Non-interactive ciphertext connection keyword retrieval method with hidden result mode

Abstract

The invention relates to a result mode hidden non-interactive ciphertext connection keyword retrieval method, which comprises the following steps of: in an initial stage, respectively storing a system key and an index structure generated by a data owner in a local data owner and a cloud server; the data owner locally generates trap door information according to the system key and sends the trap door information to the cloud server to execute retrieval operation on the index structure, and after the encrypted data meeting the retrieval condition are returned, the data owner decrypts the encrypted data meeting the retrieval condition according to the system key to obtain a retrieval result. Through a symmetric subset predicate encryption protocol (S2PE) based on an XOR filter and a novel non-interactive index structure generation method, ciphertext connection keyword retrieval in which a keyword pair result mode (KPRP) and a cross-query intersection result mode (IP) are hidden at the same time is achieved, the number of interaction rounds of a cloud server and a data owner is reduced to one, retrieval efficiency is improved, and retrieval time is shortened. The data owner and the cloud server can realize KPRP and IP hiding without extra communication.

Description

Technical Field

[0001] The present invention relates to a technology in the field of information security, in particular to a non-interactive ciphertext connection keyword retrieval method with hidden result patterns. Background Art

[0002] Ciphertext-linked keyword retrieval technology allows cloud servers to perform efficient linked keyword queries on ciphertext data while ensuring the privacy of data and queries. However, the main shortcomings of existing technologies are: 1) There is leakage of keyword pair result pattern (KPRP) and cross-query intersection result pattern (IP). 2) In order to hide KPRP, cloud servers and data owners need to perform an extra round of communication, resulting in large computational and communication overheads. Summary of the invention

[0003] In view of the above shortcomings of the prior art, the present invention proposes a non-interactive ciphertext connection keyword retrieval method with hidden result patterns. Specifically, a symmetric subset predicate encryption protocol (S 2 PE) and a new non-interactive index structure generation method are used to achieve ciphertext connection keyword retrieval with both keyword pair result pattern (KPRP) and cross-query intersection result pattern (IP) hidden at the same time, reducing the number of interaction rounds between cloud server and data owner to one round, improving retrieval efficiency, and enabling data owner and cloud server to achieve KPRP and IP hiding without additional communication, that is, for a query, the result intersection of the first keyword and any other keyword; IP refers to the result intersection of the first keyword in two queries when the first keyword is different and at least one of the remaining keywords is the same.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a non-interactive ciphertext connection keyword retrieval method with result pattern hiding. In the initial stage, a system key and an index structure generated by a data owner are respectively stored in the data owner's local computer and a cloud server. When performing a connection keyword retrieval, the data owner generates trapdoor information locally according to the system key and sends it to the cloud server to perform a retrieval operation on the index structure. After the encrypted data satisfying the retrieval condition is returned, the data owner decrypts the encrypted data satisfying the retrieval condition according to the system key to obtain a retrieval result.

[0006] In the initial stage, the data owner Generate the system key K and index structure cEMM, where: For keywords, To include The set of all document identifiers of It is the set of all keywords in the plaintext document set.

[0007] The system key K includes: the key K of the first pseudo-random function F1 and the second pseudo-random function F2 E , K X , K T and msk, which are obtained in the following way: the data owner first randomly selects the key K E and K X , and then use the encryption multi-value mapping algorithm TSet to generate the key K according to the security parameter λ and the multi-value mapping T T , using S 2 PE.Setup algorithm generates symmetric subset predicate encryption S 2 PE's master key msk, the final output system key K = (K E ,K E ,K T ,msk).

[0008] The index structure cEMM includes: encrypted mapping TS and ciphertext ct, which are obtained in the following way: the data owner generates TS by running TSet.Setup algorithm according to security parameter λ and multi-value mapping T, and generates TS according to S 2 The master key msk of PE and the set XSet generate the ciphertext ct of the set XSet, and finally output the index structure cEMM=(TS,ct).

[0009] The trapdoor information is based on the system key K and the connection keyword to be retrieved. Using the token generation algorithm in the encrypted multi-value mapping protocol And run the key generation algorithm S in the symmetric subset predicate encryption protocol 2 PE.KeyGen(msk,X) generates. The specific process includes: the data owner runs the TSet.GetTag algorithm to generate keywords The trapdoor stag is output to the cloud server; for each Data Owner Computing And store qtag in the set QSet; further, the data owner calculates the trapdoor xtoken i =S 2 PE.KeyGen(msk,QSet) and output to the cloud server. Contains keywords A collection of document identifiers.

