EP1676394A1

EP1676394A1 - Cryptographic method and device

Info

Publication number: EP1676394A1
Application number: EP04763626A
Authority: EP
Inventors: Gerd Dirscherl; Berndt Gammel; Rainer GÖTTFERT; Steffen Sonnekalb
Original assignee: Infineon Technologies AG
Current assignee: Infineon Technologies AG
Priority date: 2003-09-30
Filing date: 2004-07-29
Publication date: 2006-07-05
Also published as: WO2005043803A1; DE10345378A1; DE10345378B4; US20060265604A1

Abstract

The invention relates to a cryptographic method and device. The encryption unit (12) and the decryption unit (14) provided in a cryptographic device (10) can be used both for encryption and decryption, without their effects mutually canceling each other, if an encryption linking device (16) is provided between the decryption input of the decryption device (12) and the encryption output of the encryption device (14), said encryption linking device mapping the encryption result data block at the encryption output onto a mapped encryption result data block according to an encryption linking mapping which is for example used in encryption. A decryption linking device (18) maps the encryption result data block at the encryption output onto an inversely mapped encryption result data block according to a decryption linking mapping which is inverse relative to the encryption linking mapping and which is for example used in decryption.

Description

description

Method and device for encryption / decryption

The present invention relates generally to

Encryption / decryption scheme, as can be used, for example, to protect memory contents against unauthorized reading.

In the case of data storage secured against unauthorized spying, the data to be stored is not in plain text, i.e. unencrypted, but saved in encrypted form, as a so-called ciphertext or so-called ciphertext. If the data is to be read at a later point in time, it naturally has to be decrypted again before it can be processed further. Examples of applications in which this effort for storage is worthwhile are diverse and include, for example, chip cards, smart cards or magnetic cards, on which information to be protected, such as monetary amounts, keys, account numbers, etc., is to be protected from unauthorized access.

5 illustrates the situation again. Data to be protected will not be unprotected potential

Deliver to attackers, stored in encrypted form, referred to in Fig. 5 as a cipher domain. Outside the cipher domain, the data to be protected is in plain text, referred to as plain text domain in FIG. 5. The border between plain text and cipher domain is shown in FIG. 5 with a semicolon. An interface between the plain text and cipher domain is formed by an encryption / decryption device 900. The encryption / decryption device 900 is used to encrypt unencrypted data to be stored from the plain text domain and to use it in encrypted form

Output storage to the cipher domain, and vice versa, when accessing or reading out this data, again in decrypt the data to be read out in encrypted form in order to output it in plain text to the plain text domain. The underlying encryption scheme is symmetrical encryption, ie one in which the inverse encryption, ie the decryption, can be carried out with approximately the same effort as the encryption. The encryption / decryption device 900 therefore consists of two parts which are approximately the same size or have the same complexity in their implementation, namely an encryption unit or an encryption part 902 and a decryption unit or a decryption part 904. The encryption unit 902 simulates data at an encryption input thereof a specific encryption algorithm in blocks on encrypted data and outputs it to an encryption output of the same. The encryption unit 902 is provided in the device 900 in such a way that it stores data blocks Bj _. , ..., B _N , with N e IN, which are in plain text, receives at its encryption input, so that the encryption unit 902 outputs encrypted data blocks Ci, ..., C _N at the encryption output, the so-called cipher. The decryption unit 904 is responsible for the reverse direction, namely not for storing data but for reading data from the memory in the cipher domain into the plain text domain.

Accordingly, the decryption unit 904 is designed to map data at its decryption input to decrypted data according to a decryption algorithm which is inverse to the encryption algorithm of the encryption unit 902, and outputs this decrypted data at a decryption output thereof. In the device 900, the decryption unit 904 is provided in such a way that it receives data blocks Ci,..., C _N stored in encrypted form at the data input, decrypts this cipher Ci,..., C _{N in} blocks and the data blocks Bi at the decryption output , ..., B _N in plain text to the plain text domain. A disadvantage of the procedure described with reference to FIG. 5, namely to provide separate hardware specifically for decryption and encryption, is that one part is idle when encryption or decryption is carried out. The effectiveness of such an encryption / decryption device is therefore low in that it has a poor relationship between security on the one hand and chip area on the other.

The object of the present invention is to provide an encryption / decryption scheme that is more effective.

This object is achieved by a device according to claim 1, 13 or 14 and a method according to claim 15, 16 or 17.

The knowledge of the present invention is that the encryption unit and decryption unit present in an encryption / decryption device can both be used for both encryption and decryption, without their effects canceling each other out, if between the decryption input of the decryption device and the encryption output the encryption device is provided with an encryption link device which maps the encryption result data block at the encryption output to a depicted encryption result data block in accordance with an encryption link mapping and is used in, for example, encryption, and a further decryption link device which converts the encryption result data block at the encryption output to an inverse-depicted encryption result data block depicts encryption link mapping that is inverse to the encryption link mapping and is used, for example, in decryption. The complexity of the structure does not increase enormously, since the actual encryption or decryption is carried out by the two devices, namely the encryption and decryption device, with a correspondingly high non-linearity of the underlying images. The encryption link and decryption link mapping are only there to ensure that the effects of the encryption mapping and the decryption mapping as implemented by the encryption and decryption device do not cancel each other out. Encryption can now be effected in that a data block to be encrypted runs through at least once the sequence of encryption device, encryption link device and decryption device or is processed serially by these devices. The decryption can then be carried out based on the same encryption and decryption device in that a data block to be decrypted runs through at least a sequence of encryption device, decryption link device and decryption device.

Consequently, both encryption and

Decryption used both devices, encryption and decryption device, whereas in the past one of the two devices was only responsible for encryption and the other was exclusively responsible for decryption. In addition, two different encryption or decryption processes are effectively carried out in series, which conventionally had to be achieved by two rounds through the encryption or decryption device.

A special form of encryption or decryption link mapping according to an exemplary embodiment of the present invention is, for example, an implementation This mapping in the form of suitably routed conductor tracks, such that they perform a permutation of the bits of the encryption result data block from the encryption output to the decryption input or a reverse or inverse permutation. Such an implementation hardly costs any chip area.

Preferred exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. Show it:

1 is a block diagram of an encryption / decryption device according to a general embodiment of the present invention;

FIG. 2 shows a schematic illustration of an encryption process and a decryption process, as is possible with the device from FIG. 1 according to a further exemplary embodiment of the present invention;

3a shows a schematic illustration of an encryption process according to a further exemplary embodiment of the present invention;

3b shows a schematic illustration of a decryption process for decrypting a ciphertext encrypted after the encryption of FIG. 3a according to an embodiment of the present invention;

FIG. 4 shows a block diagram of an encryption / decryption device which implements the encryption according to FIG. 3a and decryption according to FIG. 3b, according to an embodiment of the present invention; and 5 shows a block diagram of an encryption / decryption device with an encryption unit for encryption and a decryption unit for decryption.

Before the present invention is illustrated in more detail with reference to the figures in exemplary embodiments, it is pointed out that the same elements or similar elements in these figures are provided with the same reference symbols or similar reference symbols and that a repeated description of these elements is omitted.

1 shows an encryption / decryption device 10 according to an embodiment of the present invention. The encryption / decryption device 10 is able to encrypt incoming data blocks which are to be encrypted into encrypted data blocks and to decrypt data blocks to be decrypted into decrypted data blocks.

For this purpose, the encryption / decryption device 10 has an encryption device 12, a decryption device 14, a permutation device 16, an inverse permutation device 18 and a control device 20. Furthermore, the encryption / decryption device comprises a data input 22 for those to be encrypted Data blocks, a data input 24 for the data blocks to be decrypted, a data output 26 for the encrypted data blocks and a data output 28 for the decrypted data blocks.

In FIG. 1, solid arrows indicate the path that a data block to be encrypted takes in device 10, ie which sequence of devices it goes through. The dashed arrows indicate the sequence of data blocks to be decrypted by devices of the device 10. This is controlled by the control device 20, which for this purpose has, for example, switches, multiplexers or the like, as is illustrated in more detail by way of example with reference to the exemplary embodiment of FIG. 4.

After the structure of the device 10 has been roughly described above, its mode of operation is described in more detail below. The encryption device 12 is designed to map data blocks at its encryption input block-wise according to an encryption mapping to encryption result data blocks and to output the latter at its encryption output. The encryption map is preferably a non-linear map that maps n-bit data blocks to m-bit data blocks, where n and m are integers, i.e. m, n IN. In the present exemplary embodiment, n = m, although m> n could also apply if special further conditions are placed on the plain text blocks and the mapping E. As will become clear in the exemplary embodiments of FIGS. 3a, 3b and 4, the encryption mapping can be implemented, for example, using one or more S-boxes. The encryption by the encryption device 12 is expressed below with E (E for encryption), an n-bit data block B being mapped onto a ciphertext C, which is expressed by E (B) = C.

The decryption device 14 is designed to map data blocks at its decryption input in accordance with a decryption map to decryption result data blocks and to output the latter at its decryption output, the decryption map being inverse to the encryption map. The decryption device 14 consequently implements a mapping D (D for decryption), for which it applies that for every possible unencrypted n-bit data block B it applies that D (E (B)) = B, that is to say that the decryption device 14 would always map an original data block at its decryption input E (B) to a data block B at its decryption output, which the encryption device 12 maps to the original data block E (B). At the same time, this means that E (D (E (B))) = E (B) for all possible B applies. If m> n, the decryption map would therefore be a map D that maps m-bit data blocks to n-bit data blocks and would only be defined for E (B) blocks. If the illustrations are connected in series, it should therefore be ensured that the

Figure D only affects E (B), i.e. the image set in Figure E. For m = n, as is the case here, E (D (B)) = B also applies to all possible n-bit blocks, since the image set of Ξ is equal to the definition set of D. Loosely E should preferably be different from D, i.e. E should not be self-inverting.

If the encryption result data blocks at the encryption output of the encryption device 12 of the decryption device 14 or their decryption input are fed directly, their effects would cancel each other out, i.e. a data block at the encryption input of the encryption device 12 would be output unchanged at the decryption output of the decryption device 14. As will be described in the following, this is avoided by the permutation devices 16 and 18, respectively. Like the encryption device 12, the decryption device 14 can be implemented by one or more S-boxes, namely by. S-boxes that are inverse to those that form the encryption device 12.

The permutation device 16 comprises an n-bit permutation input and an n-bit permutation output. The permutation device 16 is provided in order to permute the bits of an n-bit data block at the permutation input, ie to rearrange them, and to permute the permuted n-bit data block at the permutation input. output output. In other words, the n-bit data block at the permutation input consists of a sequence of n bits, the sequence of which is changed by the permutation by the permutation device 16. The permutation device 18 also has a permutation input and a permutation output. It is provided in order to permute the n bits of an n-bit data block at the permutation input exactly inversely to the permutation of the permutation device 16. This means that if an n-bit data block with the order of the bits were applied to the permutation input of the inverse permutation device 18, as it resulted after permutation by the permutation device 16, the n would again arise at the permutation output of the inverse permutation device 18 -Bit data block with the bit sequence as it was present at the permutation input of the permutation device 16.

Both permutation device 16 and inverse permutation device 18 can be implemented as conductor tracks which connect the individual n bit inputs at the permutation input to different ones of the n bit outputs at the permutation output.

The control device 20 is now able to encrypt data blocks at the input 22 and to decrypt them

Routing data blocks 24 through devices 12, 14, 16 and 18 in various ways. According to the exemplary embodiment of FIG. 1, the control device 20 ensures that a data block to be encrypted at the data input 22 runs through the sequence of encryption device 12, permutation device 16 and decryption device 14. The data block 22 to be encrypted is processed in succession by the encryption device 12, the permutation device 16 and the decryption device 14. First of all, the data block to be encrypted - it is denoted by B - arrives at the encryption input of the encryption device 12. Key mapping E is mapped to an encryption result data block C = E (B). An order is of course defined via the n bits of the n-bit encryption result data block C. In this sequence, the encryption result data block C is applied to the permutation device 16. The permutation is referred to in the following P.

At the permutation output there is a data block with the order of the bits changed to the encryption result data block C, i.e. C = P (C). With this changed order, the data block C is applied to the decryption input of the decryption device 14. As said, without the permutation, the decryption device 14 would now map the block to B. But now this forms the

Data block C according to the decryption mapping D on a decryption result data block from which simultaneously represents the final result of the encryption according to the present embodiment, and is shown here with C b _n is _Erge. C _result i _s = D (C ') or, expressed for the entire sequence of images that have been run through, C _result i _s = D (P (E (B))) _applies .

