SE540742C2

SE540742C2 -

Info

Publication number: SE540742C2
Application number: SE1751014A
Authority: SE
Priority date: 2017-08-23
Filing date: 2017-08-23
Publication date: 2018-10-30
Also published as: WO2019038252A1; SE1751014A1; EP3673479A1

Abstract

There is provided a device for encrypting and storing a string of characters, such a PIN number or a password, the device comprising at least three circular discs placed on top of each other, forming a stack of discs, the stack of discs being connected in the centre of the discs with a hub enabling the discs to rotate relative to each other, the stack having an upper side and a reverse side, where the discs have openings and fields for writing numbers or letters by hand.

Description

DEVICE FOR ENCRYPTING AND STORING A NUMBER TECHNICAL FIELD This invention relates to devices and methods for encrypting and storing a number, for example a four-digit PIN-code.

BACKGROUND Most persons need to remember a large number of numbers in particular four-digit PIN-codesfor use with credit cards. This is a hassle, since it is necessary to use the number when making purchases or withdrawing cash. Six digit numbers are frequently also used as passwords for digital certificates for banking apps. Moreover, it is often necessary to enter a four digit number when entering doors locked by electronic locks.

The numbers must be kept secret so that unauthorized used is avoided. However, some people still, against firm advice from banks, write down the numbers, for example on a piece of paper, and store the paper together with the credit cards in the wallet.

In addition, a large number of passwords for websites, email and computers have to be remembered.

There is thus a need for a system that helps people remember their PIN-codes and passwords without disclosing them to others.

The cipher disc was proposed in 1467 by the Italian architect and author Leon Battista Alberti. It is a simple cryptographic device that consists of two concentric circular plates mounted one on top of the other, where one disc displays the letter of the alphabet and the other displays the code, on letter for each clear text letter.

However, the Alberti cipher disc is used only for translating a message from clear text to code and vice versa. It cannot be used for storing information.

Paper disc calculators are known, for example for calculating body mass index, calculating photo exposure times, betting odds, and patent filing dates.

SUMMARY OF INVENTION In a first aspect of the invention there a device for encrypting and storing a string of characters, such a PIN number or a password, the device comprising at least three circular discs placed on top of each other, forming a stack of discs, the stack of discs being connected in the centre of the discs with a hub enabling the discs to rotate relative to each other, the stack having an upper side and a reverse side, where the middle disc has a plurality of openings arranged at an equal distance from the centre of the stack, where the upper disc has at least one first opening arranged at the same distance from the hub as the openings of the middle disc, thereby enabling viewing of parts of the upper surface of the bottom disc from the upper surface of the stack when the first opening of the upper disc is aligned with an opening of the middle disc, where the upper disc has a second opening, which may be an opening that is separate from the first opening or an opening that is a part of the first opening, said second opening enabling the viewing of a part of the upper surface of the middle disc from the upper side, said second opening arranged at a different distance from the hub than the first opening, where the stack has visible marks that uniquely identify a set of positions of the three discs relative to each other, or means for making such marks, thereby identifying a plurality of key positions, where the number of key positions is equal or greater than the number of openings in the middle disc; and where each position of the discs relative to each other where an opening of the middle disc is aligned with the first opening of the upper disc, so that a part of the upper surface of the lower disc can be seen from the upper side, is defined by one and only one key position, where the areas of the lower disc and the middle disc that is visible from the upper side when the device is set in the various key position has fields suitable for manually writing characters on.

The device makes it easy to encrypt a string of characters, such as PIN-codes or a password in a manner that is sufficiently safe, so that the codes or passwords does not have be remembered. It is very difficult for a person that does not know which key position the user has used to encrypt and store the number, to obtain the string of characters. The device cannot be hacked by computer hackers. The device is easy to carry around in a wallet or a handbag, or can be kept in an office drawer.

The upper disc has plurality of sets of first and second openings in the upper disc, where the first and second opening of each set are visibly connected on the surface of the upper disc to form a reading position, where each reading position can be used to store and encrypt one string of characters. This makes it possible to store a plurality of strings of characters and also increases the safety of the device.

The first openings of the different reading positions may be placed along the same distance from the hub of the device and where the second openings are placed at the same difference from the hub of the device. This makes it possible to reuse the non-coding characters.

Each reading position may be visibly associated with a writing space or a unique identifier on the upper surface of the upper disc. This makes it easier for the user to find the correct code.

