DE102007018802B3 - Method for tap-proof transmission of character string from client to server through computer network, involves producing shadow image on screen through server according to visual cryptography process - Google Patents

Abstract

The method involves producing a shadow image on a screen through a server according to visual cryptography process. An image with pushbutton is produced and marked corresponding to the random principle with characters, by the server. An input of string of characters on the marked pushbutton is realized by the client through mouse clicks to reconstruct the character string entered from the client over a server. Independent claims are also included for the following: (1) a computer program for executing the method (2) a plastic or paper screen for executing the method.

Description

The The present invention relates to a method for tamper-proof and tamper-proof encoding for online accounts, especially for Online bank accounts, using visual cryptography.

The tap- and Tampering Security of Online Accounts - Especially which is from online bank accounts - will through the ever-growing quantity and harmfulness of malware (ie viruses, etc.) on the PC's of bank customers. Procedures that both the interception to prevent the PIN as well as a so-called man-in-the-middle attack, are technically complex and need special Hardware and software on the PC used by the bank customer.

The interceptibility the PIN is obvious in the PIN / TAN procedure: the malware on the client's computer secretly observes when entering the PIN the keyboard input. Later will the intercepted PIN secretly passed through computer network to another computer. From there, the user can access the account at least by reading become. Procedures where the PIN with mouse clicks on the screen are also not tap-proof: the malware is listening at the same time Screen and mouse movement.

method like HBCI-2 for Bank accounts (with external number field for entering the PIN) or the clock-controlled Security Tokens for Corporate accounts protect the PIN from eavesdropping but expensive in production and use.

Of the sees man-in-the-middle attack on an online account as follows from: Bank customer X wants Transfer 50 euros to Y's account. Fulfills the corresponding online form and send it off. The malware on the computer begins this transfer order before he is sent to the bank, converts him into a transfer from 5000 euros to Z, and sends this manipulated transfer order to the bank. The demand of the bank to X for a TAN for the transfer order from 5000 euros to Z will be from the malware in the reverse direction as well intercepted, and it becomes the bank customer X on the screen the demand the bank for a tan for a transfer order from 50 euros to Y reflected. Unknowingly, X confirms this with a TAN pre-mirrored order, and the malware sends the entered by X. TAN to the bank further to the fraudulent transfer order of 5000 Euro to Z to confirm.

The PIN / TAN, PIN / iTAN, HBCI-1, HBCI-2, and Security Token procedures protect not sure of the man-in-the-middle attack. Also the encryption protects the connection (eg SSL) not sure, because the malware can be down even before the connection encryption turn on and the manipulations even before the connection encryption (or in the other direction: after the connection decryption) carry out.

Method, that protect against the man-in-the-middle attack are the ones that both a numeric keypad as well as a display outside the client computer attach, such. B. HBCI-3. However, because of the physically existing Cable connection between this extra hardware and the computer of the Clients still have a residual risk that malware on the machine Clients spied on the actions on the extra hardware. A another possibility is it, the transfer data confirm by SMS from the bank ("mTANs") this solution is that a cell phone must be present and that the reception of the SMS may take a while. Besides, it's just a question the time until mobile phones are hit by malware and so on this possibility also becomes uncertain.

Some of the known encryption methods are based on the method of visual cryptography. Visual cryptography is a method of dividing a black-and-white image into two black-and-white images (partial secrecy images) so that both partial secrecy images individually have no information, but when one of the two partial secrecy images is printed on clear foil and on top of that, the original image is rediscovered (with 50% loss of contrast: white turns gray) (Naor & Shamir "Visual Cryptography", Advances in Cryptology, EUROCRYPT, Springer Verlag, 1994, pp. 12, see 1 ). One of the two partial secrecy pictures can be generated in advance, ie without the original picture being known. The second partial secret image is then generated depending on the original image and the first partial secret image (that this second image is a noise image, ie contains no information, so in particular does not contain the information of the original image is surprising, but mathematically easy detectable).

The patents EP1472584B1 and US2005 / 0219149A1 describe an application of visual cryptography in which a transparent electronic display is mounted on the screen. A major disadvantage of this method is that the development and manufacturing costs for this technique are very high.

The patent EP1487141A1 describes the possibility of creating one of the two partial secret images of visual cryptography in advance and distributing it to the recipient, even if the image to be encrypted is not yet known is (this possibility is given with each one-time pad procedure, thus in particular also with visual cryptography). The method indicates applications of this principle to the secure transmission of messages, but not in the context of online accounts.