The control device 20 ensures that data blocks to be decrypted at the input 24 run through a different sequence of devices, namely the sequence of encryption device 12, inverse permutation device 18 and decryption device 14. It is assumed, for example, that the data block to be decrypted is the encrypted data block just received C _e esult is. This data block

C _result i _s is carried out by the input 24 to the encryption input of the encryption _device 12. This applies the encryption mapping E to the data block. At the encryption _{output of the encryption device} 12 there is therefore an encryption _result _{data block} of C _result '= E (C _result is) = E (D (P (E (B)))) = P (E (B)) = C. The encryption device 12 thus makes the mapping exactly the decryption mapping that was carried out at the end of the encryption. At the output of the encryption device 12 there is thus an encryption result data block C, as well as by sequential application of the encryption mapping E and

Permutation P would be obtained on the originally encrypted data block.

The result encryption data block C at the output of the encryption device 12 is now fed to the permutation input of the inverse permutation device 18. This operation changes the order of the n bits of the n-bit encryption result data block in a manner exactly the opposite of that used to obtain the intermediate encryption result C during encryption. The result at the permutation output 18 is C _result ni _s "= P ^-1 (C) = P ^_1 (P (E (B))) = E (B) = C. The encryption _{result data block} C is consequently not in order when decrypting of the bits as present at the encryption _{output are} applied to the decryption _{input of the} decryption device 14, but with the order changed by the inverse permutation _device 18, namely as C result '' = C. The decryption device 14 forms this data block C at its decryption selection input according to decryption mapping D to D (E (B)) = B, again the data block in plain text.

Consequently, the device 10 of FIG. 1 is able both to encrypt data blocks in plain text in cipher data blocks and to decrypt cipher data blocks back into data blocks in plain text, encryption device 12 and decryption device 14 both for encryption and for decryption are involved in the processing of the data blocks to be decrypted or encrypted. With reference to the description of FIG. 1, it is briefly pointed out that it would of course be possible to “lock” data blocks to be decrypted and encrypted not first through the encryption device 12, but through the decryption device 14, and only at the end through the encryption means 12, so that for a data to be encrypted block B, the ciphertext C _OUTCOME = e (P (D (B))) and vice versa for the ciphertext C _He result back to the plaintext data block B from e (P ^-1 ( D (C _result i _s ))) would result as long as only n = m.

With reference to FIG. 1, it is pointed out that by appropriately restricting the permitted n-bit plaintext data blocks among the possible n-bit combinations and appropriately defining E as a mapping from n- to m-bit data blocks and from P, it could be achieved that also for m> n E (D (P (B))) = P (B) for all permitted B and all possible P, for example with n = 3 and m = 6, if ensured is that all 8 permitted 3-bit data blocks are mapped by Ξ only to 8 of the 68 possible 6-bit data blocks, and the permutation only takes place in such a way that the permuted block P (B) is again one of the eight among the 120 possible or with n = 5 and m = 6, if only 30 of the 32 possible 5-bit data blocks are allowed and by E these are only mapped to the 30 of the 68 possible 6-bit data blocks, two bits with 1 and 4 Have bits with 0 or vice versa, because then by permutating each 6-bit data block back to one with the same same property is mapped.

In the following it is further assumed that n = m. In this case, it is possible for the control device 20 to have the data blocks to be encrypted run through the sequence of encryption device 12, permutation device 16 and decryption device 14 more than once, and accordingly the data blocks to be decrypted also repeat the sequence of encryption steps. direction 12, inverse permutation device 18 and decryption device 14. The security of the encrypted, stored data can be increased by going through it several times.

2 schematically shows sequences of processing which the control device 20 provides for encryption or decryption according to an exemplary embodiment of the present invention. 2, it is assumed as an example that n = m = 32, that is to say that the data block to be encrypted and the data block to be decrypted and the encrypted and decrypted data block are 32 bits long.

The upper line of FIG. 2 represents the encryption process as it is effected by the control device 20. A data block to be encrypted (far left) is successively subjected to the same serial processing 30 times in iterative or repeated fashion. Each round 30 comprises a sequence of an encryption map E, a permutation P, a decryption D and one

Permutation P. Briefly referring again to FIG. 1, this meant that the control device 20 repeatedly passed data blocks to be encrypted through the encryption device 12, the permutation device 16, the decryption device 14 and the permutation device 16, sequentially in this order. At the end (on the far right in FIG. 2), the encrypted data block would result at the output 26.

The decryption is shown in Fig. 2 in the lower line. A data block to be decrypted is subjected to a sequence of images which results when the top line is read upside down, ie from the right, ie the processing sequence is reversed, and each image is inverted, ie P ^_1 reads instead of P, E reads instead of D. and D reads instead of E, which means that each device is exchanged for its inverse device. Data blocks to be decrypted consequently also processed in rounds 32, each round 32 having a sequence of maps P ^"1 , E, P ^-1 and D. At the end (far right in FIG. 2) there is a decrypted data block.

It can be clearly seen from FIG. 2 that the rounds 30 and 32 actually represent double rounds in which encryption E and decryption or decryption mapping D 'is carried out. 2, the encryption device and the decryption device or the underlying hardware are therefore used with a time offset, both in the case of encryption and decryption. Encryption according to the upper line in FIG. 2 can of course be carried out in the device of FIG. 1 at the same time as decryption according to the lower line in FIG. 2 if the two processes are carried out in a pipelined manner, so that the encryption device E is straight is used for encryption while the decryption device is currently working for decryption.

The embodiment of FIG. 2 can of course be varied as desired. It is not imperative that only the permutation P is used for the encryption, while only the inverse permutation P ^{_1 is} used for the decryption. Alternatively, for example, an encryption round 30 could also be E, P, D, P ^_1 , while the corresponding decryption round 32 was P, E, P ^_1 , D.

In the preceding exemplary embodiments of FIGS. 1 and 2, little attention has been paid to the implementation of the encryption and decryption device. In the following, exemplary embodiments are described with reference to FIGS. 3a, 3b and 4, in which the encryption mapping and the decryption mapping are implemented by 4x4-S boxes which each map four different bits of the data block at the encryption input to four different bits of the data block at the encryption output. The advantage here is that the implementation of an S-Box, such as a 32-bit S-Box, means less hardware effort if it is implemented by smaller S-Boxes, such as eight 4x4-S-Boxes.

