DE19936998A1 - Method for filing digitally autographed documents onto paper or similar material creates digitally autographed documents able to be reproduced on paper and similar material using standard printer - Google Patents

Abstract

With an electronic document an electronic representation is created in the form of a black/white bitmap printed out with a digital signature. The print-out is scanned in for checking the digital signature later and the original bitmap and digital signature are reconstituted by means of electronic image processing methods.

Description

The invention relates to a method for archiving digitally signed documents on paper and similar materials. With its help, the advantages of digitally signed documents can be Connect the benefits of paper documents. Digitally signed documents are particularly characterized by the fact that they can be sent electronically nen. The physical transport of evidence that is necessary today is therefore no longer necessary stands. Digitally signed documents are also considered to be highly forgery-proof. For documentation Paper-based elements are easy to access in many areas of application information without technical aids as well as the possibility of safe and reliable casual storage.

State of the art

Electronic commerce is becoming an increasingly important economic factor. in the Ideally, a deal is concluded and - provided the nature of the deal is not an exchange material goods requires - the provision of services solely through the exchange of electronic ones Documents, d. H. digital data. For sending conventional paper documents, e.g. B. Offer letter, is waived. This has numerous advantages. So is the transmission of digital data usually much faster and cheaper than that Sending conventional documents. If the computer systems of both transaction partners the transactions can also be processed automatically become, which results in considerable rationalization effects.

However, the lack of conventional documents also has disadvantages, in particular obviously the evidence. If no special precautions are taken, digital ones can be used Falsify data relatively easily without it being recognizable afterwards. Under this prerequisite tongues is the evidentiary value of the electroni transmitted in the course of electronic transactions documents are relatively small. The evidence for electronic transactions is therefore in generally unsatisfactory.

Digital signature processes offer a solution to this problem. They are based on asymmetry encryption processes, which are also known as public key processes (cf. e.g. B. W. Diffie and M. E. Hellman, "New directions in cryptography", in IEEE Transactions on Information Theory, Vol. IT-22, November 1976, pages 644-654; and R. Rivest, A. Shamir and L. Adleman, "A method for obtaining digital signatures and public-key cryptosystems", in Communications of the ACM, Vol. 27, No. 2; February 1978, pages 120-126). Everyone will Person who wants to digitally sign electronic documents, a pair of corresponding keys sel assigned. The first key is used to generate a signature and is only that respective owner known. The second key is used to check one with the first Key generated signature and is often given the owner of the key pair publicly announced.

The signing of an electronic document is done by using the secret key of the signer is encrypted. Using the signer's public key  anyone can decrypt the document again. H. check the signature. Only if the public keys used for decryption with the set used for encryption If the key corresponds, the decryption process results in the original one Document. If the document was changed after it was signed, or the wrong one becomes public Using the key for decryption results in an apparently random sequence of data without recognizable sense. Because only the owner of the secret key encrypts a document in this way It can be that the corresponding public key makes sense again can be encrypted, he must successfully decode the content of a public key the encrypted document.

With digital signature processes, there is the possibility of intangible evidence to generate. Various procedures and protocols based on the basic concept of digital Build signature, z. B. in the book "Applied Cryptography - Protocols, Algorithms, and Source Code in C "(Bruce Schneier, 2nd edition 1996, New York, Chichester et al.). Provided that digital signature processes are widely used and digitally signed Documents in case law are of a similarly high probative value to conventional documents paper is allowed, despite the absence of conventional documents achieve high legal security in electronic commerce.

However, digitally signed documents have the following problems, among others:

a) handling problem

To display digitally signed documents, technical aids, e.g. B. Personalcom computer, indispensable. In areas where appropriate systems are always available, e.g. B. in the office area, this does not cause any problems. However, especially among home users Personal computers or similar systems, if they are available at all, are not always ready for use. In such cases, the information is on a paper document faster and less complicated than viewing it the information in a digitally signed document.

b) Storage problem

A big problem is the secure storage of digitally signed documents are technically no different from other digital data and are therefore exposed to the same dangers. Through computer viruses, accidental deletion of files, a hard drive defect or similar undesirable event can be important pieces of wisdom are irrevocably lost. For this reason, backup copies of the of these documents. In practice, however, backup copies are made based on the associated effort is usually only made sporadically. It is also about the devices used are often systems that do not always work reliably or where the long-term stability of the archived data is not guaranteed, e.g. B. CD-ROM burners or read / write devices for magnetic removable media. Beyond that not ensured in all systems that after a period of several years Readers for the corresponding media are manufactured. If in such a case the reader used no longer works and no replacement can be obtained, the archived data is worthless. Paper documents, on the other hand, can easily Can be stored for decades and read without technical aids.  

