Method of block encryption with private key

The invention relates to computer technology, to methods of cryptographic protection of data transmitted over a radio channel.

Encryption and decryption is performed in order to protect data from unauthorized access by third parties.

The methods of encryption are divided into symmetric and asymmetric. With a symmetric method, one private secret key is used to encrypt and decrypt messages. In an asymmetric method, two keys are used: an open public key for encryption and a private secret key for decrypting messages.

The claimed invention relates to encryption methods with a private key.

The technical result of the claimed invention is an increase in the cryptographic strength of the block ciphering method.

Crypto-resistance is the ability to withstand the cracking of an encrypted message by third parties. The cryptographic strength is measured by the number of elementary operations that must be performed to restore the original information message when you know the conversion method, but without knowing the key. The method of conversion during encryption is usually known. The key is calculated by brute force, using hardware and software. The key is called key breaking. The cryptographic strength depends on the encryption method and the length of the key used.

To increase the cryptographic strength, block data encryption is used when the message is divided into separate blocks, and the key is divided into subkeys, then these blocks are encrypted, and each subblock uses its own subkey to encrypt each block.

The methods of block encryption with a private key include standards GOST 28147-89 (Russia), DES (Data Encryption Standard, USA), etc.

The algorithm of cryptographic transformation GOST 28147-89 uses block cipher with 256-bit key and 32 conversion cycles, operating with 64-bit blocks. For encryption, the plaintext is first divided into two halves (low-order bits-A, high-bits-B). The i-th cycle uses the Ki subkey. To generate a subkey, the original 256-bit key is divided into eight 32-bit blocks: K1 ... K8. Ai and Ki add together modulo 2³². The result is divided into eight 4-bit sub-sequences (tetrads), each of which enters the input of its node in the replacement table, called the S-block. The table defines a substitution when one tetrad is replaced by another tetrad. The outputs of all eight S-blocks are combined into a 32-bit word, then the whole word is cyclically shifted to the left (to the upper bits) by 11 bits. (Standard USSR GOST 28147-89, Information processing systems, cryptographic protection, cryptographic transformation algorithm, Moscow, USSR CC under the 1989 standards). GOST-28147-89 does not describe the process of generating keys and replacement tables. There are "weak" keys and replacement tables, but the standard does not describe the criteria for selecting and dropping "weak". The disadvantage of this algorithm is the use of a permanent 256-bit key, from which the subkeys are formed. With software implementation, this method of encryption does not provide high speed (more than 1 Mbps) because of the need to convert data formats. Modern processors operate on data whose size is a multiple of 1 byte (8, 16, 32, 64, 128 bits). This method uses a large number of substitution operations over 4-bit data blocks (up to 32 rounds). In each round, the processor performs the conversion of the data formats. First, 4-bit tetrads (8 bits → 4 + 4 bits) are selected in the byte, and then a table substitution and an inverse transformation (8 × 4 bits → 32 bits) are performed. Converting data formats reduces the speed of encryption. the size of which is a multiple of 1 byte (8, 16, 32, 64, 128 bits). This method uses a large number of substitution operations over 4-bit data blocks (up to 32 rounds). In each round, the processor performs the conversion of the data formats. First, 4-bit tetrads (8 bits → 4 + 4 bits) are selected in the byte, and then a table substitution and an inverse transformation (8 × 4 bits → 32 bits) are performed. Converting data formats reduces the speed of encryption. the size of which is a multiple of 1 byte (8, 16, 32, 64, 128 bits). This method uses a large number of substitution operations over 4-bit data blocks (up to 32 rounds). In each round, the processor performs the conversion of the data formats. First, 4-bit tetrads (8 bits → 4 + 4 bits) are selected in the byte, and then a table substitution and an inverse transformation (8 × 4 bits → 32 bits) are performed. Converting data formats reduces the speed of encryption.

In the US DES standard, data block encryption is performed by generating a key, splitting the converted data block into two sub-blocks L and R, and alternately modifying the latter by performing a bitwise sum operation modulo two above the subblock L and a binary vector that is generated as the output value of some function F from values ​​of the subblock R. After that, the blocks are rearranged in places. The function F in this method is implemented by performing permutation and substitution operations performed on subblock R (National Bureau of Standards, Federal Data Processing Standard, 46, January 1977). In DES, each data block is encrypted independently of the others. This allows you to decrypt individual blocks of encrypted messages or data structures, and therefore, opens the possibility of independent transfer of data blocks or random access to encrypted data. DES uses a small-sized key, which makes it vulnerable to cryptanalysis based on key selection. In DES, the encryption key is represented as a set of subkeys. For all i-input data blocks, the same i-th subkey will be used, which reduces the level of crypto protection.

