Data encryption method of proxy re-encryption and direct revocation mechanism based on attributes

Abstract

The invention relates to a data encryption method of an attribute-based proxy re-encryption and direct revocation mechanism, which belongs to the field of information security, and comprises the following steps: S1, carrying out system initialization by a permission center PKG; s2, each user applies for a private key from the PKG according to the attribute of the user, and the PKG generates a secret key bound with the identity of the user according to the attribute set of the user; s3, the data owner DO encrypts the data; s4, when the DO hopes to share encrypted data with other users, generating a re-encryption key, and sending the re-encryption key to the cloud server; s5, the cloud server processes the original ciphertext by using the re-encryption key, converts the original ciphertext into a new ciphertext, and makes the new ciphertext accord with a specified sharing strategy; s6, after the target user receives the new ciphertext, decrypting the original ciphertext or the new ciphertext by using the private key of the target user; and S7, when the access authority of a certain user needs to be removed, the cloud server generates an updated re-encryption key according to the new access strategy.

Description

Technical Field

[0001] The invention belongs to the technical field of information security and relates to a data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism. Background Art

[0002] Cloud computing has become a mainstream information infrastructure because it provides sufficient storage and computing power. Using cloud services, users can get rid of the burden of setting up local servers and data maintenance. However, since cloud services are usually provided by third parties, data security and privacy issues arise. In order to protect private data, users can encrypt it before uploading it to the cloud server, that is, only the ciphertext will be outsourced to the cloud server.

[0003] Encryption is a fundamental method to protect the confidentiality of data, and attribute-based encryption (ABE) is a technique that can perform fine-grained access control on outsourced data. In a typical ABE setting, the data owner is assigned a list of attributes and uses these attributes to encrypt the data to be uploaded to the cloud. ABE operations should ensure the confidentiality of the data so that only designated users who have access to the data can access the encrypted data. In general, ABE is divided into key policy ABE (KP-ABE) and ciphertext policy ABE (CP-ABE). In KP-ABE, the user's private key is generated under the access policy and the ciphertext is encrypted using the attribute set, which is suitable for scenarios where the authority decides who is the authorized user, such as pay-TV systems. In contrast, in the CP-ABE setting, the attribute set is used to generate the user's secret and the ciphertext is encrypted using the access policy, which is suitable for cloud sharing systems because users can specify the access rights of the recipient.

[0004] Although ABE provides fine-grained access control, it cannot support data sharing in ciphertext form, which is crucial when collaboration is required. To solve the above problem, we can first generate a re-encryption key that can be used to convert the access policy P 1 The ciphertext under is converted into access policy P 2 Later, this re-encryption key will be sent to the cloud, and the cloud server can convert the ciphertext of the original recipient OR into a new ciphertext encrypted using the shared policy. When revocation occurs, the recipient OR needs to generate a new re-encryption key for the remote cloud server to perform the accurate conversion. Therefore, attribute-based proxy re-encryption (ABPRE) technology is an effective solution. In 2009, Liang et al. introduced the above concept into the attribute-based setting, thereby achieving a flexible description of user identity.

[0005] While existing ABE and ABPRE schemes can protect data confidentiality and enable data sharing of outsourced encrypted data, they do not scale well on the revocation problem. Revocation mechanisms are a basic requirement for data sharing in collaborative systems because many users may withdraw from collaboration. In 2006, Goyal et al. associated each attribute with a timestamp. When a new time period arrives, the authorization center will issue new private keys for non-revoked users. However, this approach increases the burden on the authorization center because it needs to issue private keys for all non-revoked users in each time period. In addition, the revocation in their scheme has a hysteresis, which means that revocation can only be performed when each time period arrives. In 2010, Liang et al. proposed a new ABE that claims to support efficient revocation by assigning a unique identity to each user's private key. By doing so, the authorization center knows to issue private keys only for non-revoked users with unique identities. Meanwhile, Yu et al. proposed a new revocation mechanism in 2010 by re-encrypting the ciphertext that only non-revoked users can decrypt. Subsequently, Attrapadung et al. proposed an attribute-based encryption scheme in 2019 that supports unbounded dynamic predicate combinations in attributes. Unfortunately, these works cannot support ciphertext sharing, which is a basic requirement when collaboration is needed. Therefore, studying data encryption schemes with attribute-based proxy re-encryption and direct revocation mechanisms has important theoretical and application value. Summary of the invention

[0006] In view of this, an object of the present invention is to provide a data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism, comprising the following steps:

[0009] S1: The authority center PKG performs system initialization;

[0010] S2: Each user applies for a private key from PKG based on his / her attributes. PKG generates a key bound to his / her identity based on the user's attribute set.