In many areas of application is therefore the storage of documents in paper form advantageous compared to storage in digital form. In order to take advantage of the Use electronic commerce and rely on the physical transport of conventional documents To be able to do without it, it makes sense to use paper as a data carrier for digitally signed data the.

In order for the digital signature to keep its meaning with this form of storage, both the signed data and the signature must be stored on paper in such a way that the original digital data can be reconstructed bit by bit on the basis of the paper. This is why necessary because the change of a single bit on the signed document to invalid digital signature. Several are available for the bit-true storage of digital data on paper re procedures available. The easiest way is to use alphanumeric characters to print on the paper. If necessary, these can be entered manually into a computer system or be read in by optical text recognition (OCR). Unless it is clearly regulated how If these characters are to be interpreted, the original digital data can be re-created to construct.

US Patent 5,214,702 describes a variant in which a digital text document is based on well defined way to remove any characters that are not in an expression at all or are not clearly recognizable, e.g. B. tab characters, multiple spaces or Zei lenumbrungen. The resulting text document is digitally signed. After that, the original text document can be printed out together with the digital signature. To become one To check the signature later, the text on the paper and the digital signature entered into a computer system. The entered text becomes the same type as in the original text document, all tab characters, multiple spaces Chen, line breaks etc. removed, so that exactly the same text document results that before An production of the printout was digitally signed. Based on the resulting text document can the digital signature is checked. With the help of the method z. B. Emails before Shipping will be digitally signed. The recipient of the email can use it along with the digital one Print out the signature and keep the printout safe. Anyone can use the expression time the digital signature are checked, without having to look at the originally received, in digi e-mail must be used.

The procedure is therefore suitable for a large part of the digitally signed documents that are in the Archiving electronic transactions are archived on paper. After Part of this process is that only very simply structured text is digitally signed can. Text documents, the formatting (e.g. underlining or bold), complex Text arrangements (e.g. multi-column text) or graphic elements can be used with Not be digitally signed using the procedure. Theoretically, the method can be expanded in this way be tert that such text documents can also be digitally signed; the complexity of the However, the process increases many times over. Another disadvantage is the limitation to a certain set of characters. Characters that are not part of the procedure, e.g. B. the new one Euro symbol, can not be used as long as the procedure does not correspond to it is continued.

Another way of storing data on paper in a bit-true manner is to use it like this named barcodes. Here, digital data is converted into a linear one in a precisely defined manner Sequence of white and black rectangles transformed. This pattern can be traded with a printer on paper. To reconstruct the original digital data structure, the pattern is optically scanned with a so-called barcode scanner. Out of it  results in an electronic replica of the pattern, from which with the help of electronic image processing the original digital data can be reconstructed. The stake this method is common. So there is z. B. on the packaging of most Le food and consumer goods a barcode pattern that with the help of appropriate checkout systems can be used.

An extension of these methods are so-called two-dimensional barcodes, which, for. Tie U.S. Patents 5,504,322 and 5,862,270. You can think of this as one Large number of conventional barcode patterns of the same length, without any space between them to be ordered. The individual barcode patterns are compressed. While at her conventional barcode patterns, the individual rectangles usually have the shape of elongated vertical stripes , their shape is generally square with two-dimensional bar code patterns. The advantage of two-dimensional barcodes lies in the larger amount of data that is on one counter ben area can be accommodated. However, more complex devices are used for optical Scanning necessary because a two-dimensional pattern must be captured. Also have to more complex image processing methods are used to get from the electronic Ab image of the pattern to reconstruct the original digital data.

Since any digital data can be clearly stored on paper with barcodes, are suitable In principle, this method also applies to any digitally signed data. An application example game is the electronic franking of mail pieces, in which, among other things, the address data of the recipient and a unique serial number digitally signed and in the form of a two-way code dimensional barcodes on a normal envelope using a commercially available printer be printed out (see United States Postal Service, Information-based indicia pro gram (IBIP) - Performance criteria for information-based indicia and security architecture for open IBI postage evidencing Systems (PCIBI-O), draft from June 25th, 1999, page A-3; available at http://www.usps.gov/ibip/documents/specs/pcibi-625.pdf).

