fig2 illustrates the general inventive idea of the present invention . protected content 205 arrives at a processing device 200 that checks 210 who issued the content and , depending on the identity of the issuer or group of issuers in case more than one issuer may prepare the same kind of content , extracts a rule ( further discussed hereinafter ). the processing device 200 then unprotects 220 the content and uses the unprotected content 225 for processing 230 , which often produces modified , unprotected content 235 . the new content 235 is then protected 240 based on the extracted rule that enforces 250 a certain protection of the new content 235 , which outputs new protected content 245 . it should be noted , however , that processing need not necessarily modify the content ; the content may remain the same , such as for example during emission clearance . the preferred embodiment uses cryptography to control the process . each device stores a number of input keys to decrypt incoming content and a number of output keys to re - encrypt outgoing content . the stored keys are normally a subset of all the keys used in the system . fig3 illustrates a device implementing the invention of the preferred embodiment , while fig4 illustrates a method according to a preferred embodiment of the invention . the processing device 300 comprises at least one processor 372 , at least one memory 374 , communication means 376 that may comprise separate input and output units , and a user interface 378 . it is preferred that the at least one processor 372 performs the functions performed by the functional units described hereinafter . the skilled person will appreciate that the processing device 300 comprises further hardware and software units , such as for example at least one data bus , although these are beyond the scope of the invention and have not been indicated in the figure for reasons of clarity of illustration . every content in the system is associated with a license , preferably in the same file as the content , but it is also possible that the content and the associated license are stored in two different files linked to each other . a license l may be written as { s , comments } ki , where s is a , preferably , symmetric key with which the content was scrambled , “ comments ” is digital information such as for instance time stamps and names input by one or more users , and { } ki indicates the data within parenthesis is encrypted by an asymmetric encryption algorithm , such as rsa , using asymmetric key k i . the comments may also comprise drm rights , e . g . as expressed in a right expression language ( rel ). it will be appreciated that the present invention enables the integrity of such data along the processing workflow . further , the decryption key that corresponds to an encryption key k i is denoted as k − 1 i . a rule preferably comprises a type of processing device for which the rule is intended , an input decryption key k − 1 i and a corresponding output encryption key k i . the rule states that , for the processing device , content that was decrypted using the input key shall be encrypted using the output encryption key given by the rule . it should be noted that it is possible to use rules with one input decryption key that is linked to more than one output encryption key . this may for example be the case if content from a certain source shall be output in more than one copy , or if the operator of the processing device is to be given a choice of output encryption keys . especially if the latter cases are used simultaneously in the system , it is advantageous to include an operator in the rule to inform the device if multiple outputs and / or operator choice is to be used . the processing device 300 receives 402 data l [ c ] s comprising content [ c ] s scrambled ( indicated by the square brackets ) using scrambling key s , which preferably is symmetrical , and a license l comprising the scrambling key s and possibly one or more comments , the license being encrypted using an asymmetrical encryption algorithm with key k i . it should be noted that the data may be sent to the processing device 300 as well as requested by it . upon reception of the data l [ c ] s , a decryption and key check module 310 selects 404 a rule among the plurality of rules stored in a memory 372 . as is shown in the figure , the processing device 300 comprises three rules , for which input decryption keys k − 1 a , k − 1 b , and k − 1 c are indicated in the decryption and processing module 310 , output encryption keys k d and k e are indicated in the protection module 340 , and a rule enforcing module 350 verifies that the rules are enforced . in the example , the first rule r 1 has k − 1 a as input decryption key and k d as output encryption key , rule r 2 has keys k − 1 b and k e , and rule r 3 has keys k − 1 c and k e . it should be noted that r 2 and r 3 have different input decryption keys and the same output encryption key . the rules are preferably distributed in the system by a policy server ( not shown ) in order to make sure that one or more processing chains is created . if no such rule is available or if all of the stored rules have already been tried (“ not ok ”), then the method stops at step 406 “ end ”. however , if the decryption and key check module 310 successfully selects a rule (“ ok ”), the method goes on in step 408 in which decryption of l with the decryption key k − 1 i of the rule . the decryption and key check module 310 may verify if the decryption was successful , for example by including a standard public value , such as “ deadbeef ” at a predetermined place in the license ; if the value is found , then the license was successfully decrypted . in case the decryption was not successful (“ not ok ”), then the method goes back to step 404 where another , preferably the next , rule is chosen . on the other hand , if the decryption was successful (“ ok ”), then the decryption and key check module 310 extracts the scrambling key s and any comments therein and descrambles 410 the scrambled content [ c ] s using the scrambling key s . once the content c has been descrambled , it is forwarded to the processing module 330 for processing 412 and , possibly , addition of and / or suppression of comments . the processing module 330 then forwards the normally modified content c ′ to a protection module 340 that first scrambles 414 the modified content c ′ with a new scrambling key s ′ and then creates 416 a new license l ′ by encrypting the new scrambling key s ′ and any comments by the key imposed by the rule enforcement module according to the selected rule that successfully decrypted the license . the new data , comprising the license l ′ and the content [ c ′] s ′ , is then sent for example directly to a further device for further processing or to an external storage unit . while the embodiment above describes the use of asymmetrical keys to protect the licenses , the person skilled in the art will appreciate that it is also possible to use symmetrical keys for this purpose , although such use reduces the security of the system somewhat . fig5 illustrates the use of the invention in an exemplary network 500 comprising an import device 510 , a digital special effects device 520 , a colour management device 530 , and an export device 540 connected by a local area network 550 . the import device 510 imports content that , in the present example , is unprotected — illustrated by “ no key ”— scrambles the content and creates a license using key k 0 . the content may then be used by the digital