The first and second opening of each reading position may be visibly connected by being aligned along a line drawn from the hub towards the circular edge of the upper disc.

The visible marks on the rims may be characters or symbols, where each character or symbol occurs at most once on each rim, and where each key position is defined by aligning three characters or symbols.

The device may have a covering disc being placed on top of the other discs, the covering disc enabling the viewing of only one key position, the other key positions being hidden. This makes it difficult to other than the user to see the characters on the disc, and also protects the device from wear.

In a second aspect of the invention there is provided a method for encrypting and storing a string of characters, such as a password or a PIN code, where the string of characters are made up from a predefined group of allowed characters, comprising using a device according to claim 1 the method comprising the steps of a) writing a first part of the string of characters on a field of the bottom disc so that the characters are visible from the upper side when the device is set in a key position, b) writing the remaining part of the string of characters on the surface of the middle disc, so that the characters are visible from the second opening of the upper disc when the device is set in the same key position as in step a), and c) writing strings of characters, where the characters are selected from the predefined group of allowed characters, in at least one other field of the middle disc and in at least one other field of the bottom disc, where the two strings of characters together form a different string of characters than the encrypted string of characters, where the fields are associated with one other key position.

Step c) may preferably be carried out for all key positions other than the key position used in step a) and b).

In a preferred embodiment, the string of string of characters is a four-digit number and the first two or the last two digits are written in step a) and the other two digits are written in step b), and where two-digit numbers are written in step c).

DRAWINGS The accompanying drawings form a part of the specification and schematically illustrate preferred embodiments of the invention, and serve to illustrate the principles of the invention.

DRAWINGS Fig. 1 is a side view of a device.

Fig. 2 is a perspective view of a device.

Fig. 3 is an exploded view of a device.

Figs. 4-5 are exploded views of a device, before and after numbers have been written. Fig. 6 is a side view of a device with a cover disc.

Fig. 7 is a perspective view of a device with a cover disc.

Fig. 8 is a flowchart showing a method.

DETAILED DESCRIPTION The device may be used for encrypting and storing a string of character, such as letters, digits or symbols that constitute a password or a PIN number. A system that uses passwords usually define which characters are allowed in a password. Examples of characters that can be included in the password are the digits 0-9, the letters of an alphabet, for example the letters A-Z (upper case or lower case) and symbols such as ~ ! @ # $ % ^ & * ( ) _ - = { } [ ] \ | Image available on "Original document" The characters may also be characters from other alphabets, such as the Cyrillic, Greek, Arabic, Chinese, Japanese, or Hindi alphabet, that are allowed in passwords or PIN-codes. In a preferred embodiment, the string of characters is a PIN-code, such as four-digit PIN code.

The device 1 comprises at least three circular discs 5, 6, 7 placed on top of each other such that a stack of discs is formed.

The discs 5, 6, 7, 20 may be circular or essentially circular. For example, the discs 5, 6, 7, 20 may have jagged edges or have protruding parts such as small handles or knobs to facilitate turning of discs 5, 6, 7, 20 by hand.

The discs 5, 6, 7 are preferably made of a thin and stiff material such as polymer material, paper or metal. The device 1 is preferably of a size that is can be carried in a handbag, pocket or even a wallet. Thus, it is a portable device. At least some surfaces of the discs should be suitable for manually writing on (see below).

The discs 5, 6, 7 are connected in the centre of each disc with a hub 2 enabling the discs 5, 6, 7 to rotate relative to each other. Thereby the user can rotate the discs 5, 6, 7 relative to each other.

The stack has an upper side 3 and a reverse side 4, and when it is referred to "upper surface" it is the surface that is closest to the upper side 3.

The discs 5, 6, 7, are referred to as the lower disc 5, the middle disc 6 and the upper disc, where the upper disc 7 is closest to the upper side 3 of the stack.

The middle disc 6 has a plurality of openings 8 arranged at an equal distance from the centre of the stack, thus forming a ring of openings. The plurality of openings 8 are preferably evenly spaced.

The upper disc 7 has at least one first opening 9 arranged at the same distance from the hub as the openings 8 of the middle disc 6, thereby enabling viewing of parts of the upper surface 10 of the bottom disc 5 from the upper surface 3 of the stack when the first opening 9 of the upper disc is aligned with an opening 8 of the middle disc 6.