The patent US2006 / 0098841A1 describes a method by which n characters of an alphabet with m characters can be transmitted tap-proof with n mouse clicks. The method requires n times many buttons. This results in practice in a number of buttons that can no longer be displayed (example: text length n = 20, alphabet size m = 50 gives 1000 buttons).

The patent US20060020559A1 describes an encryption method for online accounts. In this method, punched paper cards are placed on the screen to show the user secret information (this principle is also known as "Richelieu board"). The method has, among other things, the drawback that the security against eavesdropping is obscured because of large parts of the card Another disadvantage of this method is that when used against the man-in-the-middle attack, the die-cut paper card would become very large.

In the essay (Naor & Pinkas "Visual Authentication and Identification ", CRYPTO, Springer Verlag, 1997, pp. 322-336) becomes the application of Visual Cryptography for The security of smart cards is proposed and analyzed.

It However, no method is known with which at low Production costs the online accounts absolutely safe from malware Intervention protected can be.

The It is therefore the object of the present invention to provide a secure encryption method for online accounts to provide, in which the development and production costs are much lower compared to known methods.

To solve this problem, the method of visual cryptography is used, according to the invention, the new principle "slide-on-screen" is used: one partial secret image is printed on a transparent film, the other is displayed on the screen the user can see the original information on the screen, and it is crucial that no malware on the user's computer or in the computer network can recognize the original information: there is no way for malware to "see" the slide on the screen. to scan". 4 shows the situation: the partial secrecy image on the screen and the partial secrecy image on the slide of the user superimposed indicate a number field with randomly exchanged numbers. For malware on the computer or in the computer network, there is no way to recognize the interchange of 10 digits.

Under The term "film" is used here and below a thin one Layer of a suitable material understood, the printed or can be punched out. In particular, this can be a substantially transparent material such as plastic or transparent paper can be used, which prints with one of the partial secrecy pictures becomes. Alternatively, an opaque material is used, which is punched out according to the invention for generating a partial secret image becomes. Particularly suitable for this purpose is dark, z. B. black Paper, so that the optical contrast is greatest.

In order to the printed on the slide part secret image with the on the Screen displayed partial secret picture matching one above the other can be placed, the film is fixed on the screen. For this can individual films are provided with adhesive strips on one or more edges. In this case, you can the slides over each other like sticky notes in a notebook stacked and held together by the adhesive strips become.

It Below is an example of the inventive method for tap-proof Handling an online account described. In this example becomes a text with n characters from an alphabet consisting of m characters absolutely mouse-safe with just a mouse click from a client through a computer network to a server. Here and in the following, under alphabet, as is customary in computer science, one finite amount of characters understood. In this sense, for example the set of digits 0, ..., 9 is an alphabet with 10 characters.

First the server creates for every client a lot of sub-secret images that he names (eg numbers) provides and saves. Then he prints these pictures on transparent Slides out, prints in addition their respective name (number) visible on it, and send them Films physically, so z. By mail, to the client. Initially have all these partial secret pictures the status "unused", which can later change to "used up". This Status is managed by the server.

Client X now has these slides and wants to send the server a secret message of n characters (example: he wants to send him an 8-digit bank code). He steps on his computer online via the computer network with the server in connection and introduces itself to the server as client X before.

The server takes note of this and then creates an original image for transferring n characters from an alphabet with m characters in the following manner. At least n + m - 1 buttons are created on the image. The buttons serve as surfaces for later clicking through the mouse and do not overlap. The buttons carry labels. Each character of the alphabet appears as a caption on at least one of the buttons. In addition, there are n-1 buttons, each for a reference to a character that has already occurred. Ideally, these references are labeled numbered 1 through n-1 (if the numbers themselves are part of the alphabet, these numbers are specially marked, eg, by a prefix P for "position"). 2A shows a picture with buttons for the numbers 0, ..., 9 and text length n = 8, 2 B shows an image for an alphabet with the letters A ..., Z, the digits 0, .., 9 plus a few special characters, and text length n = 10. The assignment of labels and buttons is generated randomly. The server remembers this random assignment.

A special case occurs when the text to be transmitted is guaranteed to have no character repetition (a plausible example: only PINs consisting of the digits 0, ..., 9 are permitted, in which no digits occur multiple times). In this case, the buttons with the references to the previous position can be omitted, and one button is enough for each character of the alphabet. 2C shows an image with a randomly generated array of buttons for the numbers 0, ..., 9.