A method for block encryption of binary information is known, which includes generating an encryption key in the form of a plurality of subkeys, splitting the data block into N≥2 subblocks, and alternately converting the subblocks by performing an encryption operation on the subblock and the subkey. Before executing the two-seater encryption operation on the i-th sub-block and the subkey, perform the substitution operation on the subkey, depending on the j-th sub-block, where j ≠ i. As the substitution operation depending on the j-th subblock, the control operation or substitution operation depending on the encryption key is used (Eremeev MA, Moldovyan AA, Moldovyan NA Method of cryptographic transformation of digital data blocks , Patent No. 2211541, RU, priority 08.11.2001 Moldovyan AA, Moldovyan NA, Savlukov NV Method of block encryption of binary information, Patent No. 2140712 RU, priority 08.11.2001, Moldovyan AA and others. The method of encryption of information represented by a binary code. Patent №2103829, priority 02.04.1997).

A disadvantage of the analogs of the claimed method is the use of one original private key, from which by substitutions, permutations, transformations, functions, and so on. get a subkey sequence for subblocks. Knowing the conversion rule, you can pick up the key.

The secret key can not only be picked up, but it can also be stolen or lost as a result of a failure of a computer or storage device.

The closest to the claimed (prototype) is the method of block encryption of messages and transmission of encrypted data with a private key, including splitting the message in the transmitter into a set of individual k-bit data subblocks in the number (N≥2), initiating the private key before sending the message (when i = 0) in the transmitter and receiver with the same value of the private key, the formation of the subkey sequence from the private key, and then encrypting them in the transmitter of the corresponding transmitted blocks data, block-by-block transmission of the generated data on the communication channel and subsequent decoding of the transmitted data blocks in the receiver by means of said subkeys, including various modifications, (i) the transmitted data block from the i-th data sub-block and the attached (i) th sub-block of the service information in the form of (i + 1) th modifier of the closed sub-key, and encrypt it (i) - m the appropriate subkey from the set of subkeys; generate control bits in the transmitter and attach them to the i-th data block being transmitted; then the (i) th control data block is sent to the receiver and the correctness of the received (i) th data block is checked by the check digits, after which the received (i) th data block is decrypted (i) by the closed subkey and separated it to the i-th sub-block of data and (i) -th sub-block of the service information; in the receiver, for each correctly received data block, a receipt is generated and transmitted over the communication channel to the transmitter, and from the (i) th subkey and (i + 1) th plugin modifier, the value of the next (i + 1) -th closed subkey is formed, with the possibility of decrypting the next (i + 1) -th data block; after transmission and control of the correct reception of each i-transmitted data block in the transmitter and receiver, a new value of the closed sub-key from the private key of the private encryption key is synchronously set, after which the transmission of the next (i + 1) th data block with (i + 2) the modifier of the closed subkey; (IS Kabak, NV Sukhanova, B.M. Pozdneev, "Method of Block Encryption of Messages and Transmission of Encrypted Data with a Private Key." Patent for invention No. 24593367, priority 16.07.2010) . with the possibility of deciphering the next (i + 1) -th data block; after transmission and control of the correct reception of each i-transmitted data block in the transmitter and receiver, a new value of the closed sub-key from the private key of the private encryption key is synchronously set, after which the transmission of the next (i + 1) th data block with (i + 2) the modifier of the closed subkey; (IS Kabak, NV Sukhanova, B.M. Pozdneev, "Method of Block Encryption of Messages and Transmission of Encrypted Data with a Private Key." Patent for invention No. 24593367, priority 16.07.2010) . with the possibility of deciphering the next (i + 1) -th data block; after transmission and control of the correct reception of each i-transmitted data block in the transmitter and receiver, a new value of the closed sub-key from the private key of the private encryption key is synchronously set, after which the transmission of the next (i + 1) th data block with (i + 2) the modifier of the closed subkey; (IS Kabak, NV Sukhanova, B.M. Pozdneev, "Method of Block Encryption of Messages and Transmission of Encrypted Data with a Private Key." Patent for invention No. 24593367, priority 16.07.2010) . after transmission and control of the correct reception of each i-transmitted data block in the transmitter and receiver, a new value of the closed sub-key from the private key of the private encryption key is synchronously set, after which the transmission of the next (i + 1) th data block with (i + 2) the modifier of the closed subkey; (IS Kabak, NV Sukhanova, B.M. Pozdneev, "Method of Block Encryption of Messages and Transmission of Encrypted Data with a Private Key." Patent for invention No. 24593367, priority 16.07.2010) . after transmission and control of the correct reception of each i-transmitted data block in the transmitter and receiver, a new value of the closed sub-key from the private key of the private encryption key is synchronously set, after which the transmission of the next (i + 1) th data block with (i + 2) the modifier of the closed subkey; (IS Kabak, NV Sukhanova, B.M. Pozdneev, "Method of Block Encryption of Messages and Transmission of Encrypted Data with a Private Key." Patent for invention No. 24593367, priority 16.07.2010) .