The upper disc 7 also has a second opening 11, which may be an opening that is separate from the first opening 9 or an opening that is a part of the first opening 9. The second opening 11 is arranged at a different distance from the hub 2 than the first opening 9, and enables the viewing of a part of the upper surface 23 of the middle disc 6 from the upper side 3 of the stack. The first 9 and second 11 opening in the upper disc 7 forms a set 12 of openings being a reading position 12. Again, the first 9 and second 11 openings in the upper 7 disc may be comprised in one opening, where a part of the opening enables viewing of the openings 8 in the middle disc 6 and the other part enables viewing of the upper surface 23 of the middle disc 6.

In the figures openings 8 and 9 are placed further from the hub 2 than opening 11, but opening 11 can just as well be placed further out from the hub 2 than openings 8 and 9.

The stack of discs 5, 6, 7 also has visible markings 13 that uniquely identify a set of positions of the three discs 5, 6, 7 relative to each other, or means for making such markings, thereby identifying a plurality of key positions, where the number of key positions is at least the number of openings in the middle disc 8 and where each position of the discs 5, 6, 7 relative to each other where an opening 8 of the middle disc 6 is aligned with the first opening 9 of the upper disc 7 is defined by one, and only one, key position. Thus, every position where the upper surface 10 of the lower disc 5 can be seen from the upper side 3 is defined by one key position.

The user can select the different key positions by rotating the discs relative to each other.

The areas of the lower disc 5 and the middle disc 6 that is visible from the upper side 3 when the device 1 is set in the various key position has fields 14, 19 suitable for manually writing visible characters on such as letters or digits on, for example with a pen. The fields 14, 19 should be large enough so that the characters can be written and are legible. Writing can for example be done by hand, with a pen or a pencil. When the discs 5, 6, 7 are made of paper the fields 14, 19 may be blank, for example white or light grey, surfaces suitable for writing on. The openings 8, 9, 11 should be large enough so that the digits that are written in the fields 14, 19 can be seen from the upper side 3 of the stack.

The key positions can be obtained in different ways. In one embodiment, the key positions are arranged as follows. The uppermost disc 7 has a smaller diameter than the middle disc 6 and the middle disc 6 has a smaller diameter than the lowest disc 5, thereby constantly enabling the viewing of at least a part the upper surface 15 of rim of the middle disc 6 and at least a part of the upper surface 16 of the rim of the lowest disc 5 from the upper side 3 of the device 1. There are visible marks 13 that uniquely identify a set of positions of the three 5, 6, 7 discs relative to each other, or means for making such markings, thereby identifying a plurality of key positions, are arranged along said visible rims.

Any legible symbols, letters or words can be used to identify the key positions. For example, letters from Chinese, Japanese, Cyrillic, Arabic, Greek, Hindi or any other alphabet may also be used. Symbols such as emojis may also be used. For example, the emojis "smiley" "dog" and "sun" may define one position, and the emojis "smiley', "dog" and "flower" may define another position. The rim may alternatively comprise spaces for the user to make his or her own visible marks 13 for identifying the key positions.

The marks 13 for uniquely identifying a set of positions of the three discs 5,6,7 relative to each other may for example comprise or consist of the letters of the English alphabet from A to Z, written on each of the rims of the discs. The key positions are identified by threeletter combinations, formed by aligning one letter from each disc.

The key positions may also be viewed by openings in the discs, in a similar manner as openings 8, 9 and 11.

The key positions can be created by the user, for example by writing the letters or writing symbols in predefined areas on the rims 16 and 15.

The key positions are preferably evenly spaced, and spaced such that each key position is associated with one writing field 14 of the middle disc 6 and one writing field 19 of the lower disc 5 (see below), such that one, and only one, writing 14a field of the middle disc 6 and one writing field 19a of the lower disc 5 are visible in each key position.

As shown in Figs. 2, 4, 5 and 7, the device can be arranged to store more than one number. Thus, there may be more than one reading position 12, where each reading position 12 is used for storing one number. Thus, device may have a plurality of sets 12 of first 9 and second 11 openings in the upper disc 7, where the first 9 and second 11 opening of each set 12 are visibly connected on the surface 17 of the upper disc to form a reading position 12, where each set 12 can be used to store and encrypt one number. All of the first openings 9 and the second openings 11 are preferably placed at the same distance from the hub 2, thereby forming rings of openings in the upper disc 7. This has the advantage that the fields 14, 19 can be reused for all reading positions 12. Thus, the first openings 9 can form one circle and the second openings 11 can form another circle. For example, the first opening 9 of the upper disc 7 can form an outer circle of openings in the upper disc 7. The second openings 11 may form an inner circle of openings in the upper disc 7. This is explained in more detail below.

