in a predefined number of transformed key parts (PRK i n) of equal bit length; and for each transformed key part: a block cipher round module (4111, 4112) configured to receive an input ciphertext data and to generate the output data by selecting from a preconfigured lookup table the output ciphertext data at a location indicated by a first bit pattern of the input ciphertext data and a second bit pattern based on the transformed key part, wherein a first block cipher round module is configured to use a part of a predefined bit length of the ciphertext as the input ciphertext data and wherein subsequent block cipher round modules are configured to use an output data of a previous block cipher round module as the input ciphertext data. receive a personalized transformed key (T i) from an external preprocessing module, the personalized transformed key (T i) being personalized for the descrambler; receive the input ciphertext; generate an output data by applying a second part of a descrambling operation to the input ciphertext using the personalized transformed key (T i) as an input to the second part of the descrambling operation; and apply an inverse transformation to the personalized transformed key (T i) before generating the output data, wherein the personalized transformed key (T i) comprises preprocessed data as a result of applying a first part of the descrambling operation in the external preprocessing module; wherein the descrambler is a whitebox block cipher based descrambler, the descrambler comprising: a key partitioning module configured to split the personalized transformed key (T i) in a predefined number of transformed key parts (PRK i n) of equal bit length; and for each transformed key part: a block cipher round module (4111, 4112) configured to receive an input ciphertext data and to generate the output data by selecting from a preconfigured lookup table the output ciphertext data at a location indicated by a first bit pattern of the input ciphertext data and a second bit pattern based on the transformed key part, a key partitioning module configured to split the personalized transformed key (T i) in a predefined number of transformed key parts (PRK i n) of equal bit length; and for each transformed key part: a block cipher round module (4111, 4112) configured to receive an input ciphertext data and to generate the output data by selecting from a preconfigured lookup table the output ciphertext data at a location indicated by a first bit pattern of the input ciphertext data and a second bit pattern based on the transformed key part, wherein a first block cipher round module is configured to use a part of a predefined bit length of the ciphertext as the input ciphertext data and wherein subsequent block cipher round modules are configured to use an output data of a previous block cipher round module as the input ciphertext data.