Analogues and prototype of the claimed method have the following drawbacks:

To generate block subkeys, one original private key and one random number generator are used. The first subblock is encrypted with a private key. Then, using the private key and the random number generator, a sequence of subkeys is formed for the i-th sub-blocks (i≥2).

Kidnapping or selecting a private key allows you to determine the value of the subkey for the first sub-block. The remaining subkeys are obtained by deciphering the successive sub-blocks of the service information.

The purpose of the invention is to increase the cryptographic stability when transmitting data over a radio channel.

The cryptographic stability is enhanced by the following transformations, which prevent the accumulation of consecutive message blocks by third parties, with the aim of cryptanalysis:

1. mixing message blocks with blocks of the same size, containing random numbers;

2. choice of different frequencies for transmission of blocks;

3. Select the frequency to transmit each block based on the random number generator.

The essence of the invention is as follows. The claimed method uses a private key. The private key consists of a finite sequence of subkeys.

At the beginning of the message transmission, the private key is initialized at the transmitter and receiver. enter the initial value of the subkey (with i = 0) for encryption-decryption of the first block. This connection is the same for the receiver and transmitter. The message is then divided into separate data blocks. The blocks of data are mixed with blocks containing random numbers.

In the transmitter, the overhead information for the next (i + 1) th block is attached to the i-th block, which includes the block number, the key modifier, a code mark indicating that the block contains random numbers, and also a code specifying the transmission frequency over the radio channel. Then the i-th block and the service information are encrypted with the i-th subkey and transmitted in an encrypted form on the radio channel at the selected frequency.

Thus, blocks containing data or random numbers are encrypted using a private key.

Each block is encrypted with its own unique subkey.

The first subblock of data is encrypted with the entered closed subkey. Then a new (i + 1) th subkey is formed for each (i + 1) th block, which functionally depends on the previous i-th subkey and (i + 1) -th modifier of the key. The key modifier is a random number. The key modifier is generated in the transmitter using a random number generator.

The receiver receives and decrypts the i-th block using i-th subkey. After i-th decryption, the block is divided into the i-th data block and the service information for the next (i + 1) th block, which includes the block number, the key modifier, the code character indicating that the block contains random numbers, Specifying the frequency of transmission over the radio channel. The receiver then creates a new subkey value for the (i + 1) th data block, using the i-th subkey and the key modifier for the (i + 1) -th data block. Thus, after transmitting and receiving the next data block, the private key synchronously changes between the transmitter and the data receiver.

The receiver skips blocks that contain random numbers without processing. The receiver decrypts by connecting blocks containing data, and connects to the message in the order of their numbers.

The claimed method provides equivalent variable keys for the transmitter and the data receiver. When receiving and transmitting data, the private key is constantly changing, and at random. On the radio channel, only certain fragments of the private key are transmitted in encrypted form. In this case, the private key remains secret and is not available to third parties. Blocks of data are mixed with blocks of random numbers. An attempt to decipher a block containing random numbers gives a meaningless set of characters and makes cryptanalysis difficult.

Usually messages are transmitted on the same frequency. The frequency change results in the loss of information that is transmitted at other frequencies. When receiving information on one frequency, third parties can receive only a part of the total sequence of encrypted data blocks that are mixed with blocks of random numbers.