[0010] The encrypted data that meets the retrieval conditions is retrieved by the cloud server by executing the retrieval algorithm TSet.Retrieve(TSet,stag) in the encrypted multi-value mapping protocol to obtain the encrypted data containing the keyword The corresponding ciphertext document identifier set For each Cloud server based on trapdoor xtoken i , the ciphertext ct of the set XSet, execute the decryption algorithm S in the symmetric subset predicate encryption protocol 2 PE.Dec(xtoken i ,ct), if the input result is msg='True', then the corresponding e i Output to the data owner.

[0011] The decryption is done by the data owner calculating the key For each e returned, the data owner executes the decryption algorithm in the symmetric encryption algorithm SE Output the corresponding plaintext v as the final retrieval result.

[0012] The present invention relates to a system for implementing the above method, comprising: an index generation module, a trapdoor generation module, a retrieval module and a decryption module, wherein: the data owner uses the index generation module to generate a data according to the plaintext document set. Generate the index structure cEMM and store it in the cloud server; the data owner generates a module through the trapdoor according to the retrieval conditions Generate the corresponding trapdoor and output it to the cloud server; the cloud server uses the retrieval module to perform a retrieval operation based on the trapdoor information, obtains the ciphertext document identifier that meets the retrieval conditions and returns it to the data owner; the data owner obtains the final plaintext document identifier through the decryption algorithm in the symmetric encryption algorithm. Technical Effects

[0013] The present invention uses a symmetric subset predicate encryption protocol S 2 PE realizes efficient subset member determination; through the index structure generation method, the data owner can generate a trapdoor for search keywords and a non-interactive ciphertext connection keyword search scheme with hidden result patterns in a non-interactive manner, thereby protecting the keyword pair result pattern (KPRP) and the cross-query intersection result pattern (IP) at the same time. Compared with the prior art, the present invention can reduce the communication between the data owner and the cloud server to one round and greatly improve the search efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of an application scenario of an embodiment;

[0015] Figure 2 It is a flow chart of the present invention;

[0016] Figure 3 and Figure 4 It is a schematic diagram of the effect of the embodiment. DETAILED DESCRIPTION

[0017] like Figure 1 As shown, the application scenario of this embodiment includes: a data owner and a cloud server, wherein: the data owner generates an index structure for a plaintext document set and uploads it to the cloud server, the data owner generates a search trap based on the connection keywords to be searched, decrypts the ciphertext search results that meet the search conditions from the cloud server, and obtains the plaintext search results; the cloud server stores the index structure sent by the data owner, receives the search trap sent by the data owner, performs a search in the index structure, and returns the search results that meet the search conditions to the data owner.

[0018] like Figure 2 As shown, a non-interactive ciphertext connection keyword retrieval method based on result mode hiding in the above scenario involved in this embodiment includes the following steps:

[0019] Step 1: Generate index structure: The data owner generates an index structure based on the plaintext document set and outsources the index structure to the cloud server, including:

[0020] Step 1.1: Select a key K for the first pseudo-random function F1 E , select the key K for the second pseudo-random function F2 X , initialize an empty multi-value map T, run S 2 PE.Setup(1 λ ) to obtain the key msk, specifically: select an integer b, generate the key sk∈{0,1} of the pseudo-random function F λ ; Run the initialization algorithm in the XOR filter algorithm Generate an XOR filter with parameters (b,3) and output S 2 PE master key Message Space and complete works in: Contains 3 hash functions (h0,h1,h2),

[0021] Step 1.2: Generate a multi-valued mapping T and a set XSet, including:

[0022] 1.2.1 First cycle: for Each keyword Initialize an empty set t and run the first pseudo-random function Generate keywords Key

[0023] The pseudo-random function is implemented using HMAC-SHA 256.

[0024] 1.2.2 Second cycle: For Encrypt using the encryption algorithm SE.Enc in the symmetric encryption algorithm SE The corresponding document identifier set The i-th document identifier in gets the document identifier ciphertext e, that is, Then add e to the set t.

[0025] 1.2.3 The third cycle: for Remove All keywords except Right now when Then run the second pseudo-random function Generate a value xtag, then store xtag in the set XSet, and end the third loop, ending the second loop.

[0026] 1.2.4 Store the set t generated in the second loop in In And end the first cycle operation.

[0027] Step 1.3: Generate TS and K T , that is (TSet,K T )←TSet.Setup(1 λ ,T).

[0028] Step 1.4: Generate ct, i.e. ct←S 2 PE.Enc(msk,msg='True',XSet), specifically: for each element xtag in XSet i , calculate its pseudo-random function xtag' i ←F(sk,xtag i ), forming a new set XSet' = (xtag'1, xtag'2, ..., xtag' |XSet| ), call the update algorithm in the XOR filter algorithm After inserting the set XSet' into the XOR filter, the ciphertext ct of XSet is outputted as B Y' .

[0029] The initialization algorithm XF.Setup and the update algorithm XF.Update in the XOR filter algorithm are implemented using, but not limited to, the technology described in "Practical Volume-Hiding Encrypted Multi-Maps with Optimal Overhead and Beyond" by Wang et al.

[0030] Step 1.5: Set the system key K = (K E ,K X ,K T ,msk) is stored locally in the data owner, and the index structure cEMM=(TS,ct) is output to the cloud server;

[0031] Step 2: Generate retrieval trap information, including:

[0032] Step 2.1 When the data owner retrieves documents containing q keywords at the same time, that is, The data owner first runs the token generation algorithm TSet.GetTag in the encrypted multi-value mapping protocol to generate The trapdoor stag, that is Output stag to the cloud server.

[0033] Step 2.2 For Perform steps 2.2.1-2.2.3:

[0034] Step 2.2.1 For 2≤j≤q, generate by the second pseudo-random function F2 Qtag j Stored in a QSet i middle.

[0035] Step 2.2.2: For the set QSet generated in step 2.2.1 i Run the key generation algorithm in the symmetric subset predicate encryption protocol to generate a trapdoor xtoken i , that is, xtoken i ←S 2 PE.KeyGen(msk,QSet i ), specifically: for QSet i Each element in qtag j , generating a set of locations And form a multiset from the position sets of all elements Next, randomly select the key K∈{0,1} of the symmetric encryption algorithm Sym λ ,calculate Encrypt 0 using the encryption algorithm Sym.Enc in the symmetric encryption algorithm Sym λ get Right now After that, output QSet i Corresponding trapdoor information

[0036] Step 2.2.3 Output to cloud server.

[0037] Step 3: Search, including:

[0038] Step 3.1 When the cloud server receives stag, it runs the retrieval algorithm TSet.Retrieve in the encrypted multi-value mapping protocol to retrieve the All ciphertext document identifiers, that is, t←TSet.Retrieve(TSet,stag), where:

[0039] Step 3.2 For Run the decryption algorithm S in the symmetric subset predicate encryption protocol 2 PE.Dec for ct=B in the index structure Y Decrypt, that is, msg i ←S 2 PE.Dec(xtoken i ,ct), specifically: calculation Then, run the decryption algorithm Sym.Dec in the symmetric encryption algorithm Sym to decrypt Right now Finally, when τ = 0 λ , output 'True', otherwise output ⊥.

[0040] Step 3.3 When msg i ='Ture', i.e. e i is the ciphertext document identifier that meets the conditions, e i Add to the collection Res.

[0041] Step 3.4 The cloud server outputs the set Res to the data owner.

[0042] Step 4: Decryption, specifically including: the data owner runs the first pseudo-random function F1 to generate a key For each e in Res, the decryption algorithm SE.Dec of the symmetric encryption algorithm SE is used to decrypt e to obtain the plaintext document identifier v, and then v is stored in the set Rval. The data owner obtains the retrieval result Rval, and the document identifiers in Rval simultaneously contain all q search keywords.

[0043] The initialization algorithm TSet.Setup, the token generation algorithm TSet.GetTag, and the retrieval algorithm TSet.Retrieve in the encrypted multi-value mapping protocol are implemented using but not limited to the technology recorded in "Highly-ScalableSearchable Symmetric Encryption with Supportfor Boolean Queries" by Cash et al.

