KR20200136631A - Apparatus and method for recovering deleted message using cache file - Google Patents

Abstract

The present invention relates to a device and method for recovering a deleted message using cache files. A message recovery device includes: a message deletion detector which detects whether a message is deleted from an encrypted file which stores an encrypted text in the process of generating the encrypted text by encrypting a plain text corresponding to a message transmitted between users; a deletion message estimating unit for estimating a deletion message based on a timestamp in the cache file corresponding to the encrypted file when a message deletion is detected; a counter recovery unit for recovering a counter based on the plain text and encrypted text; and a deleted message recovery unit for recovering the deleted message based on the cache file using the counter and the symmetric key used for encryption.

Description

Device and method for recovering deleted messages using cache files {APPARATUS AND METHOD FOR RECOVERING DELETED MESSAGE USING CACHE FILE}

The present invention relates to a technology for recovering deleted messages using a cache file, and more particularly, an apparatus and method for recovering deleted messages using a cache file capable of recovering messages using cache data when messages exchanged between users are deleted. It is about.

Most mobile instant messengers share data by uploading/downloading data to a specific storage server when sending and receiving large data such as media data as messages. It is difficult to obtain data because the messages sent and received are stored in a database in an area that is not accessible by the user's authority in general, and even if they are acquired, it is very difficult to recover the messages since the messages arbitrarily deleted by the user may be overwritten with other data. I can.

However, when media data is exchanged using a mobile instant messenger, it may be automatically cached and stored in a specific area of the smartphone. In addition, since most of the mobile instant messengers send and receive encrypted conversation contents and media data for privacy protection, cached and stored data can also be encrypted. The data is cached data created in the process of communicating with the server, so even if the messenger user deletes the conversation, it is stored without being deleted.

In digital forensic investigation, recovering the contents of conversations deleted by users is one of the important research subjects. Therefore, if it is possible to recover some of the conversation contents deleted by the messenger by recovering the encrypted and cached data, it will be very useful in digital forensic investigation.

Korean Patent Registration No. 10-1036768 (2011.05.18)

An embodiment of the present invention is to provide an apparatus and method for recovering deleted messages using a cache file capable of recovering messages using cache data when messages exchanged between users are deleted.

An embodiment of the present invention is to provide an apparatus and method for recovering a deleted message using a cache file that can be used as evidence by recovering a message that was attempted to conceal by recovering deleted conversation contents in a digital forensic investigation.

An embodiment of the present invention is to recover deleted messages using a cache file that can recover a counter used for encryption by using a fixed value that is different according to the type of message and is fixed to a specific area of the transmitted data in the process of message transmission between users. It is intended to provide an apparatus and method.

Among embodiments, the device for recovering a deleted message using a cache file is a message deletion detection unit that detects whether to delete a message from an encrypted file storing the encrypted text in the process of generating an encrypted text by encrypting a plain text corresponding to a message transmitted between users. , When a message deletion is detected, a deletion message estimating unit for estimating a deletion message based on a timestamp in a cache file corresponding to the encrypted file, a counter recovery unit for recovering a counter based on the plaintext and the ciphertext, and And a deleted message recovery unit for recovering the deleted message based on the cache file by using the counter and the symmetric key used for encryption.

The message deletion detection unit may determine the deletion of the message when it is found in the encrypted file that the continuity of the message identifier (ID) identifying the message between the users is disconnected.

The message deletion detection unit may store a transmission/reception time of each of the preceding and following messages adjacent to the disconnection point of the continuity in association with the deletion of the corresponding message.

The deletion message estimator may compare the transmission/reception time and the timestamp to estimate a plurality of candidate cache data existing between the transmission/reception time as the deletion message.

The counter recovery unit includes determining a first block of the ciphertext, determining a second block of the plaintext corresponding to the first block, and recovering the counter based on the first and second blocks. Can be done.

The counter recovery unit may determine the first and second blocks by using a fixed value that is different according to a type of message and is fixed to a specific area of transmission data during a message transmission process between users.

The counter recovery unit may determine the sizes of the first and second blocks as 16 bytes, respectively.

The deleted message recovery unit may perform decryption on the estimated deleted message by applying the symmetric key to a decryption algorithm corresponding to an encryption algorithm.

The encryption algorithm may include AES-256-GCM, and the decryption algorithm may include AES-256-CTR mode.

Among the embodiments, the method of recovering a deleted message using a cache file includes detecting whether a message is deleted from an encrypted file storing the encrypted text in the process of generating an encrypted text by encrypting a plain text corresponding to a message transmitted between users. Step, when a message deletion is detected, estimating a deletion message based on a timestamp in a cache file corresponding to the encrypted file, restoring a counter based on the plaintext and the ciphertext, and the counter and the And recovering the deleted message based on the cache file by using the symmetric key used for encryption.

The disclosed technology can have the following effects. However, since it does not mean that a specific embodiment should include all of the following effects or only the following effects, it should not be understood that the scope of the rights of the disclosed technology is limited thereby.

The apparatus and method for recovering deleted messages using a cache file according to an embodiment of the present invention may recover deleted conversation contents in a digital forensic investigation, recover a message attempted to conceal, and use it as evidence.

An apparatus and method for recovering deleted messages using a cache file according to an embodiment of the present invention is a counter used for encryption using a fixed value that is different depending on the type of message and is fixed to a specific area of the transmitted data in a message transmission process between users. Can be restored.

1 is a diagram illustrating a configuration of a system for recovering a deleted message using a cache file according to the present invention.
FIG. 2 is a block diagram illustrating the physical configuration of the message recovery device of FIG. 1.
FIG. 3 is a block diagram illustrating a functional configuration of the message recovery device of FIG. 1.
4 is a flowchart illustrating a process of recovering a deleted message using a cache file performed by the message recovery apparatus of FIG. 1.
5 is an exemplary diagram illustrating an embodiment of fixed data according to media.
6 is a flowchart illustrating a process of recovering a deleted message using a cache file according to an embodiment of the present invention.

Since the description of the present invention is merely an embodiment for structural or functional description, the scope of the present invention should not be construed as being limited by the embodiments described in the text. That is, since the embodiments can be variously changed and have various forms, the scope of the present invention should be understood to include equivalents capable of realizing the technical idea. In addition, since the object or effect presented in the present invention does not mean that a specific embodiment should include all of them or only those effects, the scope of the present invention should not be understood as being limited thereto.

Meanwhile, the meaning of terms described in the present application should be understood as follows.

Terms such as "first" and "second" are used to distinguish one component from other components, and the scope of rights is not limited by these terms. For example, a first component may be referred to as a second component, and similarly, a second component may be referred to as a first component.

When a component is referred to as being "connected" to another component, it should be understood that although it may be directly connected to the other component, another component may exist in the middle. On the other hand, when it is mentioned that a certain component is "directly connected" to another component, it should be understood that no other component exists in the middle. On the other hand, other expressions describing the relationship between the constituent elements, that is, "between" and "just between" or "neighboring to" and "directly neighboring to" should be interpreted as well.

Singular expressions are to be understood as including plural expressions unless the context clearly indicates otherwise, and terms such as “comprise” or “have” refer to implemented features, numbers, steps, actions, components, parts, or It is to be understood that it is intended to designate that a combination exists and does not preclude the presence or addition of one or more other features or numbers, steps, actions, components, parts, or combinations thereof.

In each step, the identification code (for example, a, b, c, etc.) is used for convenience of explanation, and the identification code does not describe the order of each step, and each step has a specific sequence clearly in context. Unless otherwise stated, it may occur differently from the stated order. That is, each of the steps may occur in the same order as specified, may be performed substantially simultaneously, or may be performed in the reverse order.

The present invention can be embodied as computer-readable codes on a computer-readable recording medium, and the computer-readable recording medium includes all types of recording devices storing data that can be read by a computer system. . Examples of computer-readable recording media include ROM, RAM, CD-ROM, magnetic tape, floppy disk, and optical data storage devices. Further, the computer-readable recording medium is distributed over a computer system connected by a network, so that the computer-readable code can be stored and executed in a distributed manner.