For cryptanalysis, it is required to know all frequencies at which transmission is possible, and continuously record all information at these frequencies.

Thus, the claimed method of block encryption of data with a private key makes cryptanalysis difficult and provides a high level of cryptographic stability.

In the claimed method, a significant difference from analogs and prototypes is the transmission of additional service information as part of each transmitted block, including:

- a code mark that indicates that this block contains random numbers;

- block number;

- transmission frequency for the next block.

The service information is transmitted in encrypted form. In this case, the length of the key exceeds the volume of the entire block of information.

The claimed method of block encryption with a private key is explained by graphical materials, see FIG. 1-3.

In Fig. 1 shows an example of information transfer, where 1 is a message, 2 is a data block, 3 is a block of random numbers, 4 is service information, 5 is a random number generator, 6 is a private key, 7 is an encoder, 8 is an encrypted data block, control device, 10 - control bits, 11 - transmitter of radio signals with the choice of frequency f_i.

In Fig. 2 shows a method of block encryption with a private key, where the transmission of blocks 8 with check bits 10 occurs simultaneously at several frequencies f_i during the time intervals f_i.

In Fig. 3 shows an example of receiving information, where 1 is a message, 2 is a data block, 3 is a block of random numbers, 4 is service information, 6 is a private key, 7 is a decryptor, 8 is an encrypted data block, 9 is a monitoring device; digits, 11 - receiver of radio signals with the possibility of tuning to frequencies f_i, 12 - receipt.

An example of using the claimed block encryption method with a private key is shown in FIG. 1-3.

Before transmitting the message (at i = 0), the private key is initialized at the transmitter and receiver. Then, when each data block is transmitted, the value of the private key in the transmitter and the receiver is synchronously changed. The transmitter and receiver are implemented in symmetrical circuits (see Figures 1, 3). In the transmitter, the message 1 is divided into separate data blocks 2. The data blocks 2 are mixed with blocks containing random numbers 3. Then, the i-th service information block 4 is attached to each i-th block, where the (i + 1) -th modifier of the key . Each block is encrypted with its closed connection. 6. The connection for the (i + 1) -th block is functionally dependent on the connection of the previous i-th block 6 and the (i + 1) -th modifier of the key 4, which is generated at the output of the random number generator 5. The data blocks and associated service information blocks are input to the encoder inputs 7, at the output of which encrypted blocks 8 are formed. The output of the encoder 7 is connected to the input of the monitoring device 9, in which test bits 10 are formed and are connected to the i-th unit. Then the frequency f_i and at this frequency, the encrypted blocks 8 are transmitted with control bits 10, see FIG. 2. In the receiver (see Figure 3), the reception frequency is selected, radio blocks are received on the radio channel, which are then fed to the input of the monitoring device 9, where the receipt 12 is generated for the blocks received with distortions. The decoder 7 is connected to the output of the monitoring device 9, and (i + 1) -th closed connection 6 is used to decrypt the (i + 1) -th block, which is functionally dependent on the i-th closed connection and (i + 1) -th modifier key.

The claimed method of block encryption of messages and transmission of encrypted data with a private key is as follows, see FIG. 1, 2, 3.

1. At the beginning, before sending the message (with i = 0), initialize secret private keys in the transmitter and receiver. At the initialization, the private key 6 is entered. This key is the same for the transmitter and receiver.

2. In the transmitter, the message is divided into a set of individual k-bit data blocks 2. The number of data blocks N≥2. The size of the data block 2 must be less than the length of the closed subkey that is used to encrypt it.

3. Using the random number generator 5, k-bit random number blocks 3 are formed. After this, the data blocks are mixed with blocks of random numbers. Then all the blocks are numbered.

4. For each i-th block, the service information is formed. Using the random number generator 5, the private key modifier is formed. (I + 1) -th modifier of the private key is entered in the i-th service information block 4. The code information is also added to the service information block, which indicates that this block contains random numbers; number of the next block; the transmission frequency for the next block. The service information is attached to the i-th unit.

5. The I-th data block is encrypted using the i-th closed subkey.

6. Over the i-th closed sub-key and i-th private key modifier perform the functional transformation and get a new value (i + 1) -th closed subkey. In the transmitter, control bits are created for each data block, which are attached to this data block. For example, this may be a parity bit or a block checksum bit.