[0044] The encryption algorithm Sym.Enc and the decryption algorithm Sym.Dec in the symmetric encryption algorithm Sym adopt, but are not limited to, the symmetric encryption algorithm that meets PCPA security requirements and is recorded by Curtmola et al. in Searchable Symmetric Encryption: Improved Definitions and Efficient Constructions.

[0045] The encryption algorithm SE.Enc and the decryption algorithm SE.Dec in the symmetric encryption algorithm SE adopt a symmetric encryption algorithm that meets IND-CPA security.

[0046] The specific experiment was carried out on a machine with Intel(R) Core(TM) i9-11900K 3.50GHz CPU core and 128GB memory running Ubuntu 20.04, and was implemented using Python programming. The experimental parameters were set as follows: The false positive probability of the Bloom filter ε = 10 -6, using HMAC-SHA256 to implement pseudo-random function, SHA-256 to implement hash function, AES with a key length of 256-bit and CBC mode to implement symmetric encryption, and Type A pair operations in the Pypbc library to implement group operations. First, the method of the present invention (named Doris) is theoretically analyzed with the OXT scheme [Highly-Scalable Searchable SymmetricEncryption with Support for Boolean Queries], the HXT scheme [Result Pattern HidingSearchable Encryption for Conjunctive Queries], and the ConjFilter scheme [EfficientBoolean Search over Encrypted Data with Reduced Leakage], as shown in Table 1. Then, two real data sets Enron and Enwiki are used to experimentally compare the method Doris of the present invention with the OXT, HXT and ConjFilter schemes. Table 2 is a parameter comparison of the experimental data sets Enron and Enwiki used.

[0047] Table 1 Performance comparison of existing solutions

[0048] for implement Connect keyword search, Exp is the modular exponential operation, Prf is the pseudo-random function operation, H is the hash operation, Dec is the decryption operation of the symmetric encryption algorithm, Xor is the exclusive OR operation, O(T Stp ) is the initialization operation in the encrypted multi-value mapping protocol, O(T Srch ) is the retrieval operation in the encrypted multi-value mapping protocol.

[0049] Table 2 Comparison of dataset parameters Dataset Number of documents Number of keywords Number of keyword / document pairs Enron 446131 96980 3611634 Enwiki 50285 1294263 5025847

[0050] The non-interactive ciphertext connection keyword retrieval method with result pattern hiding in the present invention (named Doris) is experimentally simulated and compared with the OXT scheme [Highly-Scalable Searchable Symmetric Encryption with Support for Boolean Queries], the HXT scheme [Result Pattern Hiding Searchable Encryption for Conjunctive Queries], and the ConjFilter scheme [Efficient Boolean Search over Encrypted Data with Reduced Leakage].

[0051] like Figure 3 As shown, when searching for two keywords, the number of different search results for the first keyword is It can be seen that the retrieval efficiency of the present invention is the highest, which is about 6 times that of the HXT solution and 1.07 times that of the ConjFilter solution.

[0052] like Figure 4 As shown, this is the number of search results for the first keyword. Schematic diagram of search time comparison under different numbers of search keywords. It can be seen that the search efficiency of the present invention is the highest, which is about 42 times that of the HXT solution and 1.1 times that of the ConjFilter solution.

[0053] Compared with the existing technology, this method adopts the proposed symmetric subset predicate encryption protocol and index structure generation method to realize non-interactive keyword trapdoor generation and reduce the communication overhead of trapdoor generation. At the same time, it realizes the hiding of keyword pair result pattern (KPRP) and cross-query intersection result pattern (IP) for the first time and improves retrieval efficiency.

[0054] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A non-interactive ciphertext connection keyword retrieval method with result pattern hiding, characterized in that: In the initial stage, the system key and index structure generated by the data owner are stored locally in the data owner and in the cloud server respectively. When performing a connection keyword search, the data owner generates trapdoor information locally based on the system key and sends it to the cloud server to perform a search operation on the index structure. After the encrypted data that meets the search conditions is returned, the data owner decrypts the encrypted data that meets the search conditions based on the system key to obtain the search results. The system key K includes: the key K of the first pseudo-random function F1 and the second pseudo-random function F2 E , K X , K T and msk, which are obtained in the following way: the data owner first randomly selects the key K E and K X , and then use the encryption multi-value mapping algorithm TSet to generate the key K according to the security parameter λ and the multi-value mapping T T , using S 2 PE.Setup algorithm generates symmetric subset predicate encryption S 2 PE's master key msk, the final output system key K = (K E , K E , K T ,msk).