The device 1 is used in the following way for encrypting and storing a number, in this case a four-digit number, such as a PIN code. The method can be modified to be used with for example a six-digit number or passwords consisting of other types of characters such as letters or other characters. The invention is most easily understood with reference to Figs 3 and 8, but the method applies to Figs 2, 4, 5 and 7 also.

In step 100 the user writes the first or the last digits of the number on of the fields 14a of the middle disc 6 so that they are is visible from the upper side 3 when the device 1 is set in one of the key positions. Writing can for example be done with a pen or a pencil. The user may select the key position himself. For example, when the key positions are formed by three letter combinations, the user may select a three letter-letter combination that is easy to remember, for example the three-letter combination CAT. The three-letter combination is kept secret since it is the key for retrieving the four-digit PIN code. For example, if the PIN code to be stored is 3277, the user writes 32 on the writing field 14a of the middle disc 6 which is visible in the second opening 11 when the key is set to CAT. The number 32 is thus visible from the upper side 3 of the device 1 because the field 14a is aligned with the second opening 11 in the upper disc 7.

In step 101 the user, while the device is still set in the same key position (CAT) writes the remaining digits (77) of the PIN-code in a writing field 19a on the upper surface 10 of the lower disc 5 so that the digits 77 are visible through opening 8 in the middle disc 6 and the aligned first opening 9 in the upper disc 7.

The numbers 32 and 77 are now visible from the upper side of the disc, trough openings 11 and 9, respectively.

Suitably the user writes the numbers so that they are arranged so that the combination 3277 are seen when the device is set in the CAT position. This depends on the direction of writing and if the first or second opening is arranged furthest away from the hub. Alternatively, 32 is seen in the openings 8 and 9 and 77 is seen in opening 11.

The user may choose to invert the numbers so that the digits 7732 is seen when the device is set in the CAT position. This increases security even more, but the user off course has to remember that the two numbers are inverted when using the PIN-code.

When the device is adapted for more than one PIN-code, steps 100 and 101 is repeated for the other codes. The remaining free fields 14,19 are used. The same key position (CAT) may be used, but this is not necessary.

A plurality of strings may be stored for each reading position by using more than one key position. For example, one PIN code may be read in the reading position CAT and one other PIN code may be read in the reading position when the selected key position is RAT. Thus, more than one string may be stored in the device with one reading position (Fig. 3) by using one key position for each string.

In step 102, the user writes two digit numbers in at least one other field 14 of the middle disc 6 and writes digits in at least one other field 19 of the bottom disc 5 (unoccupied fields), where the two-digit numbers are different from the numbers in steps 100 and 101 and are associated with one other key position, thus a key position other than CAT. In this example the user writes numbers that are different from 32 or 77. Preferably the user then fills all the remaining writing fields 14, 19 associated with key positions with random two-digit numbers that does not form the four digit number 3277, thus other two digit numbers than 32 and 77. This can be done by rotating the device to the various key positions so that the fields is visible in the openings, and then writing in the fields. When a plurality of strings of characters are encrypted and stored, the strings of characters selected in this step are preferably selected so that none of them form the encoded strings. Fig. 4 shows a device 1 for storing eight different strings of characters before the user has written any number on the device. Fig. 5 shows the device 1 of Fig. 4 when the user has carried out step 102 for all fields 14, 19.

Steps 100, 101 and 102 can be carried out in any order, although it is probably easier for the user to carry out steps 100 and 101 before 102.

Preferably a step 103 is carried out where the user rotates the discs so that the device is not set in the key position that discloses the four digit number, thus a position different from CAT in this example. Thereby the four digit number is not seen in the first 9 and second 11 openings of the first disc. The device 1 is now in the encrypted state.

In step 104 the user retrieves the number. When the user wants to retrieve the four digit number the user only has to remember the visible marking 13 that indicates the key position, which in this example is the three-letter combination CAT. The user rotates the discs so that the key position CAT is obtained and reads the digits 32 in the second opening 11 in the upper disc and the digits 77 in the first opening 9 in the upper disc. When the four digit number has been retrieved and use, for example at a credit card purchase, step 103 is preferably carried out again.