special effects device 520 and the colour management device 530 , but not the export device 540 , as the latter does not have the requisite input decryption key . the digital special effects device 520 may , using key k − 1 0 , access content coming from the import device 510 , in which case the content is output encrypted using key k 1 for use by the colour management device 530 , but not by the export device 540 . the digital special effects device 520 may also , using key k − 1 2 , access content coming from the colour management device 530 , in which case the content is output encrypted using key k 3 for use by the export device 540 , but not by the colour management device 530 . analogously , the colour management device 530 may , using key k − 1 0 , access content coming from the import device 510 , in which case the content is output encrypted using key k 2 for use by the digital special effects device 520 , but not by the export device 540 . the colour management device 530 may also , using key k − 1 1 , access content coming from the digital special effects device 520 , in which case the content is output encrypted using key k 3 for use by the export device 540 , but not by the colour management device 530 . the export device 540 may , using key k − 1 3 , use content coming from either the digital special effects device 520 or the colour management device 530 , provided that the content was encrypted using key k 3 . the export device 540 then exports the content , in the example unencrypted as is indicated by “ no key ” in the figure . it will thus be appreciated that the invention enforces the workflow in the exemplary system . content that is introduced into the system by the import device 510 must pass through both the digital special effects device 520 and the colour management device 530 , although the order is in the present case irrelevant , before the export device 540 can export the content . while the preferred embodiment described video processing , it will be appreciated that the invention may also be applied to other environments in which it is important or desirable to impose strict work flows , such as in programming where the different files may be protected , for printed media where for example different articles and pictures may be protected , or for documents to which modifications should be made and / or digital signatures should be affixed in a certain order . fig6 illustrates an alternate use of the invention . a mail handling device 600 comprises at least one processor 672 , at least one memory 674 , communication means 676 that may comprise separate input and output units , and preferably a user interface 678 . the mail handling device 600 is adapted to perform a sort of “ out of office ” function , i . e . automatic transfer of incoming mails . while the exemplary embodiment is shown with encrypted incoming mails , it is to be understood that it may also be used with non - encrypted incoming mails ( in which case an input decryption key may be said to be zero , i . e . the output of the decryption equals the input ). in the mail handler , the rules r stipulate that mail coming from a certain sender is to be decrypted using a certain input decryption key , encrypted using a certain output encryption key , and sent to the destination provided by the rule . it should be noted that it is possible to have one input message give rise to a plurality of encrypted output message , each output message being destined for a specific destination ( illustrated in the figure by having input key k − 1 c linked to output keys k e and k f . it will thus be appreciated that the invention may also be used for secure automatic transfer of electronic mail . the embodiment uses a robust watermark for traceability and confidentiality , and a fragile watermark that ensures integrity together with the robust watermark . it should be noted that the invention may employ any state of the art robust and fragile watermarking algorithms . in the alternate embodiment , constant messages are given in a processing system . for example , the robust watermark may use “ dead ” and the fragile watermark “ beef ”. the robust watermark of content c using key k may thus be denoted { c , dead } k . the fragile watermark is denoted { c , beef }. licensed content l may be denoted {{ c , dead } k , beef }, i . e . the fragile watermark is inserted in the content already watermarked with the robust watermark , following well - known good practice . fig7 illustrates a flow chart illustrating content processing according to the alternate embodiment . the processing device receives licensed content l in step 702 . in step 704 , the fragile watermark is searched for by verifying the equality { l }= beef . in step 706 , the robust watermark is searched for iteratively using rules stored in the device , each rule comprising an input key k i and an output key k j . this is done by verifying the equality {{ c , dead } k a } k i = dead , where k a is the key used to insert the robust watermark and k i is the input key of the current rule . in step 708 , it is checked to see what watermark , if any , was found . if no watermark was found , the method ends in step 710 . if one of the two watermarks was found , but not the other , then an error is reported in step 712 . finally , if both the robust watermark and the fragile watermark were found , then the method goes on to remove the robust watermark in step 714 to produce unprotected content c . as the fragile watermark is designed to be destroyed as soon as the content is modified , there is no need to specifically remove this watermark . once the unprotected content c is obtained , it may be modified in step 716 to obtain new content c ′. after processing , a new robust watermark is added to the new content c ′, using the output key that according to the rule corresponds to the correct input key . then , in step 720 , a fragile watermark is added to the robustly watermarked content so as to create new licensed content l ′ that is sent in step 722 . a person skilled in the art will appreciate that it is possible to combine embodiments of the invention , for example by using the cryptographic solution of the preferred embodiment in a digital environment , the watermark solution of the alternative embodiment , when the content “ goes analogue ”, and then back again to the cryptographic solution when the content re - enters the , or another , digital environment . the skilled person will also appreciate that it is possible to use both the cryptographic solution and the watermark solution at the same time and for the same content . this requires successful double verification of the protection in order to be able to access the content . it will thus be appreciated that the method provides an alternate way of protecting content and imposing strict a work flow . as will be appreciated , the present invention enables enforcement of strict workflows , for example for processing video content , and also protection of such content . it will be understood that the present invention has been described purely by way of example . each feature disclosed in the description and ( where appropriate ) the claims and drawings may be provided independently or in any appropriate combination . features described as being implemented in hardware may also be implemented in software , and vice versa . connections may , where applicable , be implemented as wireless connections or wired , not necessarily direct or dedicated , connections . reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims .