KR102157836B1 - Deduplication security method for privacy protection in cloud environment - Google Patents

Abstract

The present invention is a file deduplication method for protecting privacy in a cloud environment that solves the privacy problem by preventing the user's information from being inferred from the file information while preventing duplicate storage of the same file uploaded by a plurality of users in a cloud environment The purpose is to provide. To this end, a) a meta server generates an SID, encrypts the SID, and transmits the encrypted SID to the client device; and b) the client device decrypts the encrypted SID; and c) the client device sends a file to the meta server. Requesting a file path for uploading; and d) generating the file path by the meta server; and e) generating an FHV by double hashing the uploaded file by the client device; and f) the client device Encrypting the FHV with the SID and transmitting it to the meta server; And g) Decrypting the FHV encrypted with the SID by the meta server; And h) The meta server transmits the FHV to the storage server and the upload file is Requesting whether the file is already registered; And i) the storage server analyzing the FHV to check whether or not the uploaded file is an already registered file, and notifying the meta server and the client device; file for privacy protection in a cloud environment including Deduplication method is provided.

Description

Deduplication security method for privacy protection in cloud environment}

The present invention relates to a file deduplication method for privacy protection in a cloud environment. More specifically, a user uploads and downloads a file based on a PIN, RefID, and FHV, thereby mapping a user-file list and a file. In a cloud environment that prevents users from being inferred only through metadata and file structure analysis on the server by fundamentally blocking the relationship, the file deduplication method for privacy protection in a cloud environment that prevents even server administrators from knowing the files uploaded by users. For.

As cloud services have recently become popular, various cloud service providers are emerging. Cloud platforms can provide various types of services, and among them, storage-based cloud services are the most widely used service types, and the market is expected to continue to expand in the future.

In particular, data size will increase exponentially in the upcoming 4th industrial era, which will eventually lead to an increase in cost due to storage expansion, which will put a heavy burden on service providers.

Therefore, in a cloud environment, deduplication technology is one of the very important element technologies. The deduplication technology refers to a technology that does not redundantly store the same data uploaded by a plurality of users, and has the advantage that it is possible to dramatically reduce storage capacity through such a deduplication technology.

Therefore, deduplication technology is already applied in many cloud storage environments. Due to the nature of the cloud environment, a large-capacity storage server is required, and if the storage capacity increases, additional storage expansion is required. However, using deduplication technology can fundamentally solve this cost problem.

However, the deduplication technology has a structural privacy problem. This is because if the deduplication technology is to be applied, the mapping structure of users and files is stored as meta information, and in this process, a list of users who uploaded a specific file can be secured through meta-analysis on the server. In particular, in the case of files with sensitivity such as political orientation, thoughts, specific diseases, and sexual life, the problem can be very serious. This is because if a list of users uploading the file can be secured from the cloud server, the risk that the cloud environment will act as a kind of monitoring system in the future cannot be excluded.

Therefore, it is time to find a way to solve the privacy problem while enacting files in the cloud server so as not to be duplicated.

Korean Registered Patent No. 10-1422759 (Data storage and sharing method to prevent collusion in data consignment environment)

The present invention is a file deduplication method for protecting privacy in a cloud environment that solves the privacy problem by preventing the user's information from being inferred from the file information while preventing duplicate storage of the same file uploaded by a plurality of users in a cloud environment The purpose is to provide.

The present invention is a client device that generates a RefID by receiving a PIN from a user and generates a RefID and double hashed a file stored in the cloud to generate an FHV, a meta server that receives and stores the RefID, and a storage that receives and stores the FHV. A method for deduplication of files for protection of privacy in a cloud environment including a server, the method comprising: a) generating an SID by a meta server, encrypting the SID, and transmitting it to a client device; And b) the client device transmits the encrypted SID. Decrypting; And c) A client device requesting a file path required for file upload to the meta server; And d) A meta server generating the file path; And e) A client device doubles the upload file. Generating an FHV by hashing; and f) encrypting the FHV with the SID by a client device and transmitting the FHV to the meta server; and g) decrypting the FHV encrypted with the SID by the meta server; and h) requesting whether the uploaded file is a registered file by transmitting the FHV to the storage server by the meta server; And i) the storage server analyzing the FHV to check whether or not the uploaded file is an already registered file, and notifying the meta server and the client device; file for privacy protection in a cloud environment including Deduplication method is provided.