[0011] S3: The data owner DO encrypts the data;

[0012] S4: When the data owner DO wants to share the encrypted data with other users, a re-encryption key is generated and sent to the cloud server;

[0013] S5: After receiving the re-encryption key, the cloud server uses the re-encryption key to process the original ciphertext and convert the original ciphertext into a new ciphertext so that it complies with the specified sharing policy;

[0014] S6: When the target user receives the new ciphertext, he uses his private key to decrypt the original ciphertext or the new ciphertext;

[0015] S7: When it is necessary to remove a user's access rights, the cloud server generates an updated re-encryption key based on the new access policy, so that the removed user cannot decrypt the shared ciphertext.

[0016] Furthermore, the authority center PKG in step S1 performs system initialization, which specifically includes:

[0017] PKG constructs a bilinear pairing group and selects necessary random numbers to generate the core parameters of the system; the core parameters include the master key and public parameters; the master key is used for subsequent key generation and management, and the public parameters are distributed to all users as the basis for the encryption and decryption process; PKG defines a hash function to ensure consistency and security during data transmission.

[0018] Further, in step S2, the PKG generates a key bound to the user's identity according to the user's attribute set, specifically including:

[0019] PKG randomly selects a value as the basic parameter, and then combines the system master key and the user's attribute set to calculate the user's exclusive private key.

[0020] Furthermore, in step S3, the data owner DO encrypts the data, which specifically includes:

[0021] First, the data owner DO defines an access policy with attributes as the core to clarify which users can decrypt the data; the access policy is represented as a matrix structure, with each row corresponding to an attribute;

[0022] Then, the data owner randomly generates a set of parameters for each row and associates them with the attributes in the matrix;

[0023] Finally, the random parameters are processed and combined with the original data to generate the final ciphertext.

[0024] Furthermore, the re-encryption key in step S4 is generated based on the private key of the data owner and a shared access policy; the shared access policy specifies which users can access the shared data.

[0025] Further, in step S5, the cloud server verifies the matching of the original ciphertext and the re-encryption key, and converts the original ciphertext into a new ciphertext after the matching is successful.

[0026] Further, the decryption in step S6 includes the following steps:

[0027] Verify whether the user's attribute set meets the encryption policy or re-encryption policy; if so, the private key can be used to successfully decrypt and restore the original data; if not, the decryption fails.

[0028] Further, in step S7, the cloud server re-encrypts the original ciphertext in combination with the new random parameters to generate a new ciphertext that complies with the updated access policy. The new ciphertext can only be decrypted by users who comply with the updated policy and cannot be decrypted by users whose permissions have been revoked.

[0029] During the updating process, an additional randomization operation is introduced to readjust the structure of the ciphertext by adding random elements, making it impossible for external attackers to identify it as the original ciphertext or the revoked ciphertext.

[0030] The beneficial effects of the present invention are as follows: the present invention allows the data owner to securely share the encrypted data with multiple users through a proxy re-encryption mechanism, without the need for the data owner to manually decrypt and re-encrypt each time, thus achieving flexible control over data access rights, allowing the cloud server to directly revoke the access rights of a specified user, avoiding the complex process of regenerating keys and re-uploading data. It also ensures that unauthorized users cannot access or restore the encrypted data, increasing data security and reducing computing costs, and is suitable for large-scale data sharing applications in actual cloud computing environments.

[0031] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0033] Figure 1 This is a system architecture diagram of the data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism of the present invention;

[0034] Figure 2 This is a timing diagram of the data encryption method process of the attribute-based proxy re-encryption and direct revocation mechanism described in the present invention. DETAILED DESCRIPTION

[0035] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0036] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and thus the drawings only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0037] In the following description, numerous details are discussed to provide a more thorough explanation of the embodiments of the present invention. However, it is obvious to those skilled in the art that the embodiments of the present invention can be implemented without these specific details. In other embodiments, well-known structures and devices are shown in the form of block diagrams rather than in detail to avoid making the embodiments of the present invention difficult to understand.

[0038] like Figure 1-2 As shown, the present invention provides a data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism, and the method includes the following roles: data owner DO, original receiver OC, cloud server AWS, shared receiver SR and authority center PKG;

[0039] The authority center PKG generates and manages the security parameters and keys of the scheme, such as the private keys of participants;

[0040] The data owner DO uses access policy P 1 Encrypt their personal data and obtain the ciphertext C 1 Upload to the cloud server AWS;

[0041] The original recipient OR wishes to share the encrypted data originally sent to him / her with other sharing users. DO generate a re-encryption key Enables the cloud to send his / her ciphertext C 1 Convert to satisfy P 2 Shared users;

[0042] The cloud server AWS stores the ciphertext and performs re-encryption and revocation operations. The re-encryption algorithm converts the original ciphertext C 1 Convert to strategy P2 The ciphertext C 2 , the undo operation undoes the satisfaction of P 2 But not satisfying P 3 but strategy P 3 P 2 More stringent;

[0043] The shared receiver SR can use its own private key to decrypt the re-encrypted ciphertext normally.

[0044] This method is mainly divided into seven stages, namely, system initialization stage, key generation stage, encryption stage, re-encryption key generation stage, re-encryption stage, decryption stage, and revocation stage.

[0045] 1) System initialization phase: In the system initialization phase, the authority center PKG is responsible for initializing the entire system. First, PKG builds a bilinear pairing group and selects the necessary random numbers to generate the core parameters of the system. These parameters include the master key and public parameters. The master key is used for subsequent key generation and management, and the public parameters are distributed to all users as the basis for the encryption and decryption process. In addition, PKG defines a hash function to ensure consistency and security during data transmission. These initialization operations provide guarantees for the security and efficiency of the system.

[0046] In this embodiment, this step is specifically implemented as follows: the authority center PKG executes the system initialization algorithm Setup(λ,U), inputs the security parameter λ and the attribute domain U, and generates a bilinear pairing tuple (e,G,G T ,p), and select a random value α, g,f 1 ,...,f U , Q∈G and a hash function Generate master key msk=g α and public parameters PP = (e, G, G T ,g,f 1 ,...,f U ,Q,g a ,e(g,g) α ,H).

[0047] 2) Key generation phase: Each user applies for a private key from PKG based on their attributes. PKG will generate a key bound to their identity based on the user's attribute set. To this end, PKG will randomly select a value as the basic parameter, and then combine the system master key and the user's attribute set to calculate the user's exclusive private key. This phase ensures the uniqueness and security of the key, so that only users with specific attributes can decrypt the corresponding data.

[0048] In this embodiment, this step is specifically implemented as follows: the authority center PKG executes the key generation algorithm KeyGen(msk,Att) and selects a random value Input the attribute set Att and the master key msk to calculate the generated key

[0049] 3) Encryption phase: In the encryption phase, the data owner DO needs to encrypt the data for secure storage in the cloud server. The data owner first defines an access policy that is attribute-centric and specifies which users can decrypt the data. This policy can be represented as a matrix structure, with each row corresponding to an attribute. Then, the data owner randomly generates a set of parameters for each row and associates them with the attributes in the matrix. These random parameters are processed and combined with the original data to generate the final ciphertext. The ciphertext is uploaded to the cloud server to ensure the security of the data during storage and transmission.

[0050] In this embodiment, this step is specifically implemented as follows: the data owner DO executes the encryption algorithm Enc(m,(M,π)), inputs a message m and an access policy (M,π), M l×n is a matrix, π associates each row of M with an attribute in U, and the algorithm selects a random vector For each row of M j ,calculate j∈[1,l], randomly select r for each j∈[1,l] j ∈Z p , then calculate C = m·e(g,g) αr , C 1 =g r , C 2 =Q r , Output the original ciphertext CT.

[0051] 4) Re-encryption key generation phase: When the data owner DO wants to share encrypted data with other users, a re-encryption key needs to be generated. The generation of the re-encryption key is based on the data owner's private key and the shared access policy. The shared access policy specifies which users can access the shared data. The data owner calculates a specific re-encryption key by randomly selecting a set of parameters, combining the shared access policy and its own private key. This key is then sent to the cloud server to convert the original ciphertext into a new ciphertext that satisfies the sharing policy. The entire process is completed locally by the data owner, ensuring the security of the re-encryption key.

[0052] In this embodiment, this step is specifically implemented as follows: the data owner DO executes the re-encryption key generation algorithm Input private key sk=(Att,sk1 ,sk 2 ,sk x x∈Att) and access strategy in yes Matrix, randomly selected χ∈G T , And calculate rk 2 =g δ , Pick a random vector for Each line of calculate For each Random Selection Then calculate Output the re-encryption key rk.

[0053] 5) Re-encryption phase: After receiving the re-encryption key, the cloud server processes the original ciphertext according to the key. The core of the re-encryption algorithm is to convert the original ciphertext into a new ciphertext so that it complies with the specified sharing policy. The cloud server verifies the matching of the ciphertext and the re-encryption key, and performs the conversion after the match is successful. After the new ciphertext is generated, the target user can decrypt it using his private key, and other users cannot read the content even if they obtain the ciphertext.