With the help of barcode processes, all data stored in electronic Transactions occur, are archived on paper. Everyone is at a disadvantage However, the barcode patterns are not directly legible for people.

Another method to use digital signatures in conjunction with paper or similar mate rialien is in the international patent publication DE 44 90 836 T1 Application WO 94/19770 described. The subject of this publication is a safe person sun identification tool. Both the text data and the photo of the owner are included this invention by the digital signature of the issuing authority against counterfeiting protects. The photo is used for the signing process on a clearly reproducible bit follow reduced by extracting several global features of the image and in small numbers be encoded. The resulting bit sequences can be combined into a single bit sequence summarize. The same process is carried out when checking the digital signature.

While this method is suitable for the area of personal identification tools, However, it is only to be used for a fraction of the documents that are part of the electronic transactions are exchanged. So from printing a photo on paper the same feature values as can be extracted from the original photo are also to make high demands on the printing process used.  

task

The methods mentioned are only conditionally suitable for use in the context of electronic transactions archived exchanged digitally signed documents in paper form. The invention is therefore based on the task of generating digitally signed documents that are as few as possible are subject to restrictions regarding their content or their graphic design and that are directly legible for people, as well as using a commercially available computer system in conjunction with a commercially available printer on paper or similar materials give.

So that a printout of a digitally signed document generated in this way is archived and can later serve as evidence, the invention is also based on the object to check the digital signature of the printout in a simple way without a Recourse to the document originally present in the computer system is necessary.

solution

The generation of digitally signed documents and their output on paper or similar materials is solved according to the invention by

• a) that a graphic representation of its content in an electronic document in in the form of a two-dimensional matrix of pixels with a defined color - one like this called bitmap - is generated,
• b) that an electronic document based on the digital representation of this bitmap and, if necessary, based on further data, digitally signed,
• c) that this electronic document, the digital signature and, if necessary, further data the computer system of a recipient are transmitted
• d) and that with the computer system of the recipient in connection with a commercially available Printer makes a printout on paper or similar material that has the bit map, the digital signature and part or all of the additional information, if applicable contains data.

The verification of the digital signature of an expression generated by this method is solved according to the invention by

• a) that the printout using a commercially available flatbed scanner or other equipment is optically scanned,
• b) that from the resulting digital image by means of electronic image processing and, if necessary, optical text recognition processes the original digital representations tion of the bitmap, the digital signature and any additional data that may be included be reconstructed,
• c) that from this data the electronic document is reconstructed, which originally was digitally signed,
• d) that the digital signature is decrypted using the signer's public key,  
• e) and that the results of steps c) and d) are compared.

The bitmap that is generated in the context of the method according to the invention is marked with a compare two-dimensional barcode patterns. The key difference, however, is that the pattern can be read and interpreted directly by people.

When generating the bitmap, a compromise must be made, particularly with regard to the resolution chooses. The higher the resolution, the more appealing the expression is from an aesthetic point of view, but the more difficult it is to later correctly reconstruct the original bitmap the expression. Therefore, the playback capabilities of the receiver should be used printer. For example, B. he bitmap with 100 dpi resolution be witnessed when a laser printer with a resolution of 600 dpi on the receiver side is available. In this case, every single pixel of the bitmap would be printed processing resolved into a rectangle of 6 by 6 points. Accordingly, one Inkjet printer with a resolution of 360 dpi a resolution of 72 dpi for the bitmap can be selected so that each pixel of the bitmap is printed into a rectangle of 5 times 5 points. Unless there is information about the output device used are available, a resolution can be specified for the bitmap by default, those with most of the usual devices in terms of later reproduction Results, e.g. B. 60 dpi. However, aesthetic quality has to be compromised of expression.

It is a good idea to check the digital signature of a printout immediately after it was fathered. This ensures that errors in the printout or contamination of the Paper does not prevent the confirmation of the digital signature. Only then should the document be kept safe. If the digital signature cannot be confirmed, a new one should be sent Printout.

In theory, the bitmap can contain different shades of gray or colors. The only The condition is that every single pixel of the original bitmap is correct again allows the expression to be reconstructed. The quality of the printing processes available today leaves However, it seems advisable to limit yourself to black and white printouts ken. A "black" pixel results from the application of the respective print process characteristic color or color combination with minimal light reflection. On "White" pixel results from the non-printing of those provided for the pixel Surface on the paper and therefore has the color of the paper. When using the for each The appropriate contrast should be achieved for a recon structure of the bitmap are sufficient.

