Arrangement for protecting against the unauthorized reading of program words stored in a memory

Claim 1

A device for protection against the unauthorized reading of program words stored in a (14) memory which forms part of a memory unit 20, said protection device comprising: a data processor unit having an address output which is connected to an address input of said memory in order to address the stored program words during a processing operation in a given sequence which is determined by the data processor unit; said memory unit (20) comprises a nuisance word source which is separate from the memory (14), and which serves to supply at least one nuisance word which is unrelated to said program words, a verification unit and a selection unit; said selection unit comprises a control unit, a first data input connected to a first output of the memory and a second data input connected to an output of the nuisance word source; said memory having a program field comprising the program words and an additional field in which there is stored an additional information for each program word, said additional information being addressed simultaneously with its respective program word; said additional field being connected to a second output of the memory to which a first input of the verification unit is also connected; a second input of the verification unit being connected to a further connection of the memory; said verification unit comprises a storage element connected to said first input for storing an indicator derived from the additional information of a first program word when read from the memory; said stored indicator indicates a program word subsequent to the first program word as determined by said sequence; said verification unit comprising verifying means for verifying, when a second program word is read from the memory, whether the program information in respect of the read second program word corresponds to the stored indicator; said verification unit being further provided for generating a first signal when the verification results in correspondence and a second signal when the verification results in non-correspondence; the selection unit being provided for supplying the read program word from the memory on an output under the control of the first signal received at its control input and to block the supply of the read program word from the memory under the control of a received second signal and to replace this read program on the output by a nuisance word generated by the nuisance word source.

Claim 2

A device for protection against the unauthorized reading of program words stored in a (14) memory which forms part of a memory unit 20, said protection device comprising: a data processor unit having an address output which is connected to an address input of said memory in order to address the stored program words during a processing operation in a given sequence which is determined by the data processor unit; said memory unit (20) comprises a nuisance word source which is separate from the memory (14), and which serves to supply at least one nuisance word which is unrelated to said program words, a verification unit and a selection unit; said selection unit comprises a control unit, a first data input connected to a first output of the memory and a second data input connected to an output of the nuisance word source; said memory having a program field comprising the program words and an additional field in which there is stored an additional information for each program word, said additional information being addressed simultaneously with its respective program word; said additional field being connected to a second output of the memory to which a first input of the verification unit is also connected; a second input of the verification unit being connected to a further connection of the memory; said verification unit comprises a storage element connected to said first input for storing an indicator derived from the additional information of a first program word when read from the memory; said stored indicator indicates a program word subsequent to the first program word as determined by said sequence; said verification unit comprising verifying means for verifying, when a second program word is read from the memory, whether the program information in respect of the read second program word corresponds to the stored indicator; said verification unit being further provided for generating a first signal when the verification results in correspondence and a second signal when the verification results in non-correspondence; the selection unit being provided for supplying the read program word from the memory on an output under the control of the first signal received at its control input and to block the supply of the read program word from the memory under the control of a received second signal and to replace this read program on the output by a nuisance word generated by the nuisance word source.