[0054] In this embodiment, this step is specifically implemented as follows: the cloud server executes the re-encryption algorithm ReEnc(rk,CT), inputs the original ciphertext CT and the re-encryption key rk, and if R(Att,(M,π))=0, then output ⊥, otherwise, let Find the element So that∑ j∈J θ j ·M j =(1,0,...,0), then calculate Output the re-encrypted ciphertext CT'.

[0055] 6) Decryption phase: When the target user receives the ciphertext, he can use his private key to try to decrypt it. The decryption process includes two steps: first, verify whether the user's attribute set meets the encryption policy or re-encryption policy; second, perform the decryption operation based on the private key that meets the conditions. If the user's attributes meet the encryption policy, the original data can be successfully decrypted and restored; if not, the decryption fails. This mechanism ensures the permission control and security of data access.

[0056] In this embodiment, this step is specifically implemented as follows: the target user executes the decryption algorithm Dec(sk, CT / CT'), inputs the private key sk = (Att, sk 1 ,sk 2 ,sk x x∈Att), if CT=((M,π),C,C 1 ,C 2 ,{C 3,j ,C 4,j} j∈[1,l] ), that is, CT is the original ciphertext, check R(Att,(M,π))=1, if not, output ⊥, otherwise, let Finding Elements So that∑ j∈J θ j ·M j =(1,0,...,0), then calculate m, if That is, CT is the re-encrypted ciphertext, check If not, output ⊥, otherwise, let Finding Elements Make Then calculate χ and

[0057] 7) Cancellation phase:

[0058] When it is necessary to remove a user's access rights, the cloud server will generate an updated re-encryption key based on the new access policy. This key makes the original shared ciphertext no longer applicable to the removed user. To this end, the cloud server will re-encrypt the original ciphertext in combination with the new random parameters to generate a new ciphertext that complies with the updated access policy. The new ciphertext can only be decrypted by users who comply with the updated policy, and the user whose permissions have been revoked cannot access the updated data even if he has the original private key. In actual operation, in order to avoid exposing the update characteristics of the re-encryption key after the revocation operation, this method introduces additional randomization operations. By adding random elements and readjusting the structure of the ciphertext, it cannot be identified as the original ciphertext or the revoked ciphertext by external attackers, thereby enhancing the security of the system.

[0059] In this embodiment, this step is specifically implemented as follows: the cloud server executes the Revoke algorithm Revoke(rk,(M,π)), inputs a re-encryption key rk and a revocation access policy (M,π), where and M are and Matrix, output a revocation re-encryption key for access policy (M',π'), set (M',π') to where c 1 yes The first column of , and M' is a l'×n' matrix, where Calculate rk 5 ”=rk 5 , rk 6 ”=rk 6 , 1 of them G is the identity element of the group G, and then choose a random vector For each row M of M' j ',calculate j∈[1,l'], randomly select r for each j∈[1,l'] j ”'∈Z p , and calculate rk 5 ”'=e(g,g) αr ”',rk 6 ”'=g r ”', Then, calculate rk 5 '=rk 5 ”·rk 5 ”',rk 6 '=rk 6 ”·rk 6 ”',rk 7,j '=rk 7,j ”·rk 7,j ”',rk 8,j '=rk 8,j ”·rk 8,j ”', The output revoked re-encryption key is rk'. Since the element rk 5 ” and RK 6 " is equal to rk 5 andrk 6 , so the attacker can distinguish whether the re-encryption key is the revoked re-encryption or the original re-encryption. To avoid this problem, the random element rk is introduced 5 ”' and rk 6 ”' to re-randomize rk 5 ” and RK 6 In addition, the same method was used to re-randomize rk 7,j ” and RK 8,j ”.

[0060] In the above embodiments, the description's reference to "this embodiment" indicates that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least some embodiments, but not necessarily all embodiments. Multiple occurrences of "this embodiment" do not necessarily all refer to the same embodiment.

[0061] In the above-described embodiments, although the invention has been described in conjunction with specific embodiments of the invention, many substitutions, modifications, and variations of these embodiments will be apparent to those of ordinary skill in the art based on the foregoing description. For example, other storage structures (e.g., dynamic RAM (DRAM)) may use the embodiments discussed. Embodiments of the invention are intended to encompass all such substitutions, modifications, and variations that fall within the broad scope of the appended claims.

[0062] This embodiment further provides a computer-readable storage medium on which a computer program is stored. When the program is executed by a processor, any one of the methods in this embodiment is implemented.