So that an electronic image processing method based on an optical scanning of the Document resulting electronic image to reconcile the location and dimension of the recognizing the structuring bitmap, it is advisable to print it accordingly To create markings on the side of the bitmap. It is conceivable to border the bitmap limit. So that the image processing method the location of each individual pixel of the bit map can determine exactly z. B. all rows and columns of the bitmap on the edge are characterized by a white and a black pixel.

The highest possible resolution should be used for optical scanning, e.g. B. 600 dpi, so that as much information as possible for the subsequent electronic image processing for the reconstruction of the bitmap, the digital signature and any other data  available. For the same reason it is advisable to use optical scanning Grayscale.

The digital signature can be printed on paper in various forms. Think bar is z. B. the expression of an alphanumeric text string. To have the smallest possible area to consume on the printout, however, it lends itself to it in the form of a two-dimensional Output barcodes. The reconstruction of the digital representation of the digital signature can be done in this case with the same image processing method with which the Bitmap is reconstructed. Because the digital signature in no form can be read directly for people there is no loss of information for the user with this solution.

In general, the printout of the bitmap and the digital signature alone is not enough to check the validity of the digital signature. Usually there are additional anga ben necessary, e.g. B. the name of the signer, the signature process used, the public The signer's key and the certificate of a trusted authority that the association confirmation of the public key to the alleged signatory. Next to it further data may be of interest, e.g. B. the signature date. This and where appropriate additional data should therefore also be printed on the document. Some of them This data can also be used to protect it from manipulation. This can be by making it digital with the digital representation of the bitmap Document are put together, which instead of the digital representation of the bitmap is digitally signed. In this case, the decrypted Si when checking the signature Compare the nature with the reconstructed electronic document.

Encrypt large electronic documents with asymmetric encryption driving takes a relatively long time. Therefore, in practice, the entire document is not generally ment encrypted, but representative of its so-called hash value. So-called one-way Hash functions provide a value with constant for electronic documents of any length Length, the hash value that is characteristic of a particular document. they are thus similar to a cross-sum function, but there are two significant differences. For one thing, even the smallest changes to a document lead to a completely changed one Hash value. Second, with one-way hash functions, it is practically impossible to target one Create a document that has a certain hash value (Bruce Schneier, Applied Cryptography - Protocols, Algorithms, and Source Code in C, 2nd edition 1996, New York, Chichester et al. a., pages 30f). It can also be advantageous in the method according to the invention be based on the digital representation of the bitmap or on the basis of a digital representation sentation of the bitmap and other data containing electronic document a hash value to be calculated and digitally signed to represent the original data. In In this case, the decrypted signature with the hash value is used when checking the signature of the reconstructed electronic document.

If an electronic document cannot be meaningfully displayed on a single page, can also use the method according to the invention to multipage documents in paper form be generated. For this purpose, the current page number is on each individual page of the document and the total number of pages. In addition, e.g. B. by specifying a for assembling the entire document characteristic serial number on each page can be prevented from pages from different documents. This information must exist be part of the data to be signed to prevent manipulation.

Although the method according to the invention is preferably intended for electronic purposes Archiving digitally signed documents received from transactions are also useful  Possible applications where printing on the same computer system on which the document is digitally signed. The transmission of the relevant data to another recipient is not necessary in these cases. Such an approach is conceivable Use in all cases where the high level of security against forgery is digitally signed data should be used, e.g. B. with personal identification instruments or official documents ten.

Applications are conceivable in which both the generation of a digitally signed Aus printing as well as checking this printout on the same computer system or on a limited number of computer systems, to which only the owner of the has access to the secret key used when signing. In this case, the Er generation and verification of the document instead of asymmetrical encryption methods symmetric encryption method used. An example of such an application of the method according to the invention are personal identification instruments that are internal be used in a closed area.

Although the method according to the invention is optimally used with paper and similar data carriers can be used, use cases are conceivable in which a more stable data carrier is used det should be z. B. plastic. The inventive method is also suitable for these applications Process, if a printing process is available for the material used, with whose help is the bitmap characteristic of the method according to the invention, the associated one digital signature and any additional information in human-readable form can be. The only limitation of the materials to be printed lies in the Bedin that the plotted data can be correctly reconstructed by optical scanning. Personal identification is also for such an application of the method according to the invention citing instruments as an example.

Achieved advantages

Evidence can be generated and checked by the method according to the invention for which does not require physical transportation of material items. This takes time and save costs that would be necessary to transport conventional documents.