The encryption security of the device 1 is dependent on the number of key positions. Assuming that the user writes random numbers for the fields 14, 19 of all key positions there will be a number of combinations equal to the number of possible key positions. If the letters of the alphabet from A to Z is used for each of the discs, there will be 26 x 26 key positions = 676 key positions, which is sufficient for many purposes (There is not 26x26x26 because there is needed 26x26 to define the position of two discs relative to each other). If the first two digits are reversed with the second two digits (3277 to 7732), the number of possibilities is doubled 1352. This makes the device less secure than a four-digit PIN code itself, which has 10000 different possibilities (including the combination 0000), but still secure enough for many purposes. The embodiment shown in Figs 2-5 has 24x24 combinations = 576 possible combinations, since it uses the letters a-y.

Each reading position 12 may be visibly associated with a writing space 18 or a unique identifier on the upper surface 17 of the upper disc. The first and second openings 9, 11 of each reading position 12 may be visibly connected by being aligned along a line drawn from the hub 2 towards the circular edge of the upper disc. However, this is not necessary and the first and second openings 9,11 of a reading position 12 can for example be associated with bent lines or coloured zones on the upper surface 17 of the upper disc 7.

The embodiment shown in Figs. 2, 4-5 and 7 has eight reading positions 12, and can thus be used for, for example, to store PIN numbers for eight different credit cards or the like. Each reading position 12 may be associated with writing space 18 on the upper surface 17 of the upper disc 7 where the user may make notes for remembering what each reading positions is used for, such as "Card No. 1", Card No. 2 etc", preferably in a way that makes it impossible to identify the card in question. For example, it is unsuitable to write "Visa that ends with 66542". If this is done correctly someone who finds the device 1 and the credit card will not be able to pair the reading position 12 with the correct card. Alternatively, the different reading positions 12 can be identified with different symbols or colours. The device 1 may also be used for storing passwords for logging in to computers, email accounts web services, web shopping sites and the like, and the fields can be used for associating the correct reading position with the correct site, computer or web service.

The use of multiple reading positions increases the security with a factor equal to the number of reading positions 12. When there are eight reading positions the security is increased by a factor 8. The embodiment with letters 1-y shown in the figures and eight reading positions 12 thus has 4608 possible combinations, which makes the device 1 even safer. A device 1 with the letters a-z would have 676 x 8 = 5408 possible combinations of cards and digits.

As seen in Figs. 6-7 the device 1 may have a cover disc 20 (a fourth disc) placed on top of the upper disc 7. The cover disc 20 is made of a transparent and stiff material, for example a transparent polymer material such as polycarbonate or an acrylic polymer. The rim 21 of cover disc 20 is arranged such that all key positions but one cannot be read, thus the other key positions are hidden. The lower disc 5, the middle disc 6 and the upper disc 7 can rotate relative to the covering disc 20. For example, when covering disc 20 is made of transparent plastic, the rim 21 may be coloured so that only one key position is visible in a window 22. The covering disc 20 servers the purpose of covering the other key positions so that they are not seen by anyone but the user, for example people standing in line next to the user. This covering disc 20 also protects the upper disc 7, which may be made of paper, from damage and precipitation. In particular, the cover disc 20 protects the handwritten numbers from smearing and the like.

When the device 1 is used for encoding and storing a six digit number, the number may be divided into two groups each comprising one three-digit number, and each of the three digit numbers are written on each space 14, 19. For example, the number 123456 may be divided into 123 and 456. When the devicel is used for a five digit number it may be divided into a two-digit number and a three digit number.

The device 1 may be designed to accommodate numbers of different length. For example, it may be difficult to fit writing spaces for long numbers for many (for example eight) different key positions. Therefore, the number of reading positions may be decreased in order to accommodate longer numbers.

When device 1 is used for storing a password comprising letters from an alphabet, or a combination of letters and/or digits and/or other symbols allowed in passwords such as question marks or exclamation marks, the method described above with reference to Fig. 8 can also be used. The password is then suitably divided into two parts and a first part is written on a field 19 on the lower disc 5 and a second part is written on field 14 of the middle disc 6. For example, if the password is "rain35!" the password can, as an example, be divided into a first part "rai" and second part "n35!". The remaining fields 14, 19 are then filled with random letters and/or digits and/or symbols that are allowed in passwords. The password can be divided in any suitable way, but it is often preferably to divide in two parts of approximately equal length. A string of characters consisting of an even number of characters is preferably divided in two equal parts, such as 2+2, 3+3 or 4+4. A string of characters consisting of an uneven number of characters is preferably divided into 2+3, 3+4, 4+5 number of characters, etc.