3a shows an encryption according to an exemplary embodiment of the present invention. As in the exemplary embodiment in FIG. 1, a plurality of devices are available for encryption, with a further device being available for each device that carries out a specific mapping, which carries out the inverse mapping for this. In the embodiment of FIG. 3a, 4x4-S-boxes Si-SQ serve as encryption device 12 ', while eight S-boxes S ^ ¹ -S _g ^1, which are inverse thereto, serve as decryption device 14'. Furthermore, two identical mapping devices 40 and 42 are available which output a 32-bit data block at their 32-bit data input in accordance with a self-inverting linear mapping or linear transformation to a 32-bit data block at their data output. Furthermore, two rotation devices 44 and 46 are provided, which rotate a 32-bit data block at their rotation input according to a bit rotation R by a predetermined number of bits in a predetermined direction and output the result of the rotation at their rotation output. Finally, two 32-bit XOR logic devices are provided, each consisting of 32 XOR gates, which bit by bit the 32 bits of a 32-bit data block with the bits of a 32-bit

Round key, namely once Ki and the other time K ₂ , XOR and output the result as a 32-bit data block. These XOR links are indicated at 48 and 50, respectively.

According to the encryption example of Fig. 3a, a plaintext data block B only goes through a double round 52, i. H . a Processing sequence which has an encryption 12 'once or in one partial round and a decryption 14' the other time or in the other partial round. The double round 52 is thus divided into two partial rounds, namely 52a and 52b, which are carried out sequentially. The first sub-round 52a, which the plaintext data block B passes through, consists of the sequence of XOR link 48 with the round key Ki, encryption mapping by the S-boxes Si-S ₈ , linear transformation 40 and subsequent rotation 44. After passing through the sub-round 52a, processing is carried out by the sub-round 52b, which includes a sequence of the XOR combination with the round key K ₂ , a decryption mapping by the inverse S-boxes ST / ¹ -S ^"1 , linear transformation 42 and

Rotation 46 has. After sub-round 52b, cipher C or cipher block C results.

Expressed a little more precisely, according to the exemplary embodiment in FIG. 3a, a data block B to be encrypted passes through the XOR logic device 48. The result at the output of the XOR logic device 48 is a data block, the bits of which at the points where the round key K _x a has logic one, is inverted to the corresponding bits of data block B, while the remaining bits are identical to the corresponding bits of data block B.

Then the bits are fed to the S-box inputs of the S-boxes Si - S ₈ , namely the most significant four bits 31 - 28 of the S-Box S _x , the next least significant bits 27 - 24 of the S-Box S, etc. The S-Boxes Si - Sg map the 4-bit words present at their S-Box inputs to 4-bit words shown in accordance with a mapping rule assigned to them, which is preferably non-linear and different for all S-Boxes. The four bits at the S-box outputs of the S-boxes Si-S ₈ are then supplied as a 32-bit data block to a 32-bit data input of the linear transformation device 40, namely the four bits of the S-Box Si as the most significant four bits 31-28, the four output bits of the S-box S ₂ as the next least significant bits 27 - 24 ... and the bits of the S-Box S ₈ as bits 3 - 0.

The linear transformation device 40 maps the data block at its data input by linear mapping to another 32-bit data block. In the present embodiment, the linear mapping L is even self-inverting, so that executing L twice on a data block would result in the data block again, i.e. L ((B)) = B. The resulting data block at the data output of the linear transformation device 40 is forwarded to the rotation device 44, which shifts the bits of the data block applied to its data input to the right or left by a number of bits depending on the rotation R, and appends the bits pushed out again to the bit positions that become free. The data block at the output of the rotating device 44 thus represents the result of the first partial round 52a.

This 32-bit data block is then again an XOR

Linkage 50 is now subjected to a round key K, in turn inverting the bit positions at which the round key K _{2 has} a logical one. In each case four successive bits of the resulting data block are then transferred to the inverse S-boxes S ^"1 - S ^{" 1} on their S-box

Inputs supplied, which then perform inverse mapping on the supplied 4-bit words, namely the S-box S ^ ¹ one to the mapping of the S-box Si inverse mapping, the S-box S ^"1 one to the mapping of the S -Box S ₂ inverse mapping, etc. The 4-bit words at the S-box outputs of the S-boxes S ^ ¹ - S _g ¹ in turn form a 32-bit data block that is connected to the

Linear transformation device 42 is applied, which carries out the same linear transformation as the linear transformation device 40. The result of the linear mapping is a 32-bit data block which is fed to the input of the rotation device 46 and which rotates this data block in the same direction by the same number of bits as the Rotation device 44. The resulting 32-bit data block is the ciphertext C or the ciphertext data block C.

As in the embodiment of FIG. 2, the passage of a plurality of double rounds 52 could also be provided in order to perform encryption, as is also provided for the implementation of the encryption according to FIG. 3a according to the embodiment of FIG. 4. As can be seen from the illustration of the encryption sequence in FIG. 3a, a mapping is carried out between each encryption or decryption map 12 'or 14', which can be referred to as an encryption link map. While this link encryption mapping was, for example, the permutation P in the exemplary embodiment of FIG. 1, this is the case in the exemplary embodiment of FIG.

3a shows the sequence of the linear transformation L, rotation R and the XOR round key combination 50. While the S boxes Si - S ₈ and S ^"1 - S _g ¹ cause confusion in the cipher, that is to say that the relationship between the round keys and the Cipher is as complex as possible, ensure the linear

Mapping L through multiple XOR operations of the bits in the individual data blocks to ensure that small changes in the plaintext data block have a major impact on the ciphertext data block. Above all, however, the linear transformations L ensure that the bits that are output by the S-boxes Si-Sβ are effectively mixed with other bits of other bit positions and shifted to other bit positions, so that they are not simply rotated get to predetermined subsequent inverse S-boxes.

With reference to FIG. 3a, it is also pointed out that in the description of the encryption process it was assumed that two linear transformation devices 40 and 42 as well as two rotation devices 44 and 46 and two XOR combination devices 48 and 50 are provided. Of course, this is not necessary. With everyone Partial round 52a-52b could be run through the same device, ie in the partial round 52a the same linear transformation device as in the partial round 52b, in the partial round 52a the same rotating device as in the partial round 52b and in the partial round 52a the same XOR combination device using the key K. _x as in the partial round 52b, but in the latter the round key K ₂ is used. The multiple use of these devices would only increase the control effort for the control device (not shown) in order to ensure that the plain text data block B or the intermediate results derived from the same pass through the devices in the appropriate order. The embodiment of FIG. 4 still to be discussed relates to an implementation example for the encryption process of FIG. 3a using two devices in each case, as has been illustrated in FIG. 3a.