Nevertheless, the recipient of such a digitally signed piece of evidence can easily and cost effective way to produce a material item of evidence that will last for many years can be stored safely and reliably. Due to the digital signature, this is better before Tampering protected than conventional documents. Because ideally everyone for review the digital signature necessary data are available on the printout, such Document can still be checked after years without the need to use other data is agile. The digital signature can be checked quickly and easily using a standardized hardware or software based on the method according to the invention Product.

Should the signature fail because the document is dirty or damaged confirmation of the signature is still possible in many cases. That I if you look closely or use a magnifying glass to see the structure of the bitmap theoretically, manual reconstruction of the bitmap is possible. Requirement of it is, that only those parts of the bitmap that are missing or damaged by logical thinking or the comparison with similar documents can be reconstructed. Such a manual Reconstruction is complex, but it will be worthwhile for important documents. The poss  Manual reconstruction of partially damaged evidence data has a significant impact for the security of the archived documents.

The content confirmed by the digital signature clearly goes out of the bitmap pressure and is obvious to everyone even without technical aids. This will meaning-distorting or falsifying manipulations on the representation of a digitally signed Document effectively prevented.

By using a bitmap as the central component of the digitally signed data is a high flexibility regarding the content and the graphic representation of a document given. A company can e.g. B. use their company logo and thus the outer appearance maintain the image of their business letters. This has the advantage for the recipient that his Differentiate pieces of evidence optically and make them easier to find again.

Digital signatures on paper seem to be a step backwards, since one is usually too avoiding media breakage forms the basis of the process. However, it can have all the advantages electronic transactions, such as B. fast and error-free processing can be used. For the sole purpose of securely storing a piece of evidence, the recipient receives a Document created on paper. Since it is only used in exceptional cases, e.g. B. in disputes, still once used, the disadvantages of media disruption are limited. The advantages weigh this Disadvantage by far. Because for many private individuals - subjectively and objectively - the evidence situation in electronic transactions will become a major hurdle for the comprehensive Use of electronic transactions and the associated benefits are overcome

Embodiment

The following is an embodiment of the invention based on electronic shipping an insurance policy on the Internet. Show it:

Fig. 1 is a term of an insurance policy electronically transmitted

Fig. 2 shows a detail from Fig. 1

Generation of a digitally signed insurance policy on paper

• 1. A customer fills an application on his computer using an ordinary web browser form for travel health insurance.
• 2. The application form is processed automatically on the insurance company's computer system works. If the processing is successful, an electronic document is generated that the Insurance policy and general insurance conditions.
• 3. This electronic document is a graphic image in the form of a bitmap generated.
• 4. The bitmap comes with some additional information that is part of the digital Insurance policies to be signed are to become an electronic document  summarized. These details include e.g. B. the name of the signer, d. H. the Versi and the date.
• 5. A hash value is calculated based on the electronic document.
• 6. The hash value is digitally signed with the insurance company's secret key.
• 7. The electronic document generated in step 4 and the digital signature are combined to the customer with additional data. The additional data becomes subsequent verification of the digital signature is required, but is not itself a contract part. These data include e.g. B. the public key of insurance and that Certificate from a trustworthy body with which the public key of the insurance company tion is confirmed.
• 8. The customer receives the transmitted data on his computer. Ideally are for him only a few seconds have passed since he submitted the application form. A special Central software checks the transmitted data for plausibility, in particular whether the digi tale signature matches the transmitted data.
• 9. If this check is successful, the software prepares the data for printing and outputs it on a connected printer (see FIG. 1). The bitmap ( FIG. 1, bit map 1 ) is output in the form of a two-dimensional barcode pattern ( FIG. 2, bitmap 1 ). It is surrounded by an edge, which supports the later reconstruction of the bitmap using an electronic image processing method ( FIG. 2, edge 2 ). The additional information ( FIG. 1, additional information 2 ) is output in part in text form and in part as a two-dimensional barcode pattern. The digital signature ( FIG. 1, digital signature 3 ) is output as a two-dimensional bar code pattern.
• 10. If the customer does not find any errors in the insurance policy issued, he can file them and keep it safe. In addition, he can make another expression that he opens can carry along his journey.

The verification of the digital signature of a document created in this way is from the perspective of the contractor wenders just as easy:

Review a digitally signed paper document

• 1. The document is with a commercially available flatbed scanner or a similar Ge advises scanned. Reconstruct using electronic image processing and optical text recognition special software creates the original digital data, including the bitmap the signer's public key and the digital signature.
• 2. Based on the original data and the public key of the signatory the software verify the digital signature. Depending on whether the digital signature is valid or not, the software issues a corresponding message.