After this The server has created such an original image, he takes one more unused saved part secret image for the client X, makes of these two images according to the process of visual Cryptography is a second (electronic) partial secret image, which he then he through the computer network online on the screen of the client sends. Also, the name (number) of the stored partial secret image sent to the client's screen. The stored partial secret picture gets the status "used up" at the server.

Client X sees the partial secret image and its name (number) on the screen, see 3A , He puts the slide with the same name (number) on the screen, exactly on the partial secret picture on the screen. Because the two partial secret images were generated by the method of visual cryptography, client X can see the original image.

In In practice, the two pictures are not the same size. The picture on the screen must therefore be adjusted by the client (eg stretched) can, on best by mouse.

The client sees the original image with its buttons and their labels, see 3B , He can now enter his secret text via mouse clicks, according to the following rules.

A text with n or less characters is entered character by character as follows. For each character that has not previously existed, the button on which the character is located is clicked. If a character has already occurred, the largest position in the text where the character already stood is determined: the button whose label represents a reference to this position is clicked. For example, in 3 the bank code 20041111 is entered by clicking the following buttons: 2, 0, P2, 4, 1, P5, P6, P7.

To These simple rules guarantee that every button only a maximum of once clicked. The software on the client machine can because of this, potentially helping the user and all of them once before clicked buttons specifically show, for. B. in the partial secrecy picture on the screen invert all pixels of the button clicked. That's how it looks Users, which buttons he should not click anymore, the privacy is not impaired.

A more possible support for the user offer - so as he knows it from the bank machine - a correction button and a progress bar showing "asterisks" indicating how many characters have already been entered were.

The information about the clicked buttons and their order is sent to the server, e.g. By transmitting the pixel positions of the n mouse clicks relatively in the sub-secret image, or other unique description of the clicked buttons. For example, the in 3B clicked buttons with the labels 2, 0, P2, 4, 1, P5, P6, P7 are transmitted as coordinates of the illustrated 4 × 5 matrix (with (0,0) top left and (3,4) bottom right), ie as follows: (3,2), (3,1), (0,0), (3,4), (1,0), (0,1), (3,0), (1,1).

Because each button is clicked at most once and the label of the buttons was chosen randomly, malware on the computer of the client or in the computer network can not provide any information about the transmitted text: the malware would have to spy out the launched slide which is not possible.

The Information sent to the server can then be decrypted there: the server knows the original image and knows which labels the clicked buttons to have. So he can be transmitted from the n Descriptions of buttons reconstruct the client input message of n characters. In the example from above in which the matrix coordinates (3,2), (3,1), (0,0), (3,4), (1,0), (0,1), (3,0), (1,1), The server can because of the knowledge of the original image from it directly read out the message 20041111. The message was therefore tap-proof from Transfer clients to the server.

Vice versa A secret text can also be sent from the server to the client The server writes the text to a black and white picture, takes an unused foil, and creates a corresponding one from both second, partial electronic secret image, which he uses via computer network sends the client to the screen, including name the foil. The client places the corresponding slide on the screen and can read the text. In the event that the client at all he knows a text, he knows sure that the text is from the server, because nobody else knows his slide, and therefore no one can fake a message to him.

Of the Advantage of the method described is thus that a Online account safe from listening to the PIN and secure from the man-in-the-middle attack protected can be. The reason is that malware (on the machine the bank customer or in the computer network) the foil to be placed does not knows and definitely can not spy (a "scan" the slide via screen is absurd!).

Compared to in US2006 / 0098841A1 described method requires the inventive method only n + m - 1 buttons. For the above example (text length n = 20, alphabet size m = 50 results in 1000 buttons) this means: 69 buttons in the present method instead of 1000, those in US2006 / 0098841A1 needed.

One Another advantage is the relatively low production costs, since only the corresponding foils are produced and printed Need to become, no expensive devices are necessary.

Further Advantages, features and applications of the invention will be below with reference to the embodiments described with reference to the drawings. In the drawings show:

1 : Visual Cryptography Principle (Naor & Shamir): Two slides alone do not contain any information, but when stacked they give away information.
(Note: The picture 1 is suitable for demonstration of visual cryptography: 1 Copy on slide, and then put the picture on top of that in the middle: you can see the bottom picture.)