FIG. 3b shows a decryption round for decrypting a cipher data block C, as is obtained by an encryption round 52 from FIG. 3a. The round of decryption is generally indicated at 60. It in turn consists of two sub-rounds 62 and 64. A cipher data block C runs in a decryption round the same S-boxes Si - S ₈ or S ^ ¹ - S _g ¹ as in the encryption round of

Fig. 3a, or the same encryption and decryption device 12 'and 14'. Depending on the implementation, the other devices can be chosen to be identical to the devices in the encryption, or they can be provided specifically for the decryption. In FIG. 3b, the remaining devices are provided with their own reference numerals, as if they were different from those of FIG. 3a, the exemplary embodiment representing an opposite implementation option with respect to the linear imaging devices. A cipher block C goes through two inverse rotators 66, 68, two linear transformers 70 and 72 and two XOR links 74 and 76 during a decryption round 60.

During the decryption, the images are carried out on the cipher data block, as are also carried out on the plain text data block in the case of encryption, but in reverse order and inverted. That is, in accordance with the rotation 46 of FIG. 3a, an inverse rotation is first carried out by the rotation device 66 on the cipher data block C, i.e. a shift of the bits of the cipher frame C by a number of bits identical to that of the rotation R but in the opposite direction. The 32-bit bit-rotated

Data block is passed on to the linear transformation device 70. This performs the same linear mapping on the incoming data block as the linear transformation devices 40 and 42 and also the linear transformation device 72. This is because, as mentioned above, the linear mapping according to the present exemplary embodiment is self-inverting, so that L ( L (B)) = B applies. Thereafter, the passage of the S ^-1 boxes of Figure 3a corresponding to the at the output of the linear transformation means 70 resulting 32-bit data block in units of 4-bit words of the S-boxes Si -. S ₈ as the encryption device 12 'fed. The resulting 32 bits are XOR-linked with the round key K ₂ . This link corresponds to link 50 of FIG. 3a. Like the self-inverting mapping L, the XOR link 50 is also a self-inverting mapping, since the repeated inversion of the bits at the bit positions at which the 2-bit round key K _{2 has} a one again supplies the original data block. The result of the XOR link 74 is the result of the sub-round 62. The sub-round 64, which follows the sub-round 62, then corresponds to a reversal of the sub-round 52a of the encryption round 52 from FIG. 3a. There, the data block is then sequentially fed to the inverse rotation device 68, the linear transformation device 72, the inverse S-boxes 14 'and the XOR linkage with the round key Ki, whereupon the plain text data block M is obtained, as shown in FIG. 3a the ciphertext C has been encrypted.

4, an implementation for an encryption / decryption device capable of performing encryption and decryption in the manner described in FIGS. 3a and 3b will now be described. The encryption / decryption device of FIG. 4 comprises the devices of FIG. 3a and additionally some devices of FIG. 3b. For encryption and decryption, however, the linear transformation devices from FIG. 3a are used together, so that in FIG. 4 they only have the reference numerals from FIG. 3a, i.e. 40 and 42, and the linear transformers 70 and 72 have been implemented by the same actual devices.

The encryption / decryption device of FIG. 4 is indicated generally at 100. The encryption / decryption device 100 comprises, in addition to the inverse rotation devices 66, 68, the linear transformation devices 42, 40, the rotation devices 46, 44, the XOR combination devices 48, 50,

74 and 76, the S-boxes Si - S ₈ and the inverse S-boxes S -1 1

S _g ¹ switches 102, 104, 106, 108, 110 and 112 and a control unit 114. A data input 116 is provided for the reception of the data blocks to be encrypted, a data input 118 for the reception of the data blocks to be decrypted, an output 120 for the output of the encrypted data blocks and an output 122 for the output of the decrypted data blocks.

In Fig. 4, the lines connecting the device are each 32-bit lines and either represented by a broken line or by a solid line, wherein dashed lines indicate the data path that is relevant for decryption, while the solid lines are used for encryption. Data inputs from devices and data lines, which are used together for decryption and encryption, are shown by dashed and solid lines running in parallel. The arrows are intended to make it easier to read the encryption / decryption device.

Starting with the encryption part, the 32-bit XOR

Linking device 48 with its output connected to the input of the S-boxes Si - S ₈ . The output of the S-boxes S _x - S ₈ is connected to a 32-bit input of the 32-bit switch 106. The switch has two 32-bit outputs and is provided in order to connect the switch input to either one switch output or the other switch output in accordance with a control signal c ₀ which it receives at a control input from the control unit 114. As will be discussed in more detail below, a first of the switch outputs is assigned to encryption rounds, while the other switch output is permanently assigned to decryption rounds. The encryption switch output is connected to an input of the linear transformation device 40. The output of the linear transformation device 40 is connected to a 32-bit switch input of the switch 108.

The switch 108 also receives the signal c ₀ from the control unit 114 at a control input thereof and accordingly connects the switch input to either a 32-bit encryption switch output or a 32-bit decryption switch output.

The encryption switch output of the switch 108 is connected to an input of the rotation device 44. An output of the rotation device 44 is connected to a data input of the encryption device 50, which contains the round key K ₂ at its 32-bit key input, while at the key input of the key device 48 the round key Ki is applied. The output of the XOR logic device 50 is connected to an input of the ST / ¹ -S ^"1. The outputs of the latter are connected to a 32-bit switch input of the switch 110 which, like the switches 106 and 108, has the control signal at a control input thereof receives c ₀ from the control device 114 and, depending on it, connects the 32-bit control input to either a 32-bit encryption switch output or a 32-bit decryption switch output The encryption switch output of the switch 110 is connected to an input of the linear transformation device 42, whose output is in turn connected to a 32-bit switch input of the switch 102. This switch 102 also receives the control signal Crj from the control unit 114 at a control input thereof and accordingly switches the switch input to either a 32-bit encryption control output or a 32-bit decryption switch output The 32-bit Ve The key switch output of the switch 102 is connected to an input of the rotation device 46, the output of which is in turn connected to a 32-bit switch input of the switch 104. This switch 104 receives a control signal b _o from the control unit 114 at a control input thereof and has a 32-bit round termination switch output and a 32-bit round continuation switch output. Depending on the signal b ₀ , the switch 104 connects the switch input either to the round termination switch output or to the round continuation switch output. The lap continuation switch output is connected to the input of the XOR logic device 48, while the lap termination switch output is connected to the output 120 of the device 100.