7. The encrypted i-th data block is transmitted on the radio channel at the frequency f_i for a time interval f_i, see Fig. 2. Repeat steps 3-6 to encrypt all data blocks in the message.

8. At the receiver, the i-th data block is received and the correctness of its reception in the monitoring device 9 is checked. For example, during verification, it is possible to determine the parity bits or the checksum bits of the block and compare them with the corresponding control bits obtained in the data block.

9. For each block received with distortion, the monitoring device 8 generates and transmits the receipt 12. The receipt contains the number of the incorrectly received block. Receipt goes to the transmitter.

10. Then the receiver decrypts the i-th data block using the i-th subassembly using the decoder 7. As a result, the decrypted i-th data block is obtained. In the block of random numbers 3, only the service information 4 is used. The data block is divided into two sub-blocks-i-th data subblock 2 and i-th sub-block of service information 4. Data sub-2 is attached to message 1.

11. In the i-th sub-block of the service information 4, the private key modifier is allocated for the next (i + 1) -th block.

12. Form a new closed (1 + 1) -th subnet 6 in the receiver. The new closed subkey for the (i + 1) th block is functionally dependent on the i-th closed subkey and the private key modifier for the (i + 1) th block (which was obtained in step 11).

13. Repeat steps 8-12 to receive and decrypt all data blocks 2 as part of message 1.

The declared method of block encryption with a private key provides reliable data delivery, ensures the identity of the sent and received data blocks, their integrity and that the data blocks are received in the same order as they were sent.

Reliable data delivery implies that four conditions for unreliable delivery are excluded, namely, the blocks can not be lost or damaged, delayed, doubled (tripled), not delivered in the order they were sent. Reliable delivery guarantees that the message (as a sequence of blocks) will be delivered to the receiver in the same form as it was sent by the transmitter. In case the messages are lost, damaged, delayed, delayed, delayed, delivered in the wrong order, a receipt 12 is generated in the control unit 9. After the specified timeout time, the transmitter re-sends the blocks starting from the first block to which they received the receipt.

In the event of a transmission failure, the transmitter and receiver may have different private keys. In this case, the control unit forms a receipt, in which it indicates from which block it is necessary to repeat the transmission of the message.

The proposed method of block encryption with a private key is suitable for any methods of encryption and decryption and for any amount of information.

The technical result of the invention is to increase the level of cryptographic security and the speed of message transmission.

The technical result of the invention is achieved due to the fact that with the claimed method of block encryption with a private key, including splitting the message in the transmitter into a plurality of separate k-bit data blocks in an amount (N≥2), initializing the private key before transmitting the message (for i = 0) in the transmitter and receiver with the same value of the private key, the formation of the subkey sequence from the private key, followed by encrypting them in the transmitter of the corresponding transmitted data blocks, The transmission of the generated data over the communication channel and the subsequent decoding of the transmitted data blocks in the receiver by means of said subkeys, including various modifications, first, in the transmitter, an information unit is attached to the (i) (i + 1) th closed connection, then the data block and the service information (i) -th subkey from the set of subkeys are encrypted; After that, the control bits are formed and attached to the i-th transmitted block, then the (i) th control-check block is sent to the receiver, where the correctness of the received (i) -th block is verified by check bits, after which the received (i) - th block is decrypted by the (i) -th closed sub-node and divides it into the i-th data block and (i) the -th service information block, in the receiver from the (i) th plug-in and (i + 1) -th modifier, the value of the next (i + 1) -th closed connection with the ability to decrypt it the next (i + 1) -th block; and after transmission, reception and control of the correctness of each i-th transmitted unit in the transmitter and receiver synchronously establish a new value of the closed sub-key from the private key of the private encryption key, after which the transmission of the next (i + 1) th data block with (i + 2) - th modifier of the closed plug, and in the receiver, from the correctly received data blocks, the received message is formed, according to the invention, in the transmitter, the data blocks are mixed with blocks containing random numbers, then, after encryption, the blocks before the block number, the code flag, indicating that the block contains random numbers, as well as the code specifying the transmission frequency for the next block, are transmitted in the service information block, and the blocks are received at different frequencies after the decryption, after the decryption is separated blocks,

The technical result is achieved due to the fact that with the claimed block encryption method with a private key, according to the invention, the frequency bands for transmitting and receiving blocks are specified by a random number sensor.