2 : three original pictures with buttons to click for the mouse.

3 : Surreptitious communication of information with repetition of symbols, Example: Entering a bank code with 8 digits.

4 : Interception-proof exchange of information in which no repetitions of symbols occur, example: PIN entry (assumption: only PINs without numerical repetition are assigned)

5 : Counterfeit-proof confirmation of a transfer including a TAN (which can then be entered in plain text on the keyboard). It is important for anti-counterfeiting security that the information is not in a fixed position, eg. For example, in Figure B, the numbers displayed are horizontally randomly offset.

6 : Tamper-proof online confirmation of debits via visual cryptography.

7 : Tap-proof mediation of general text, including repetition of characters.

embodiment

The above procedure for sending secret messages between server and client is applied to the specific case of online banking ( 4 and 5 ). The method prevents PIN listening and man-in-the-middle attack.

Of the Bank server generates for bank client X a lot of partial secret images, number them, save them and send them printed on slides to bank customers by mail too (so similar how TAN lists are sent). In addition, the customer is like PIN / TAN procedure a PIN delivered, it is assumed that in the PIN no digit occurs twice (the number of possibilities is still enough to make it futile to guess the PIN)

If the customer X has received the slides and his PIN, he can start online banking. To log in, he goes to the web page of the bank and there gives his account number. at. The account no. will be sent to the bank server. The bank server now checks the authenticity of X as follows:
The bank server generates an original image with 10 buttons, eg. B. in the way that the arrangement of the buttons corresponds to the arrangement of the keys of the number field on a keyboard, see 2C , Randomly, the 10 digits 0, ..., 9 are distributed to these buttons. The server remembers this random permutation. An unconsumed stored slide is taken for the bank customer X and, according to the method of visual cryptography, a second partial electronic secret image is generated from it and the original image. This is sent to the bank customer X in his browser window, including the number of the stored partial secret image. The situation arises as in 4A ,

The bank customer sees the partial secret picture and its number and places its appropriate foil on it. He can recognize the number field, see 4B , He clicks the corresponding buttons with the mouse. Suppose his PIN is 41629, so he clicks in 4B one after the other the buttons that represent the digits 0, 5, 2, 9, 6 on the normal number field. This digit sequence 05296 is sent to the bank server.

Of the Bank server receives the number sequence 05296. Because the bank server itself the original picture me created the exchanged numbers and remembered the exchange now he can directly conclude that by the mouse clicks the number sequence 41629 was entered. He compares this number sequence with the PIN for Bank customer X (which, of course, too is stored). If that was the correct PIN, the bank customer gets X Access to the account.

listening end Malware (on the computer used by bank customer X or on the Internet) has no chance to listen to the PIN: the positions of the mouse clicks and the numbers sent to the server have no meaning unless the interchanging of digits on the number field is known. This only knows the bank server and the one who can hang up the appropriate slide on the browser.

bank customer X gets so with the knowledge of the PIN and the physical possession the slide access to his bank account. Only one of them is enough not from. The procedure thus protects the PIN twice: firstly is she not audible, and if somebody else should come into their possession he needs the right slide: without her, he will not be in the account.

in the The following shows how the method is a man-in-the-middle Thwarted attack. Bank customer X has successfully logged in and want a transfer from 50 euros to Y make. He types in plain text in a corresponding form Name, account number, bank code and amount and sends this information to the bank server.

The bank server writes this information to a black and white image. In order to guarantee the security against forgery, he does not write every single piece of information at a fixed point in the picture, but only in a certain defined area of the picture. In addition, he writes a randomly generated number sequence ("TAN") in a certain specified area of the image. He remembers this TAN. Then he takes an unconsumed slide and generates from this and the generated image according to the method of visual cryptography, the second partial secret image, which he sends including its number to the browser window in which the bank customer X sits. It is the in 5A illustrated situation.

Bank customer X places the corresponding slide on the partial secret picture on the screen and sees the transfer data confirmed again, see 5B , If the data is correct, he types in the displayed TAN in clear text in a designated input field and sends it to the bank server.

Of the Bank server compares the transmitted number with the previously given by him TAN. If both match, he gives the transfer free.

The Process protects against the man-in-the-middle attack: while it is for malware easy, the transfer data but malware does not stand a chance to the bank client displayed confirmation the transfer data too fake: That would be the knowledge of the pixels on the slide set up by the bank customer necessary. For the same reason, malware does not stand a chance to the bank client to recognize displayed TAN.