With regard to the decryption, the input 118 is connected to an input of the inverse rotation device 66. Its output is in turn connected to the input of the linear transformation device 42. The decryption switch ter output of the switch 102 is connected to the input of the S-boxes Si - S ₈ . The decryption switch output of the switch 106 is connected to a data input of the XOR logic device 74, which receives the round key K ₂ at its key input and which is connected to an input of the inverse rotation device 68 with its data output. The output of the inverse rotation device 68 is connected to the input of the linear transformation device 40. The decryption switch output of the switch 108 is connected to the input of the inverse S-boxes S ^"1 - S _g ^1. The decryption key output of the switch 110 is connected to the data input of the XOR logic device 76, which receives the round key Ki at its key input, and that is connected with its data output to a switch input of the switch 112. The switch 112 receives the control signal bo from the control unit 114 at a control input thereof and accordingly connects the switch input either to a decryption round termination switch output or to a decryption round continuation switch output. Continuation switch output of switch 112 is connected to the input of inverse rotator 66, while the decryption turn termination switch output is connected to output 122 of device 100.

After the structure of the device of Fig. 4 has been described above, its operation will be described below.

For purposes of illustration, it is assumed that the encryption / decryption device 100 of FIG. 4 is designed to carry out two encryption (double) rounds and two decryption (double) rounds, but the description can also be extended to more double rounds without further ado , First, encryption is considered. A data block to be encrypted is present at data input 116. The control unit 114 then uses the signal c _{0 to control} all the switches 102, 106, 108 and 110 in such a way that they connect their respective control input to the encryption control output. This means nothing other than that the sequence of devices through which the data block to be encoded and to be encrypted passes through to switch 104, namely to the sequence of XOR logic device 48, S-boxes 12 ', linear transformation device 40 , Rotation device 44, XOR combination device 50, inverse S-boxes 14 ', linear transformation device 42, rotation device 46, just as has already been described with reference to FIG. 3a.

The control unit 114 need not change the signal c ₀ while the data block is going through this sequence. In general, the control unit 114 does not change the signal c ₀ for the entire encryption process, ie also for the subsequent rounds. The control signal c ₀ remains the same for the entire encryption process, so that there is only a small amount of control for the control unit 114. With the control signal bo, the control unit 114 ensures that the switch 104 after the first round pass, ie after the

Processing by the rotation device 46 connects its switch input to the encryption round continuation switch output, so that the intermediate result or the data block which the rotation device 46 outputs is again applied to the XOR logic device 48 which marks the start of the operation by the switches 106, 108 , 110 and 102 defined round of encryption forms.

After the second pass or the second processing by the rotating device 46, the control unit 114 ensures that the switch 104 now switches the switch output to the encryption round termination switch output has (dashed switch position), so that the cipher or the chiff-at data block is output at the data output 120, as it results after two round passes 52, as shown in Fig. 3a.

If a decryption is to be carried out, the control unit 114 uses the control signal c _{0 to} ensure that the switches 102, 106, 108 and 110 connect their control input to the decryption control output (the switch state (not shown in FIG. 4)). As a result, a data block to be decrypted, which is present at data output 118, is easily guided by a sequence of devices that corresponds to the sequence of FIG. 3b, namely by the sequence of inverse rotation device 66, linear transformation device 42, S boxes Si - S ₈ , XOR logic device 74, inverse rotation device 68, linear transformation device 40, inverse S boxes 14 ', XOR logic device 76. The control signal bo sets the control unit 114 in such a way that the switch 112 switches the data block, which is after the first Decoding round has returned to the

Input of the inverse rotation device 66 is applied, that is, such that the switch 112 connects its switch input with the decryption round continuation switch output. By switching the signal b ₀ , the control unit 114 then ensures that after the second run through the

Decryption round the resulting data block is output as the decrypted data block at output 122, namely switch 112 switching its control input to the decryption round termination switch output (dashed switch position).

The preceding exemplary embodiments are suitable for use as encryption of memory contents as protection against unauthorized reading out of these memory contents. However, the exemplary embodiments can also be used for online or bus encryption in other applications if, for example, the underlying encryption hardware is to be kept small.

The previous exemplary embodiments of FIGS. 3a to 4 relate to encryption / decryption by a cryptographically complete block cipher. It is not possible for an attacker to calculate back or infer the data in encrypted form to the plain text, or only with disproportionate effort. For example, in the embodiment of FIG. 4 or of FIG. 2, the hardware implementation does not take up a large area, since the block cipher is designed with a variable number of rounds. This means that the cryptographic strength of the encryption is scalable at the expense of performance or performance or speed, but not at

Cost of the area. The more rounds you go through, the higher the encryption strength.

In all of the previous exemplary embodiments, the area required for implementation was kept small, although

Encryption and decryption are equally feasible. This was achieved by running through S-box layers in the exemplary embodiments of FIGS. 3a-4. If the first layer contains the S-Box S, then the second layer contains the inverse S-Box Inv (S ') = S ^"1 .

A rotation was used in the embodiments of FIGS. 3a-4. Of course, it would also be possible to generally replace the rotation with a permutation. In any case, permutation or rotation ensures that the effects of the S-boxes do not weaken each other.

3a - 4, a self-inverting linear transformation was used as a further principle. A linear transformation L is called self-inverting if (L (x)) = x applies to all input vectors x. In a second implementation variant, instead of a self-inverting linear transformation L, a pair of linear transformations i and L ₂ are used which are inverse to one another. Then Lχ (L ₂ (x)) = ₂ (ι (x)) = x for all input vectors x.

The S-boxes of the exemplary embodiments 3a-4 caused confusion, the linear transformations for diffusion of the plain text bits. By introducing a corresponding number of multiplexers or switches, one and the same module could then also perform the decryption by a control unit using these switches or multiplexers to ensure that the devices were appropriately connected to a corresponding sequence of devices. In contrast to the exemplary embodiment in FIG. 4, however, the control can also take place dynamically during a double round, so that a device is run through twice during a double round. For example, in the embodiment of FIG. 4, the linear transformation devices 40, 42, the inverse rotation devices 66, 68 and the rotation devices 46 and 44 could each be replaced by one. The disadvantage would be the increased control effort for the control unit 114, whereas the advantage would be the smaller chip area.

In the end, this means for each of the exemplary embodiments described above that the same piece of hardware is used both for encryption and for decryption.

With regard to the preceding description, it is further pointed out that, although it has been described in the foregoing that the length of the original data blocks in the encryption maps is equal to or less than that of the data blocks resulting from the encryption map S (ie n m m), it is also possible to choose n> m as for the DES algorithm, such as several 6x4-S boxes, if, for example, redundancy is then created. fende expansion of the data block before encryption S or compression after decryption S ^{-1 is} carried out.