While the invention has been described with reference to specific exemplary embodiments, the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention. The invention is generally defined by the claims.

Claims

CLAIMS 1. A device for encrypting and storing a string of characters, such a PIN number or a password, the device comprising at least three circular discs placed on top of each other, forming a stack of discs, the stack of discs being connected in the centre of the discs with a hub enabling the discs to rotate relative to each other, the stack having an upper side and a reverse side, where the middle disc has a plurality of openings arranged at an equal distance from the centre of the stack, where the upper disc has at least one first opening arranged at the same distance from the hub as the openings of the middle disc, thereby enabling viewing of parts of the upper surface of the bottom disc from the upper surface of the stack when the first opening of the upper disc is aligned with an opening of the middle disc, where the upper disc has a second opening, which may be an opening that is separate from the first opening or an opening that is a part of the first opening, said second opening enabling the viewing of a part of the upper surface of the middle disc from the upper side, said second opening arranged at a different distance from the hub than the first opening, where the stack has visible marks that uniquely identify a set of positions of the three discs relative to each other, or means for making such marks, thereby identifying a plurality of key positions, where the number of key positions is equal or greater than the number of openings in the middle disc; and where each position of the discs relative to each other where an opening of the middle disc is aligned with the first opening of the upper disc, so that a part of the upper surface of the lower disc can be seen from the upper side, is defined by one and only one key position, where the areas of the lower disc and the middle disc that is visible from the upper side when the device is set in the various key position has fields suitable for manually writing characters on. 2. The device according to claim 1 where the upper disc has plurality of sets of first and second openings in the upper disc, where the first and second opening of each set are visibly connected on the surface of the upper disc to form a reading position, where each reading position can be used to store and encrypt one string of characters. 3. The device of claim 2 where the first openings of the reading positions are placed along the same distance from the hub of the device and where the second openings are placed at the same difference from the hub of the device. 4. The device according to claim 2 or 3 where each reading position is visibly associated with a writing space or a unique identifier on the upper surface of the upper disc. 5. The device of any one of claims 2-4 where the first and second opening of each reading position is visibly connected by being aligned along a line drawn from the hub towards the circular edge of the upper disc. 6. The device of any one of claims 1 to 5 where the uppermost disc has a smaller diameterthan the middle disc and the middle disc has a smaller diameterthan the lowest disc, thereby enabling the viewing of the upper surface of rim of the middle disc and the upper surface of the rim of the lowest disc from the upper side of the device, and where the visible marks that uniquely identify a set of positions of the three discs relative to each other, or means for making such markings, thereby identifying a plurality of key positions, are arranged along said visible rims. 7. The device according to claim 6 where the visible marks on the rims are characters or symbols, where each character or symbol occurs at most once on each rim, and where each key position is defined by aligning three characters or symbols. 8. The device according to any one of claims 1 to 7 where the device has a covering disc, the covering disc being placed on top of the other discs, the covering disc enabling the viewing of only one key position, the other key positions being hidden. 9. A method for encrypting and storing a string of characters, such as a password or a PIN code, where the string of characters are made up from a predefined group of allowed characters, comprising using a device according to claim 1 the method comprising the steps of a) writing a first part of the string of characters on a field of the bottom disc so that the characters are visible from the upper side when the device is set in a key position, b) writing the remaining part of the string of characters on the surface of the middle disc, so that the characters are visible from the second opening of the upper disc when the device is set in the same key position as in step a), c) writing strings of characters, where the characters are selected from the predefined group of allowed characters, in at least one other field of the middle disc and in at least one other field of the bottom disc, where the two strings of characters together form a different string of characters than the encrypted string of characters, where the fields are associated with one other key position. 10. The method of claim 9 where step c) are carried out for all key positions other than the key position used in step a) and b).

1. The method of claim 10 or 11 where the string of characters is a four-digit number and the first two or the last two digits are written in step a) and the other two digits are written in step b), and where two-digit numbers are written in step c).