The technical result is achieved due to the fact that with the claimed block encryption method with a private key according to the invention, a code indicative that indicates that the block contains random numbers is generated by a random number sensor.

1. A method of block encryption with a private key, including splitting the message in the transmitter into a plurality of separate k-bit data blocks in an amount (N≥2), performing the initialization of the private key before transmitting the message (at i = 0) in the transmitter and receiver to the same value of the closed key, formation of the subkey sequence from the private key with subsequent encryption by them in the transmitter of the corresponding transmitted data blocks, block-by-block transmission of the generated data over the communication channel and subsequent decoding transferring data blocks in the receiver by means of said subkeys, including various modifications, first in the transmitter to the (i) th data block, a service information block containing the (i + 1) th closed connection modifier is attached, then the data block and the service information (i) - the same subkey from the set of subkeys are encrypted; After that, the control bits are formed and attached to the i-th transmitted block, then the (i) th control-check block is sent to the receiver, where the correctness of the received (i) -th block is verified by check bits, after which the received (i) - th block is decrypted by the (i) -th closed sub-node and divides it into the i-th data block and (i) the -th service information block, in the receiver from the (i) th plug-in and (i + 1) -th modifier, the value of the next (i + 1) -th closed connection with the ability to decrypt it the next (i + 1) -th block; and after transmission, reception and control of the correctness of each i-th transmitted unit in the transmitter and receiver synchronously establish a new value of the closed sub-key from the private key of the private encryption key, after which the transmission of the next (i + 1) th data block with (i + 2) - th modifier of the closed plug, and in the receiver from the correctly received data blocks a received message is formed, characterized in that in the transmitter the data blocks are mixed with blocks containing random numbers, then after encryption, the blocks before the block number, the code flag, indicating that the block contains random numbers, as well as the code specifying the transmission frequency for the next block, are transmitted in the service information block, and the blocks are received at different frequencies after the decryption, after the decryption is separated blocks,

2. A method of block encryption with a private key according to claim 1, characterized in that the frequency bands for transmitting and receiving blocks are specified by a random number sensor.

3. A method of block encryption with a private key according to claim 1, characterized in that the code character, which indicates that the block contains random numbers, is generated by a random number sensor.

In the claimed method of block encryption with a private key, any known method of encryption and decryption can be used, for example, gamming, etc. (Ivanov MA Cryptographic methods of information protection in computer systems and networks .-- M: KUDITS-OBRAZ, 2001, p. ).

A random number generator can be implemented as a hardware device or software (Ermakov VF, Gudzovskaya VA RU Sensor of random numbers with a uniform distribution of increased accuracy, RF patent №2103726, G06F 7/58).

In the monitoring device, control bits are created and attached to each block, for example, a checksum, CRC (redundancy code), etc. (see Henry S. Warren, Jr. Algorithmic tricks for programmers - M: Williams, 2007. - 288 pp.). The control of data blocks can be performed at the receiver using a hardware device or by means of a program, for example, using a checksum calculation algorithm, CRC, etc. The receiver calculates check digits for the received blocks and compares them with the control bits attached to this block. If these control bits do not coincide, the receiver generates and transmits the receipt. The transmitter receives a receipt and repeats the transmission, beginning with the distorted block.

Thus, the claimed method of block encryption of messages and transmission of encrypted data with a private key can be implemented hardware, software or hardware-software based on a computer with software installed on it.

The characteristics indicated in the independent claim are significant and interrelated to form a stable set of necessary characteristics sufficient to obtain the required technical result.

Properties regulated in the claimed method by separate features are generally known in the art and do not require further explanation. It should be noted that the claimed set of essential features provides a synergistic (super-sum result) in the mix.

Thus, the foregoing information indicates that the following set of conditions is fulfilled when the claimed method is used:

- the claimed method has practical application, is intended for cryptographic protection of information transmitted over a radio channel;

- for the claimed method as it is characterized in the independent clause of the formula below, it has been confirmed that it can be carried out with the help of the means and methods mentioned in the application or known from the prior art on the priority date;

- in the implementation of the method, the technical result seen by the applicant is achieved.

On the basis of the foregoing, the claimed method meets the requirement of the patentability condition "novelty" and "inventive step".