In contrast to Feistel ciphers and their implementing encryption / decryption devices, the exemplary embodiments of the present invention have the advantages that a high number of rounds is not required to achieve the same level of security, which in turn improves the performance or performance or effectiveness compared to these Feistel ciphers. Cipher encryption / decryption devices increased.

The previous exemplary embodiments only required a minimum of elementary components, namely for example in the exemplary embodiments from FIGS. 3a-4 S-boxes and linear transformations. With each elementary element used, the associated inverse elementary element is also built into the encryption / decryption device. This can then undo the operation of the elementary block, which is used for decryption. For encryption, care has been taken to ensure that the

The effects of the elementary building blocks and the inverse elementary building blocks do not weaken or cancel each other, but complement each other. As described above with reference to the rotation and permutation, this can be achieved by a suitable wire guide that does not take up any extra space. Mathematically, such a wire guide corresponds to a permutation or rotation of data bits.

With regard to the preceding description, it is pointed out that the number of rounds, ie the number of double rounds, is not fixed to one or two, but can take any other value. The encryption rounds of FIGS. 3a and 3b can be run through as often as desired. The cipher C then represents a 1, 2, ... N double-round encryption or a 2, 4, 6 ... 2N-round encryption, with N e IN. The encryption device can also be viewed neutrally as a first imaging device with a first image and the decryption device as a second imaging device with an associated image that is inverse to the first.

In particular, it is pointed out that, depending on the circumstances, the scheme for encryption / decryption according to the invention can also be implemented in software. The implementation can take place on a digital storage medium, in particular a floppy disk or a CD with electronically readable control signals, which can interact with a programmable computer system in such a way that the corresponding method is carried out. In general, the invention thus also consists in a computer program product with program code stored on a machine-readable carrier for carrying out the method according to the invention when the computer program product runs on a computer. In other words, the invention can thus be implemented as a computer program with a program code for carrying out the method if the computer program runs on a computer.

LIST OF REFERENCE NUMBERS

10 encryption / decryption device 12 encryption device 14 decryption device 16 permutation device 18 inverse permutation device 20 control device 22 data input 24 data input 26 data output 28 data output 30 encryption round 32 decryption round 40 linear transformation device 42 linear transformation device 44 rotation device 46 rotation device 48 XOR linking device 50 XOR linking device 52 encryption round 52a 52b partial round 60 decoding round 62 partial round 64 partial round 66 inverse rotation device 68 inverse rotation device 70 linear transformation device 72 linear transformation device 74 XOR logic device 76 XOR logic device

100 encryption / decryption device

102 switches 104 switches

106 switches

108 switches 110 switches

112 switches

114 control unit

116 Data input 118 Data input

120 data output

122 data output

900 encryption / decryption device

902 encryption unit 904 decryption unit

Claims

claims

1. Device for encrypting a data block to be encrypted into an encrypted data block and for decrypting a data block to be decrypted into a decrypted data block

an encryption device (12), which has an encryption input and an encryption output, for mapping a data block at the encryption input to a encryption result data block at the encryption output in accordance with an encryption map;

decryption means (14) having a decryption input and a decryption output, for mapping a data block at the decryption input to a decryption result data block at the decryption output in accordance with a decryption map that is inverse to the encryption map;

an encryption link device (18) for mapping the encryption result data block onto an imaged encryption result data block according to an encryption link mapping and feeding the same to the decryption input of the decryption device (14);

a decryption combiner means (18) for Abbil- is that of the encryption result data block on an inversely mapped encryption result data block according to a decryption link picture, to which the scrambling system selungsverknüpfungsabbildung inverse, and supplying dessel ^¬ ben direction to the decryption input of the Entschlüsselungsein- (14); and a control device (20; 114) which is designed to cause the data block to be encrypted to pass through the sequence of encryption device (12), encryption linkage device (16) and decryption device (14) at least once in order to obtain the encrypted data block , and that the data block to be decrypted runs through the sequence of encryption device (12), decryption link device (18) and decryption device (14) at least once in order to obtain the decrypted data block.

2. The device according to claim 1, wherein the encryption link device (16) and the decryption link device (18) are designed to feed the encryption result data block to the decryption input of the decryption device (14) such that, in the case of identity mapping, instead of the encryption link or Decryption link mapping a data block when running through the sequence of encryption device (12), encryption link device (16) and decryption device (14) and when running through the sequence of encryption device (12), decryption link device (18) and decryption device (14) on itself would be.

3. Apparatus according to claim 1 or 2, wherein the data block to be encrypted, the encrypted data block, the data block to be decrypted and the decrypted data block are n-bit data blocks, where n is a predetermined integer.

4. Device according to one of the preceding claims, wherein the encryption device (12) an n-bit encryption input and an m-bit

Has encryption output and is configured to an n-bit data block at the encryption input to a map the m-bit encryption result data block at the encryption output according to the encryption map, and in which the decryption device (14) has an m-bit encryption input and an n-bit decryption output and is designed to convert an m-bit data block at the decryption input to an n Mapping the bit encryption result data block at the decryption output according to the decryption map.

5. The apparatus of claim 4, wherein the encryption map and the decryption map are non-linear maps.

6. The device as claimed in claim 4 or 5, in which the encryption linkage device has the following feature:

a first permutation device with a first m-bit permutation input and a first m-bit permutation output for permuting an m-bit data block at the first m-bit permutation input to a permuted m-bit data block at the first m-bit permutation output a permutation rule, wherein the first permutation device can be switched in series between the m-bit encryption output and the m-bit encryption input, and

in which the decryption link device has the following feature:

a second permutation device with a second m-bit permutation input and a second m-bit permutation output for permuting an m-bit data block at the second m-bit permutation input to a permuted m-bit data block at the second m-bit permutation output a second permutation rule that is inverse to the first permutation rule, the second permutation rule tion device serially between the m-bit

Encryption output and the m-bit decryption input is switchable,

The first and second permutation devices can be switched between the m-bit encryption output and the m-bit decryption input in such a way that the data block to be encrypted runs through the sequence of encryption device (12), encryption link device (16) and decryption device ( 14) leads to a data block which, when the sequence of encryption device (12), decryption link device (18), decryption device (14) is run through, would again result in the data block to be encrypted.