Here, j) if the upload file is a file that is not registered, the client device uploading the upload file to the storage; and k) the client device receiving a PIN from the user and generating a RefID; And l) the storage server verifies the integrity of the stored upload file; and m) the storage server maps the stored file and the FHV; and n) the client device transmits the RefID to the meta server; And o) mapping the stored RefID and the file path by the meta server, and informing the client device that the file upload has been completed.

Here, the RefID may be characterized in that it is generated through the following (Equation 1).

(H(PIN)

FHV))^H(PIN) (Equation 1) E(H(UserID)

(H(PIN)

FHV)) ^H(PIN)

In addition, a client device that generates a RefID by receiving a PIN from the user and generates a RefID and double hashed a file stored in the cloud to generate an FHV, a meta server that receives and stores the RefID, and a storage server that receives and stores the FHV. A method for deduplication of files for privacy protection in a cloud environment comprising: a) generating an SID by a meta server, encrypting the SID, and transmitting it to a client device; And b) the client device decrypts the encrypted SID. And c) requesting, by a client device, to download a file for a specific path to the meta server; and d) checking whether a download file exists in the specific path by the meta server; and e) a user of the client device Receiving a PIN from, encrypting the hash value of the PIN with SID, and transmitting it to the meta server; and f) decrypting the hash value of the PIN by the meta server; and g) the meta server decrypting the RefID and the Decrypting the FHV using the hash value of the PIN; And h) The meta server transmits the FHV to the storage server and requests whether a downloaded file mapped to the FHV exists; and i) the storage server analyzes the FHV. Notifying to the meta server and the client device that the mapped download file exists; And j) connecting a client device to the storage server to download a file mapped with the FHV; a method for removing duplicate files for privacy protection in a cloud environment including.

Here, in step g), the FHV may be generated through the following (Equation 2).

UserID)

(H(PIN)))^H(PIN) (Equation 2) D((RefID

UserID)

(H(PIN))) ^H(PIN)

The present invention has an effect of preventing user information from being leaked by an outsider attack because an encrypted parameter using an encryption key shared in advance between a client device and a cloud server (including a meta server and a storage server) is used.

In addition, the present invention physically separates the meta server, which is a cloud server, and the storage server, managed by different administrators, and prevents decryption of the information used by the other party using the information used in each server. It has the effect of preventing the leakage of user information.

FIG. 1 is a diagram illustrating checking whether a file to be uploaded to a cloud server is stored in duplicate according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating a mapping for removing redundant storage and protecting privacy after uploading a file to a cloud server according to an embodiment of the present invention.
3 is a diagram illustrating a process of downloading a file stored in a cloud server according to an embodiment of the present invention.

Advantages and features of the present invention, and a method of achieving them will become apparent with reference to embodiments to be described later in detail together with the accompanying drawings.

However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms.

In the present specification, the present embodiment is provided to complete the disclosure of the present invention, and to completely inform the scope of the invention to those of ordinary skill in the art to which the present invention pertains.

And the invention is only defined by the scope of the claims.

Accordingly, in some embodiments, well-known components, well-known operations, and well-known techniques have not been described in detail in order to avoid obscuring interpretation of the present invention.

In addition, throughout the specification, the same reference numerals refer to the same constituent elements, and terms used in the present specification (referred to) are for describing exemplary embodiments and not limiting the present invention.

In this specification, the singular form also includes the plural form unless specifically stated in the phrase, and the components and actions referred to as'include (or, have)' do not exclude the presence or addition of one or more other components and actions. .

Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used as meanings that can be commonly understood by those of ordinary skill in the art to which the present invention belongs.

In addition, terms defined in a commonly used dictionary are not interpreted ideally or excessively unless defined.

Terms used in the present invention will be defined as shown in [Table 1] below.

Abbreviations used in the present invention and their meaning Abbreviation meaning UserID User ID SID Session ID PK Pre-shared encryption PK key File Files being uploaded or downloaded Filehash Hash value of the file FHV Hash value of the hash value of the file (i.e., the double hash value of the file) FilePath Access file path of destination file (stored file) PIN User authentication value RefID Reference value stored in meta server H(·) ^K Hash result of a specific target (hash value) E(·) ^K Result value of performing encryption on a specific target using key K D(·) ^K The result value of decoding a specific target using key K

A file deduplication system for privacy protection in a cloud environment of the present invention includes a client device 100, a meta server 200, and a storage server 300.

