An information processing method for encrypting encoded image data, comprising the step of: inputting, encoded image data and corresponding header data, to produce inputted encoded image data, wherein the header data includes first error-detecting encoding information and second error-detecting encoding information; characterised by the steps of: determining, based on the first error-detecting encoding information, whether the inputted encoded image data includes an error-detecting code; modifying (S122,S123), if the determining indicates that the inputted encoded image data includes the error-detecting code, the inputted header data, the modification including: modifying the first error-detecting encoding information to indicate that the encoded image data does not include the error-detecting code, whilst the error-detecting code is included in the inputted encoded image data, and modifying the second error-detecting encoding information to indicate that the first error-detecting encoding information previously indicated that the inputted encoded image data includes the error-detecting code; encrypting the inputted encoded image data including the error-detecting code; and outputting the encrypted image data and the modified header data. inputting, encoded image data and corresponding header data, to produce inputted encoded image data, wherein the header data includes first error-detecting encoding information and second error-detecting encoding information; characterised by the steps of: determining, based on the first error-detecting encoding information, whether the inputted encoded image data includes an error-detecting code; modifying (S122,S123), if the determining indicates that the inputted encoded image data includes the error-detecting code, the inputted header data, the modification including: modifying the first error-detecting encoding information to indicate that the encoded image data does not include the error-detecting code, whilst the error-detecting code is included in the inputted encoded image data, and modifying the second error-detecting encoding information to indicate that the first error-detecting encoding information previously indicated that the inputted encoded image data includes the error-detecting code; encrypting the inputted encoded image data including the error-detecting code; and outputting the encrypted image data and the modified header data. determining, based on the first error-detecting encoding information, whether the inputted encoded image data includes an error-detecting code; modifying (S122,S123), if the determining indicates that the inputted encoded image data includes the error-detecting code, the inputted header data, the modification including: modifying the first error-detecting encoding information to indicate that the encoded image data does not include the error-detecting code, whilst the error-detecting code is included in the inputted encoded image data, and modifying the second error-detecting encoding information to indicate that the first error-detecting encoding information previously indicated that the inputted encoded image data includes the error-detecting code; modifying (S122,S123), if the determining indicates that the inputted encoded image data includes the error-detecting code, the inputted header data, the modification including: modifying the first error-detecting encoding information to indicate that the encoded image data does not include the error-detecting code, whilst the error-detecting code is included in the inputted encoded image data, and modifying the second error-detecting encoding information to indicate that the first error-detecting encoding information previously indicated that the inputted encoded image data includes the error-detecting code; modifying the first error-detecting encoding information to indicate that the encoded image data does not include the error-detecting code, whilst the error-detecting code is included in the inputted encoded image data, and modifying the second error-detecting encoding information to indicate that the first error-detecting encoding information previously indicated that the inputted encoded image data includes the error-detecting code; encrypting the inputted encoded image data including the error-detecting code; and outputting the encrypted image data and the modified header data.