7. The device according to claim 6, in which the first permutation device (16) and the second permutation device 818) are each implemented as m conductor tracks which extend between the first and second permutation input on the one hand and the first and second permutation output on the other.

8. Device according to one of claims 4 to 7, in which the encryption link device (16) has the following feature:

a first linear imaging device (40) having a first m-bit linear mapping input and a first m-bit linear mapping output for mapping an m-bit data block at the first m-bit linear mapping input to an imaged m-bit data block at the first m-bit Linear image output according to a first linear image, the first linear image device being switchable in series between the m-bit encryption output and the m-bit decryption input, and in which the decryption link device has the following feature:

a second linear imaging device (72) with a second m-bit linear imaging input and a second m-bit linear imaging output for mapping an m-bit data block at the first m-bit linear imaging input onto an imaged m-bit data block at the second m Bit linear mapping output according to a second linear mapping, the second linear mapping being inverse to the first linear mapping, and wherein the second linear mapping device (72) can be connected in series between the bit encryption output and the m bit decryption input,

wherein the first and the second linear imaging device can be switched between the m-bit encryption output and the m-bit decryption input in such a way that the data block to be encrypted runs through the sequence of encryption device (12), encryption link device (16) and decryption device ( 14) leads to a data block which, when the sequence of encryption device (12), decryption link device (16) and decryption device (14) is run through, would again result in the data block to be encrypted.

9. The device according to one of claims 4 to 8, wherein the encryption linkage device further has the following feature:

a first key XOR device (50) with a first m-bit data input, a first m-bit key input and an m-bit data output for bitwise XORing an m-bit data block on the first m-bit Data input with an m-bit key at the m-bit key input, the first key XOR device being connected serially between the m-bit Encryption output and the m-bit decryption input is switchable, and

in which the decryption link device has the following feature:

a second key XOR device (74) with a second m-bit data input, a second m-bit key input and a second m-bit data output for bitwise XORing an m-bit data block at the second m-bit Data input with the m-bit key at the second m-bit key input, the second key XOR device (74) being serially switchable between the m-bit encryption output and the m-bit decryption input,

wherein the first and the second key XOR device can be switched between the m-bit encryption output and the m-bit decryption input in such a way that the data block to be encrypted runs through the sequence of encryption device (12), encryption link device (16 ) and decryption device (14) leads to a data block which, when running through the sequence of encryption device (12), decryption link device (18), decryption device (14), would again result in the data block to be encrypted.

10. The device according to one of claims 4 to 9, wherein m = n.

11. Device according to one of the preceding claims, wherein the encryption device comprises a plurality of pxp encryption S-boxes, each of which maps different p bits of the data block at the encryption input to p bits, which together form the encryption result data block, and the decryption device has a plurality of pxp decryption S-boxes, each of which has different p bits of the data block on the Maps decryption input to p bits, which together form the decryption result data block, each of the decryption S-boxes implementing a mapping inversely to a different one of the encryption S-boxes.

12. Device according to one of the preceding claims, in which the control device is designed to cause a data block to be encrypted to run through a sequence of encryption device, encryption linkage device, decryption device, encryption linkage device one or more times in order to obtain the encrypted data block and that the data block to be decrypted runs through a sequence of decryption link device, link device, decryption link device and decryption device one or more times in order to obtain the decrypted data block, or that the data block to be encrypted follows a sequence of encryption device, encryption link device, decryption device, Decryption link device one or more times to obtain the encrypted data block, and that the z u decrypting data block runs through a sequence of encryption link device, link device, decryption link device and decryption device one or more times in order to obtain the decrypted data block

13. Device for encrypting a data block to be encrypted into an encrypted data block, with

an encryption device (12), which has an encryption input and an encryption output, for mapping a data block at the encryption input to a encryption result data block at the encryption output according to an encryption map; decryption means (14), having a decryption input and a decryption output, for mapping a data block at the decryption input to a decryption result data block at the decryption output in accordance with a decryption map that is inverse to the encryption map;

an encryption link device (18) for mapping the encryption result data block onto an imaged encryption result data block according to one

Encryption link mapping and feeding the same to the decryption input of the decryption device (14); and

a control device (20; 114) which is designed to cause the data block to be encrypted to pass through the sequence of encryption device (12), encryption linkage device (16) and decryption device (14) at least once in order to obtain the encrypted data block ,

14. Device for decrypting a data block to be decrypted into a decrypted data block, with

an encryption device (12), which has an encryption input and an encryption output, for mapping a data block at the encryption input to a encryption result data block at the encryption output according to an encryption map;

decryption means (14), having a decryption input and a decryption output, for mapping a data block at the decryption input to a decryption result data block at the decryption output in accordance with a decryption map that is inverse to the encryption map; a decryption link device (18) for mapping the encryption result data block onto an inversely mapped encryption result data block according to a decryption link map to which the encryption link map is inverse and supplying the same to the decryption input of the decryption device (14); and

a control device (20; 114) which is designed to cause the data block to be decrypted to pass through the sequence of encryption device (12), decryption link device (18) and decryption device (14) at least once in order to obtain the decrypted data block.

15. A method for encrypting a data block to be encrypted into an encrypted data block, by means of an encryption device (12) which has an encryption input and an encryption output, for mapping a data block at the encryption input to an encryption result data block at the encryption output according to an encryption map, and a decryption device (14 ), which has a decryption input and a decryption output, for mapping a data block at the decryption input to a decryption result data block at the decryption output according to a decryption map that is inverse to the encryption map, with the following step:

Have the data block to be encrypted run through the sequence of encryption device (12) and decryption device (14) at least once in order to obtain the encrypted data block, by mapping the encryption result data block onto an imaged encryption result data block according to an encryption link mapping and supplying the same Decryption input of the decryption device (14).

16. A method for decrypting a data block to be decrypted into a decrypted data block, by means of an encryption device (12), which has an encryption input and an encryption output, for mapping a data block at the encryption input to an encryption result data block at the encryption output according to an encryption map, and a decryption device (14), which has a decryption input and a decryption output, for

Mapping a data block at the decryption input onto a decryption result data block at the decryption output according to a decryption map that is inverse to the encryption map, with the following step:

Have the data block to be decrypted run through the sequence of encryption device (12) and decryption device (14) at least once in order to obtain the decrypted data block, by mapping the encryption result data block to an inverse image

Encryption result data block according to a decryption link map, to which the encryption link map is inverse, and supplying the same to the decryption input of the decryption device (14).

17. Computer program with a program code for performing the method according to claim 15 or 16, when the computer program runs on a computer.