Here, the meta server 200 and the storage server 300 refer to a cloud server, and the meta server 200 stores meta data, and the storage server 300 is an actual uploaded and downloaded user corresponding to the meta data. The data is saved.

Here, the client device 100 refers to any device that can be used by a client to access a cloud server, such as a mobile device including a smartphone, a laptop computer, and a PC.

In the present invention, the meta server 200 and the storage server 300 must be strictly separated. The meta server 200 and the storage server 300 cannot be configured in one server, but must be configured in different servers. That is, it is preferable that the meta server 200 and the storage server 300 are physically separated.

In addition, managers in charge of the meta server 200 and the storage server 300 must be configured separately.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a diagram illustrating checking whether a file to be uploaded to a cloud server is stored in duplicate according to an embodiment of the present invention.

In step S101, the meta server 200 generates a session ID (SID).

In step S102, the session ID generated by the meta server 200 is encrypted using a pre-shared encryption PK key and provided to the client device 200.

Step S103 is a step of decrypting the encrypted session ID received by the client device 200 using a pre-shared encrypted PK key.

In step S104, the client device 200 requests the client server to upload a specific file.

Here, the client device 200 may request a desired upload file path.

Step S105 is a step in which the meta server 200 generates an upload file path requested by the client device 200.

In step S106, the meta server 200 informs the client device 100 that the upload file path requested by the client device 100 has been generated.

Step S107 is a step of generating a hash value of the file to be uploaded by hashing the file to be uploaded by the client device 200.

In step S108, the client device 200 hashes the hash value of the file to be uploaded generated in step S107 once more to generate the FHV, which is the double hash value of the file to be uploaded.

In step S109, the client device 200 encrypts the FHV using the SID and transmits it to the meta server 200.

In step S110, the meta server 200 decrypts the FHV.

Here, since the meta server 200 knows the SID, the FHV encrypted using the SID may be decrypted using the SID.

In step S111, the meta server 200 transmits the FHV to the storage server 300.

Here, the meta server 200 requests whether a file corresponding to the FHV exists from the storage server 300 through the FHV.

In step S112, the storage server 300 searches whether a corresponding file is stored in the storage server 300 using FHV. In addition, the storage server 300 checks whether the corresponding file is completely stored and registered as a result of the search, only partially stored, or not stored.

In step S113, the storage server 300 checks the FHV, checks whether or not the file requested by the meta server 200 is stored, and informs the meta server 200 of the result.

Here, the storage server 300 depends on whether the file is in a storage state (E: Exist), a non-storage state (N: Not Exist) that is not stored, or a partial storage state (I: Incomplete) in which only a part is stored. Information on the status is transmitted to the meta server 200.

In step S114, the meta server 200 provides information on the storage state of the file received from the storage server 300 to the client device 100.

Here, information on the storage status of the file is shown in [Table 2].

Abbreviation for information on file storage status and their meaning Abbreviation meaning E File already exists on the storage server N File does not exist on the storage server I Part of the file is stored on the storage server

FIG. 2 is a diagram illustrating a mapping for removing redundant storage and protecting privacy after uploading a file to a cloud server according to an embodiment of the present invention.

Step S201 is a step of uploading an upload file to the storage server 300 when the file to be uploaded after step S114 in FIG. 1 is not completely stored in the storage server 300 by the client device 100.

Here, the upload file is uploaded to a corresponding file path using the upload file path generated in step S105 described above.

In step S202, the storage server 300 notifies the client device 100 that the upload file has been uploaded.

Step S203 is a step in which the user inputs the user authentication value (PIN) into the client device 100.

In step S204, the client device 100 generates a reference value RefID stored in the meta server based on the user authentication value PIN.

Here, the RefID is generated by the following [Equation 1].

(H(PIN)

FHV))^H(PIN) [Equation 1] RefID = E(H(UserID)

(H(PIN)

FHV)) ^H(PIN)

RefID is generated by encrypting the hash value of the PIN and the result of XORing the FHV and the hash value of the user ID with the hash value of the PIN.