2. The non-interactive ciphertext connection keyword retrieval method with result pattern hiding according to claim 1 is characterized in that: In the initial stage, the data owner Generate system key K and index structure cEMM, where: l is the keyword, is the set of all document identifiers containing l, It is the set of all keywords in the plaintext document set.

3. The non-interactive ciphertext connection keyword retrieval method with result pattern hiding according to claim 1 is characterized in that: The index structure cEMM includes: encrypted mapping TS and ciphertext ct, which are obtained in the following way: the data owner generates TS by running TSet.Setup algorithm according to security parameter λ and multi-value mapping T, and generates TS according to S 2 The master key msk of PE and the set XSet generate the ciphertext ct of the set XSet, and finally output the index structure cEMM=(TS, ct).

4. The non-interactive ciphertext connection keyword retrieval method with result pattern hiding according to claim 1 is characterized in that: The trapdoor information is based on the system key K and the connection keyword φ=(l1∧...∧l q ), using the token generation algorithm TSet.GetTag(K T , l) and run the key generation algorithm S in the symmetric subset predicate encryption protocol 2 PE.KeyGen(msk,X) is generated. The specific process includes: the data owner runs the TSet.GetTag algorithm to generate the trapdoor stag of the keyword l1 and outputs it to the cloud server; for each i∈|MM[l1]|, j∈[q], the data owner calculates qtag=F2(K X ,l1||l j ||i), and stores qtag in the set QSet; further, the data owner calculates the trapdoor xtoken i =S 2 PE.KeyGen(msk, QSet) and output to the cloud server, where MM[l1] refers to the set of document identifiers containing keyword l1.

5. The non-interactive ciphertext connection keyword retrieval method with result pattern hiding according to claim 1 is characterized in that: The encrypted data that meets the retrieval condition is retrieved by the cloud server by executing the retrieval algorithm TSet.Retrieve(TSet,stag) in the encrypted multi-value mapping protocol to obtain a set of ciphertext document identifiers corresponding to the keyword l1. For each i∈|MM[l1]|, the cloud server uses the trapdoor xtoken i , the ciphertext ct of the set XSet, execute the decryption algorithm S in the symmetric subset predicate encryption protocol 2 PE.Dec(xtoken i ,ct), if the input result is msg = 'True', then the corresponding e i Output to the data owner.

6. The non-interactive ciphertext connection keyword retrieval method with result pattern hiding according to claim 1 is characterized in that: The decryption is done by the data owner calculating the key For each e returned, the data owner executes the decryption algorithm in the symmetric encryption algorithm SE Output the corresponding plaintext v as the final retrieval result.