If So the bank server that receives the correct TAN has the information about the indeed pending transfer reached bank client X, and no pre-mirrored information. A man-in-the-middle attack is thus repulsed with this method.

In a similar way, as the bank customer can confirm by means of visual cryptography a transfer tamper-proof, the bank customer can also confirm debits from his account tamper-proof, z. For example, when debiting from your own account when shopping online: if the bank server receives the debit order from the seller, the bank server sends (eg via email, or via link from the web page on which the purchase was made) a confirmation message to the account holder, see 6 , Only if the latter confirms the debit there with his PIN, the debit is carried out. For the same reasons as the transfer confirmation, the procedure is forgery-proof.

The illustrated method for Online bank accounts are also safe against so-called pharming: no attacker can pretend successfully via web page, to be the bank, because the partial secret image, what the user can not be forged when logging in to enter the PIN: without Knowledge of the pixels on the slide to be placed can not be a partial secret image be created on the together with the slide to recognize anything is, because a fake one Partial secret image on the screen and the user's slide lie one above the other a noise image (i.e., an image without information).

The The procedure described makes the so-called phishing for scammers uninteresting: Emails or web pages that fraudulently ask the bank customer for the PIN, can in the case that the bank customer naively reveals the PIN, with this PIN do not do anything: to get into the account is missing still give them a slide.

Claims (19)

Method for the tap-proof transmission of a string of characters from a client through a computer network to a server through the following steps: a) generating a first Partial secret image according to the procedure visual cryptography on a slide through the server, b) generating an image with buttons that randomly be labeled with characters, by the server, c) generating a second partial secret picture according to the visual method Cryptography for the image generated in b) by the server, which is on the screen the client is displayed, with the buttons of the in b) generated corresponding areas of the second partial secrecy image as buttons are clickable through a mouse, d) overlaying the film and the screen by the client, taking the picture with the clickable and buttons labeled with characters become visible to the client, e) the input of a series of n characters by the client via mouse clicks on the labeled buttons, f) the transfer the information about which buttons the client clicked in which order, to the server, G) the reconstruction of the character string entered by the client the server. Method of interception and forgery-proof transmission a string of characters from a server through a computer network to one Clients characterized by the following steps: a) the Generating a first partial secret image according to the visual method Cryptography on a slide through the server, b) generating of an image to be transmitted with the Character string is labeled by the server, c) generating a second partial secret picture according to the visual method Cryptography for the image generated in b) by the server, which is on the screen of the client is displayed, d) overlaying the film and the screen by the client, the image with the string of characters for the Clients becomes visible. Method according to one of the preceding claims, characterized that the film is made of plastic or paper. Method according to one of the preceding claims, characterized the film is printed with color for generating the partial secret image becomes. Method according to one of the preceding claims, characterized that the slide for generating the partial secret picture with black or colorful color is printed. Method according to one of the preceding claims, characterized that the film is punched out to produce the partial secret image. Method according to one of the preceding claims, characterized that the film for fixing on the screen with at least one Adhesive strip is provided. Method according to one of the preceding claims, characterized that the characters on the image with buttons numbers, letters and / or Special characters are. Method according to one of the preceding claims, characterized that the image with the clickable and labeled buttons one Number field represents, where the number is between 0 and 9 respectively the individual buttons assigned. Method according to one of the preceding claims, characterized in that the inscriptions on the buttons on the image have a Ver indicate the position of the button to be clicked. Method according to one of the preceding claims, characterized that the number of buttons n + m - 1 is, where m is the number of all for the input available standing sign is. Method according to one of the preceding claims, characterized that the character string is a password, a PIN, a TAN or similar. Computer program product for carrying out the method according to one the claims 1 to 12 when the computer program is run on a processor. Foil to carry of the method according to one of claims 1 to 12, characterized that they have a partial secrecy image according to the process of visual Includes cryptography and to fix on the screen with at least one adhesive strip is provided. A film according to claim 14, characterized in that it consists of a transparent material. A film according to any one of the claims 14 to 15, characterized in that it is printed with color. A film according to any one of the claims 14 to 16, characterized in that they are black or colored Color is printed. A film according to any one of the claims 14 to 17, characterized in that it is punched out. Use of the method, the computer program product or the film according to a of the preceding claims with online accounts.