는 XOR 연산처리를 수행하는 연산기호이다.here,

Is an operation symbol that performs XOR operation processing.

In operation S205, the storage server 300 extracts a hash value of a file uploaded from the client device 100 and stored in the storage server 300.

In step S206, the storage server 300 compares the FHV value received from the client device 100 through step S111 of FIG. 1 with the hash value of the uploaded and stored file to verify whether the stored file is a complete file.

In step S207, when the file stored as a result of the verification in step S206 is a complete file, the stored file and the FHV value are mapped.

In step S208, the client device 100 provides the RefID generated in step S204 to the meta server 200.

Step S209 is a step of storing the RefID received by the meta server 200.

In step S210, the meta server 200 maps and stores the RefID and the file path.

In step S211, the meta server 200 informs the client device 100 that information of a file for deduplication of an uploaded file and a file path are mapped.

That is, it notifies the client device 100 that the RefID and the file path have been mapped and that the stored file and the FHV value have been mapped.

Through the above process, the present invention can provide a method for removing duplicate files for privacy protection in a cloud environment.

That is, the present invention determines whether a file to be uploaded is stored in the storage server 300, and if the file to be uploaded is already stored, the same file is not duplicated and the FHV, which is the double hash value of the file, is transferred to the storage server 300. Save by mapping with the already saved file.

Accordingly, a user who uses the same file may use the same file already stored in the storage server 300 using only the FHV value of the file without needing to additionally store the same file in the storage server 300.

3 is a diagram illustrating a process of downloading a file stored in a cloud server according to an embodiment of the present invention.

In step S301, the meta server 200 generates an SID, which is a session ID.

Step S302 is a step of encrypting the SID generated by the meta server 200 using a pre-shared encryption PK key and providing the SID to the client device 200.

Step S303 is a step of decrypting the encrypted session ID received by the client device 200 using a pre-shared encrypted PK key.

In operation S304, the client device 200 requests the meta server 200 to download a specific file previously uploaded.

Here, since the client device 200 knows the file path of the uploaded file, the file path is notified to the meta server 200.

In step S305, the meta server 200 checks whether the download file requested by the client device 100 exists in the corresponding file path.

In step S306, when the download file requested by the client device 100 exists in the corresponding file path, the meta server 200 notifies the client device 100 of this.

In step S307, a user inputs a user authentication value (PIN) to the client device 100.

In step S308, the client device 100 encrypts the hash value H(PIN) of the user authentication value PIN using the SID and transmits it to the meta server 200.

Step S309 is a step of decoding the hash value H (PIN) of the user authentication value PIN received by the meta server 200.

Since the meta server 200 already knows the SID value, it can decrypt the hash value H(PIN) of the user authentication value PIN using this.

In step S310, the meta server 200 extracts the FHV value based on the RefID and H(PIN) value.

The meta server 200 extracts the FHV value using the following [Equation 2].

UserID)

(H(PIN)))^H(PIN) [Equation 2] FHV = D((RefID

UserID)

(H(PIN))) ^H(PIN)

Here, the FHV is generated by performing the XOR operation result value of the RefID and the user ID (UserID) with the hash value of the PIN and the XOR operation, and then encrypting it with the hash value of the PIN.

In step S311, the meta server 200 transmits the FHV of the file requested to be downloaded to the storage server 300.

In step S312, the storage server 300 checks whether the FHV corresponding to the download file requested by the client device 100 exists in the storage server 300.

In step S313, when the FHV corresponding to the download file requested by the client device 100 exists in the storage server 300, the storage server 300 notifies the meta server 200.

In step S314, the meta server 200 informs the client device 100 that it is in a download ready state.

In step S315, the client device 100 requests the storage server 300 to download a corresponding file.

Step S316 is a step in which the client device 100 downloads a download file from the storage server 300.

The privacy issue in the existing deduplication environment arises from the structure in which users and files are mapped. In practice, if the mapping structure between files and users is completely removed, the list of users who uploaded the file from a specific file, or vice versa, cannot be obtained. Accordingly, the present invention uses the above-described method in which the user-file list and the mapping relationship between the files are fundamentally blocked so that the user cannot be inferred only through metadata and file structure analysis on the server.