Depending on the devices used, this check only takes a few seconds customers up to a few minutes. The software could be standard software act for both private and public purposes - e.g. B. at judicial ver actions - must be used. Should z. B. the insurance when the insured event occurs the customer can deny having concluded an insurance contract with the customer de Present the digitally signed insurance policy at any time and using the procedure have it checked.

A further exemplary embodiment will be described below on the basis of the production at lower cost protection-proof user IDs explained. It shows:

Fig. 3 front and back of a user card for a public library.

Generation of a digitally signed user ID for a public library

• 1. A library employee enters the data of a library user into the library interior ne computer system. In addition, a photo of the library user is created using a suitable scanners optically scanned and read into the computer system.
• 2. For the front of the user card ( Fig. 3, front 1 ), a bitmap ( Fig. 3, bitmap front 2 ) is created, which contains the library logo and the user number of the library user.
• 3. The scanned photo is transformed using the Floyd-Steinberg algorithm or another error diffusion method into a black and white bitmap ( Fig. 3, bitmap back 3 ). This is used for the back of the user card ( Fig. 3, back 4 ).
• 4. The hash value of the two bitmaps is calculated. The two bitmaps are like one treated. The hash value thereby protects both sides of the user ID from manipulation.
• 5. The hash value is digitally signed with the library's secret key.
• 6. The bitmaps and the digital signature ( FIG. 3, digital signature 5 ) are output by means of a printer on a suitable carrier material. It can e.g. B. using a La serdruckers a double-sided print on cardboard, which is then laminated to protect against damage.

Check the user ID

• 1. On the basis of the photo, a library employee can carry out a plausibility check as to whether no unauthorized person wants to use the user ID. If in doubt, he can Show your ID card.  
• 2. If there are no doubts, the user card is optically scanned with a scanner. Here it would make sense to use a custom-made product that, on the one hand, matches the size of the ID card is optimized and on the other hand scans both sides at the same time.
• 3. Using electronic image processing, special software reconstructs the original Chen bitmaps and the digital signature.
• 4. Using the original bitmaps and the public one known in the computer system The software can use the key to verify the digital signature. Depending on whether the digital If the signature is valid or not, the software issues a corresponding message.

Claims (8)

1. A method for producing digitally signed documents and their output on paper or similar materials, characterized in that

• a) that an electronic document is used to produce a graphic representation of its content in the form of a two-dimensional matrix of pixels with a defined color - a so-called bitmap -
• b) that an electronic document, which is based on the digital representation of this bitmap and, if necessary, on further data, is digitally signed,
• c) that this electronic document, the digital signature and, if necessary, further data are transmitted to the computer system of a recipient
• d) and that with the computer system of the recipient in connection with a commercially available printer a printout on paper or a similar material is prepared, which contains the bitmap, the digital signature and part or all of the data which may be additionally transmitted.

2. The method according to claim 1, characterized in that the individual pixels of the bit map can assume the colors "black" or "white", whereby "black" for a color Application with minimal light reflection and "white" for no application of color on the printout stands. 3. The method according to claim 1, characterized in that based on one of the digital Representation of the bitmap and, if necessary, electronic data generated from further data A hash value is calculated using a one-way hash function, which represents the digital document to be digitally signed. 4. The method according to claim 1, characterized in that the computer system on which the digital signature is generated, is identical to the computer system of the recipient. 5. The method according to claim 1, characterized in that for signing the electronic Document an asymmetric encryption method is used. 6. The method according to claim 1, characterized in that for signing the electronic A symmetrical encryption method is used. 7. The method according to claim 1, characterized in that any material used is, for which a printing process is available, with the help of the bitmap, the di digital signature and, if necessary, additional data can be applied in this way, that they can be reliably reconstructed by optical scanning. 8. A method for checking the digital signature of an expression generated by means of a method according to claim 1 to 7, characterized in that

• a) that the printout is optically scanned using a commercially available flatbed scanner or other devices,
• b) that the original digital representation of the bitmap, the digital signature and any additional data which may be present are reconstructed from the resulting digital image by means of electronic image processing and, if necessary, optical text recognition processes,
• c) that the electronic document is reconstructed from this data, which was originally digitally signed,
• d) that the digital signature is decrypted using the signer's public key
• e) and that the results of steps c) and d) are compared.