An apparatus comprising: a communication interface for locally connecting the apparatus to a host computer (99; 320); and a data processing component adapted to provide a cryptographic processing result generated by cryptographically combining a cryptographic secret key with first input data; whereby the apparatus is adapted to present itself to said host computer, when it is connected to said host computer by the communication interface, as a mass storage device that an application on the host computer can access through a standard mass storage access mechanism for exchanging files; and the communication interface is adapted to return an output file to said host computer over said communication interface whereby said output file comprises the cryptographic processing result, and said host computer obtains said output file by reading said output file from the apparatus over said communication interface through a mechanism for reading files of said standard mass storage access mechanism; the apparatus characterized in that it is further adapted to generate a digital signature over at least some contents of an input file whereby: said communication interface is further adapted to receive said input file from said host computer whereby said host computer sends the input file to the apparatus over said communication interface by saving the input file to the mass storage device presented by the apparatus through a mechanism for saving files of said standard mass storage access mechanism; said first input data are based on a value that represents said at least some contents of the input file; said generating of said digital signature takes place upon said receiving of said input file; and said digital signature is comprised in said result of said cryptographically combining of said cryptographic secret key with said first input data. a communication interface for locally connecting the apparatus to a host computer (99; 320); and a data processing component adapted to provide a cryptographic processing result generated by cryptographically combining a cryptographic secret key with first input data; whereby the apparatus is adapted to present itself to said host computer, when it is connected to said host computer by the communication interface, as a mass storage device that an application on the host computer can access through a standard mass storage access mechanism for exchanging files; and the communication interface is adapted to return an output file to said host computer over said communication interface whereby said output file comprises the cryptographic processing result, and said host computer obtains said output file by reading said output file from the apparatus over said communication interface through a mechanism for reading files of said standard mass storage access mechanism; the apparatus characterized in that it is further adapted to generate a digital signature over at least some contents of an input file whereby: said communication interface is further adapted to receive said input file from said host computer whereby said host computer sends the input file to the apparatus over said communication interface by saving the input file to the mass storage device presented by the apparatus through a mechanism for saving files of said standard mass storage access mechanism; said first input data are based on a value that represents said at least some contents of the input file; said generating of said digital signature takes place upon said receiving of said input file; and said digital signature is comprised in said result of said cryptographically combining of said cryptographic secret key with said first input data. said communication interface is further adapted to receive said input file from said host computer whereby said host computer sends the input file to the apparatus over said communication interface by saving the input file to the mass storage device presented by the apparatus through a mechanism for saving files of said standard mass storage access mechanism; said first input data are based on a value that represents said at least some contents of the input file; said generating of said digital signature takes place upon said receiving of said input file; and said digital signature is comprised in said result of said cryptographically combining of said cryptographic secret key with said first input data.