That is, in the method used in the present invention, as described above, a user can upload/download a file based on PIN, RefID, and FHV. Here, the PIN is a value that only the user knows, and the RefID is a value owned by the meta server 200 and is configured based on the PIN. In addition, the FHV value is a double hash value obtained by taking a hash one more time to the hash result value of the file, and the RefID cannot be inferred based on the FHV. Conversely, the FHV and PIN cannot be inferred based on the RefID.

In the present invention, RefID is stored in the meta server 200, and the FHV is stored in the storage server 300.

Here, in order to know the user ID and FHV by decrypting the RefID, the H(PIN) value, which is the hash value of the user authentication value (PIN), is required, but the H(PIN) value is the meta server 200 and the storage server ( 300).

Therefore, the meta server 200 administrator cannot decode the RefID value and thus cannot know the user ID and FHV.

The present invention having the above-described characteristics has the following advantages.

(1) Guaranteed privacy through complete separation of user-file information

The existing deduplication method secures security for the file itself by applying the file encryption technique, but exposes the user-file mapping structure as it is. Meta-information analysis is possible according to these problems, and it can lead to serious invasion of user privacy.

In the present invention, user and file information are completely separated. The RefID is stored in the meta server 200, and the FHV value is stored in the storage server 300.

RefID is a value that is XOR-processed and encrypted by a PIN, and an administrator of the meta server 200 cannot decrypt the encrypted value. An H(PIN) value is required for decryption, but this information is not stored in the meta server 200.

Therefore, the FHV cannot be estimated based on the RefID, and conversely, the RefID cannot be found based on the FHV. If the information is completely configured and the file is downloaded, a PIN value known to the user is required. In particular, RefID is encrypted with H(PIN) value, making analysis more difficult. In other words, if the PIN value is not known, the user's privacy is secured because the mapping relationship between user information and files cannot be reconstructed using information in the server.

(2) Resolving ownership issues

The existing deduplication technology generally handles the problem of file ownership based on the user mapping structure for files. However, this method is structurally problematic because there is a possibility of privacy invasion.

In the method proposed by the present invention, the PIN value serves as a clue to prove the user's ownership of a specific file. If an unauthorized person attempts to download a file, the PIN is not known, so the normal FHV cannot be extracted from RefID, and the file download procedure cannot be performed normally. However, in the case of a normal user, the FHV value can be extracted from RefID based on the PIN applied at the time of upload and downloaded. In this case, only the user who knows the PIN can extract the FHV of the file from the RefID, so the user can prove ownership of the file. That is, the confidential information is a PIN, and the problem of file ownership can be solved while ensuring privacy through the characteristic that the secret information is not stored in the server.

(3) Insider attack

The existing deduplication technology has a mapping structure between files and users as meta information. In this case, the administrator with the browsing authority is very vulnerable to insider attacks in terms of being able to collect meta information. That is, the cloud server administrator can easily identify an uploader list for a specific file or a list of all files of a specific user.

In the present invention, even assuming that the entire meta server 200 and the storage server 300 are exposed by an insider attack, the attacker extracts a user's file list (i.e., user information) or an upload user list for a specific file. Can not. The information stored in each server is RefID and FHV, and the values themselves do not have mutual connectivity. In particular, since the RefID value is encrypted with H (PIN) as a key, this makes meta-analysis more difficult. Therefore, even if the entire data is exposed by the insider, if the PIN information is not known, the relationship between the file and the user cannot be known, so that the insider attack problem can be solved.

(4) sniffing attack

In the present invention, parameters between a client and a server are encrypted and transmitted during file upload, file mapping, and file download.

This guarantees safety even if a hacker performs a sniffing attack. In particular, the SID, which is a one-time value generated by the meta server 200, is used as a key for encrypting parameters transmitted in the protocol process. Therefore, even if a hacker performs a replay attack based on sniffing, if the attacker does not know the PK shared in advance between the server and the client, the attacker cannot normally perform the upload and download protocol for the file.

(5) Integrity aspect

In the present invention, in order to ensure the integrity of the file, a verification procedure at the server side is performed once more. The FHV value is a double hash value, which is a disaster hash value for the hash value of the file, and the server side can check whether the uploaded file is integrity by checking whether the two hash result values for the uploaded file and the FHV value match. That is, the FHV value is characterized in that it is used to prove ownership of the file during the execution of the download protocol, and at the same time, it can perform the role of verifying the integrity of the uploaded file. In particular, there is also an advantage in that it is possible to detect whether a file stored in the storage server 300 has been altered.