[0063] This embodiment also provides an electronic terminal, including: a processor and a memory;

[0064] The memory is used to store a computer program, and the processor is used to execute the computer program stored in the memory, so that the terminal executes any one of the methods in this embodiment.

[0065] The computer-readable storage medium in this embodiment can be understood by ordinary technicians in this field: all or part of the steps of implementing the above-mentioned method embodiments can be completed by hardware related to the computer program. The aforementioned computer program can be stored in a computer-readable storage medium. When the program is executed, the steps of the above-mentioned method embodiments are executed; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk and other media that can store program codes.

[0066] The electronic terminal provided in this embodiment includes a processor, a memory, a transceiver and a communication interface. The memory and the communication interface are connected to the processor and the transceiver and complete communication with each other. The memory is used to store computer programs, the communication interface is used to communicate, and the processor and the transceiver are used to run computer programs so that the electronic terminal executes each step of the above method.

[0067] In this embodiment, the memory may include a random access memory (RAM), and may also include a non-volatile memory (non-volatile memory), such as at least one disk memory.

[0068] The above-mentioned processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components.

[0069] The present invention can be used in many general or special computing system environments or configurations, such as personal computers, server computers, handheld or portable devices, tablet devices, multiprocessor systems, microprocessor-based systems, set-top boxes, programmable consumer electronic devices, network PCs, minicomputers, mainframe computers, distributed computing environments including any of the above systems or devices, and the like.

[0070] The present invention may be described in the general context of computer-executable instructions executed by a computer, such as program modules. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform specific tasks or implement specific abstract data types. The present invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices connected through a communication network. In a distributed computing environment, program modules may be located in local and remote computer storage media, including storage devices.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism, characterized in that: The following steps are involved: S1: The authority center PKG performs system initialization; S2: Each user applies for a private key from PKG based on his / her attributes. PKG generates a key bound to his / her identity based on the user's attribute set. S3: The data owner DO encrypts the data; S4: When the data owner DO wants to share the encrypted data with other users, a re-encryption key is generated and sent to the cloud server; S5: After receiving the re-encryption key, the cloud server uses the re-encryption key to process the original ciphertext and convert the original ciphertext into a new ciphertext so that it complies with the specified sharing policy; S6: When the target user receives the new ciphertext, he uses his private key to decrypt the original ciphertext or the new ciphertext; S7: When it is necessary to remove a user's access rights, the cloud server generates an updated re-encryption key based on the new access policy, so that the removed user cannot decrypt the shared ciphertext.

2. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: In step S1, the authority center PKG performs system initialization, which specifically includes: PKG constructs a bilinear pairing group and selects necessary random numbers to generate the core parameters of the system; the core parameters include the master key and public parameters; the master key is used for subsequent key generation and management, and the public parameters are distributed to all users as the basis for the encryption and decryption process; PKG defines a hash function to ensure consistency and security during data transmission.

3. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: Step S2: The PKG generates a key bound to the user's identity based on the user's attribute set, specifically including: PKG randomly selects a value as the basic parameter, and then combines the system master key and the user's attribute set to calculate the user's exclusive private key.

4. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: In step S3, the data owner DO encrypts the data, which specifically includes: First, the data owner DO defines an access policy with attributes as the core to clarify which users can decrypt the data; the access policy is represented as a matrix structure, with each row corresponding to an attribute; Then, the data owner randomly generates a set of parameters for each row and associates them with the attributes in the matrix; Finally, the random parameters are processed and combined with the original data to generate the final ciphertext.

5. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: The re-encryption key in step S4 is generated based on the private key of the data owner and the shared access policy; the shared access policy specifies which users can access the shared data.

6. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: In step S5, the cloud server verifies the matching of the original ciphertext and the re-encryption key, and converts the original ciphertext into a new ciphertext after the matching is successful.

7. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: The decryption in step S6 includes the following steps: Verify whether the user's attribute set meets the encryption policy or re-encryption policy; if so, the private key can be used to successfully decrypt and restore the original data; if not, the decryption fails.

8. The data encryption method based on attribute-based proxy re-encryption and direct revocation mechanism according to claim 1, characterized in that: In step S7, the cloud server re-encrypts the original ciphertext with the new random parameters to generate a new ciphertext that complies with the updated access policy. The new ciphertext can only be decrypted by users who comply with the updated policy and cannot be decrypted by users whose permissions are revoked. During the updating process, an additional randomization operation is introduced to readjust the structure of the ciphertext by adding random elements, making it impossible for external attackers to identify it as the original ciphertext or the revoked ciphertext.

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