7. The non-interactive ciphertext connection keyword retrieval method according to any one of claims 1 to 6, characterized in that: include: Step 1: Generate index structure: The data owner generates an index structure based on the plaintext document set and outsources the index structure to the cloud server, including: Step 1.1: Select a key K for the first pseudo-random function F1 E , select the key K for the second pseudo-random function F2 X , initialize an empty multi-value map T, run S 2 PE.Setup(1 λ ) to obtain the key msk, specifically: select an integer b, generate a key sk∈{0,1} of the pseudo-random function F λ ; Run the initialization algorithm in the XOR filter algorithm Generate an XOR filter with parameters (b, 3) and output S 2 PE master key Message Space and complete works in: Contains 3 hash functions (h0, h1, h2), Step 1.2: Generate a multi-valued mapping T and a set XSet, including: 1.2.1 First cycle: for For each keyword l in: initialize an empty set t, run the first pseudo-random function F1(K E , l) Generate the key K of keyword l l , 1.2.2 Second loop: For 1≤i≤|MM[l]|, use the encryption algorithm SE.Enc in the symmetric encryption algorithm SE to encrypt the document identifier set corresponding to l The i-th document identifier in gets the document identifier ciphertext e, that is, Then add e to the set t; 1.2.3 The third cycle: for Remove all keywords l' except l, that is, when Then run the second pseudo-random function F2(K X , l||′||i) generates a value xtag, then stores xtag in the set XSet, and ends the third loop, and ends the second loop; 1.2.4 Store the set t generated in the second loop in T[l], that is, T[l]←t, and end the first loop operation; Step 1.3: Generate Ts and K T , that is (TSet,K T )← TSet.Setup(1 λ , T); Step 1.4: Generate ct, i.e. ct←S 2 PE.Enc(msk,msg='True',XSet), specifically: for each element xtag in XSet i , calculate its pseudo-random function xtag′ i ←F(sk,xtag i ), forming a new set XSet′=(xtag′1, xtag′2, ..., xtag′ |XSet| ), call the update algorithm in the XOR filter algorithm After inserting the set XSet′ into the XOR filter, the ciphertext ct of XSet is outputted as B Y′ ; Step 1.5: Set the system key K = (K E , K X , K T ,msk) is stored locally in the data owner, and the index structure cEMM=(TS,ct) is output to the cloud server; Step 2: Generate retrieval trap information, including: Step 2.1 When the data owner searches for documents containing q keywords at the same time, φ=(l1∧...∧l q ), the data owner first runs the token generation algorithm TSet.GetTag in the encrypted multi-value mapping protocol to generate the trapdoor stag of l1, that is, stag←TSet.GetTag(K T ,l1), output stag to the cloud server; Step 2.2 For i=1, 2, ...MM[l1], execute steps 2.2.1-2.2.3: Step 2.2.1 For 2≤j≤q, generate qtag by the second pseudo-random function F2 j =F2(K X ,l1||l j ||i), qtag j Stored in a QSet i middle; Step 2.2.2: For the set QSet generated in step 2.2.1 i Run the key generation algorithm in the symmetric subset predicate encryption protocol to generate a trapdoor xtoken i , that is, xtoken i ←S 2 PE.KeyGen(msk,QSet i ), specifically: for QSet i Each element in qtag j , generating a set of locations And form a multiset from the position sets of all elements Next, randomly select the key K∈{0,1} of the symmetric encryption algorithm Sym λ ,calculate Use the encryption algorithm Sym.Enc in the symmetric encryption algorithm Sym to encrypt 0λ and get Right now After that, output the trapdoor information corresponding to QSeti Step 2.2.3 Output to cloud server, Step 3: Search, including: Step 3.1 When the cloud server receives stag, it runs the retrieval algorithm TSet.Retrieve in the encrypted multi-value mapping protocol to retrieve all ciphertext document identifiers containing l1, that is, t←TSet.Retrieve(TSet,stag), where: Step 3.2 For 1≤i≤|MM[l1]|, run the decryption algorithm S in the symmetric subset predicate encryption protocol 2 PE.Dec for ct=B in the index structure Y Decrypt, that is, msg i ←S 2 PE.Dec(xtoken i , ct), specifically: calculation Then, run the decryption algorithm Sym.Dec in the symmetric encryption algorithm Sym to decrypt Right now Finally, when τ=0λ, output 'True', otherwise output ⊥; Step 3.3 When msg i ='Ture', i.e. e i is the ciphertext document identifier that meets the conditions, e i Add to the collection Res; Step 3.4 The cloud server outputs the set Res to the data owner; Step 4: Decryption, specifically including: the data owner runs the first pseudo-random function F1 to generate a key For each e in Res, the decryption algorithm SE.Dec of the symmetric encryption algorithm SE is used to decrypt e to obtain the plaintext document identifier v, and then v is stored in the set Rval. The data owner obtains the retrieval result Rval, and the document identifiers in Rval simultaneously contain all q search keywords.

8. A non-interactive ciphertext connection keyword retrieval system for hiding the result pattern of any of the methods described in claims 1-7, characterized in that: include: Index generation module, trapdoor generation module, retrieval module and decryption module, where: the data owner uses the index generation module to Generate an index structure cEMM and store it in the cloud server; the data owner generates a trapdoor module according to the retrieval condition φ=(l1∧…∧l q ) Generate the corresponding trapdoor and output it to the cloud server; The cloud server uses the retrieval module to perform a retrieval operation based on the trapdoor information, obtains the ciphertext document identifier that meets the retrieval conditions and returns it to the data owner; the data owner obtains the final plaintext document identifier through the decryption algorithm in the symmetric encryption algorithm.

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