(6) Efficiency aspect

In general, the deduplication method creates a file hash value based on Merkle-Tree. The file hash generation part is actually the part that takes the most time in the process of overlapping processing, and in the present invention, the RefID extraction time is added to the Merkle-Tree generation time during the file overlap check.

However, in the present invention, a security function is added to the existing deduplication technology, and the processing time is substantially added to the time required for security processing such as RefID operation compared to the existing deduplication technology. It does not show significant performance degradation in the removal method, and has the advantage of having privacy protection and security functions, so it has an advantage in terms of efficiency compared to additionally performing a separate security method to the existing deduplication method. have.

The present invention is not limited to the specific preferred embodiments described above, and any person having ordinary knowledge in the technical field to which the present invention pertains without departing from the gist of the present invention claimed in the claims can implement various modifications Of course, it is obvious that such a change will fall within the scope of the description of the claims.

100: client device
200: meta server
300: storage server

Claims (5)

Includes a client device that generates a RefID by receiving a PIN from a user and generates a RefID and generates an FHV by double hashing a file stored in the cloud, a meta server that receives and stores the RefID, and a storage server that receives and stores the FHV. In the file deduplication method for privacy protection in a cloud environment,
a) generating an SID by a meta server, encrypting the SID, and transmitting the SID to a client device;
b) decrypting the encrypted SID by a client device;
c) requesting, by a client device, a file path required for file upload to the meta server;
d) generating the file path by a meta server;
e) generating FHV by double hashing the uploaded file by the client device;
f) the client device encrypting the FHV with the SID and transmitting the encrypted data to the meta server;
g) decrypting the FHV encrypted with the SID by the meta server;
h) requesting whether the uploaded file is a registered file by transmitting the FHV to the storage server by the meta server; And
i) the storage server analyzes the FHV, checks whether or not the uploaded file is a registered file, and notifies the meta server and the client device;
File deduplication method for privacy protection in a cloud environment that includes.
The method according to claim 1,
j) if the upload file is a file that is not registered, the client device uploading the upload file to the storage;
k) generating a RefID by receiving a PIN from the user at the client device;
l) the storage server checks the integrity of the stored upload file;
m) mapping the stored file and FHV by the storage server;
n) the client device transmitting the RefID to the meta server; And
o) mapping the stored RefID and the file path by the meta server and notifying the client device that the file upload has been completed;
File deduplication method for privacy protection in a cloud environment, characterized in that it further comprises.
The method according to claim 2,
The RefID is a file deduplication method for privacy protection in a cloud environment, characterized in that generated through the following (Equation 1).
(Equation 1) E(H(UserID)

(H(PIN)

FHV)) ^H(PIN)
Includes a client device that generates a RefID by receiving a PIN from a user and generates a RefID and generates an FHV by double hashing a file stored in the cloud, a meta server that receives and stores the RefID, and a storage server that receives and stores the FHV. In the file deduplication method for privacy protection in a cloud environment,
a) generating an SID by a meta server, encrypting the SID, and transmitting the SID to a client device;
b) decrypting the encrypted SID by a client device;
c) requesting, by a client device, to download a file for a specific path to the meta server;
d) checking, by the meta server, whether the download file exists in the specific path;
e) receiving, by a client device, a PIN from a user, encrypting the hash value of the PIN with SID, and transmitting it to the meta server;
f) the meta server decrypting the hash value of the PIN through decryption;
g) decoding the FHV using the hash value of the RefID and the PIN by the meta server;
h) requesting whether a download file mapped to the FHV exists by transmitting the FHV to the storage server by the meta server;
i) the storage server notifying the meta server and the client device that a downloaded file mapped by analyzing the FHV exists; And
j) accessing the storage server by a client device to download a file mapped with the FHV;
File deduplication method for privacy protection in a cloud environment that includes.
The method of claim 4,
In the step g), the FHV is generated through the following (Equation 2). Method for removing duplicate files for privacy protection in a cloud environment, characterized in that.
(Equation 2) D((RefID

UserID)

(H(PIN))) ^H(PIN)

Images