All terms used herein have the same meaning as commonly understood by one of ordinary skill in the field to which the present invention belongs, unless otherwise defined. Terms defined in commonly used dictionaries should be construed as having meanings in the context of related technologies, and cannot be construed as having an ideal or excessive formal meaning unless explicitly defined in the present application.

1 is a diagram illustrating a configuration of a system for recovering a deleted message using a cache file according to the present invention.

Referring to FIG. 1, a deleted message recovery system 100 using a cache file may include a user terminal 110, a message recovery device 130, and a database 150.

The user terminal 110 may correspond to a computing device capable of sending and receiving messages with a counterpart through a messenger and checking the recovered message, and may be implemented as a smartphone, laptop, or computer, but is not limited thereto, It can be implemented in various devices such as a tablet PC. The user terminal 110 may be connected to the message recovery device 130 through a network, and a plurality of user terminals 110 may be connected to the message recovery device 130 at the same time.

The message recovery device 130 may be implemented as a server corresponding to a computer or program capable of recovering message files exchanged during message communication between user terminals 110 when deleted. Here, the message file may correspond to a large media file. The message recovery device 130 may be wirelessly connected to the user terminal 110 through Bluetooth, WiFi, a communication network, and the like, and may exchange data with the user terminal 110 through a network.

In one embodiment, the message recovery device 130 may store information necessary for recovering a deleted message in conjunction with the database 150. Meanwhile, unlike FIG. 1, the message recovery apparatus 130 may be implemented including the database 150 inside. Further, the message recovery device 130 may be implemented including a processor, a memory, a user input/output unit, and a network input/output unit, which will be described in more detail with reference to FIG. 2.

The database 150 may correspond to a storage device for storing various pieces of information necessary in a process of encrypting and storing messages exchanged between users and recovering deleted data based on the encrypted messages. The database 150 may store message information, personal information, and information used for encryption exchanged with the other party from the user terminal 110, and is not limited thereto, and the message recovery device 130 recovers deleted data. Information collected or processed in various forms during the process can be stored.

FIG. 2 is a block diagram illustrating the physical configuration of the message recovery device of FIG. 1.

Referring to FIG. 2, the message recovery device 130 may include a processor 210, a memory 230, a user input/output unit 250, and a network input/output unit 270.

The processor 210 can execute each procedure for processing each operation in the process of recovering the deleted data, and can manage the memory 230 that is read or written throughout the process, and the memory 230 Synchronization time between existing volatile and nonvolatile memory can be scheduled. The processor 210 can control the overall operation of the message recovery device 130, and is electrically connected to the memory 230, the user input/output unit 250, and the network input/output unit 270 to control data flow between them. I can. The processor 210 may be implemented as a CPU (Central Processing Unit) of the message recovery device 130.

The memory 230 is implemented as a nonvolatile memory such as a solid state drive (SSD) or a hard disk drive (HDD), and may include an auxiliary storage device used to store all data required for the message recovery device 130, A main memory device implemented as a volatile memory such as RAM (Random Access Memory) may be included.

The user input/output unit 250 may include an environment for receiving a user input and an environment for outputting specific information to a user. For example, the user input/output unit 250 may include an input device including an adapter such as a touch pad, a touch screen, an on-screen keyboard, or a pointing device, and an output device including an adapter such as a monitor or a touch screen. In one embodiment, the user input/output unit 250 may correspond to a computing device accessed through a remote connection, and in such a case, the message recovery device 130 may be performed as a server.

The network input/output unit 270 includes an environment for connecting to an external device or system through a network, and includes, for example, a local area network (LAN), a metropolitan area network (MAN), a wide area network (WAN), and a VAN ( Value Added Network) may include an adapter for communication.

FIG. 3 is a block diagram illustrating a functional configuration of the message recovery device of FIG. 1.

Referring to FIG. 3, the message recovery device 130 includes a message deletion detection unit 310, a deletion message estimation unit 330, a counter recovery unit 350, a deletion message recovery unit 370, and a control unit 390. I can.

The message deletion detection unit 310 may detect whether a message is deleted from an encrypted file storing the encrypted text in the process of generating an encrypted text by encrypting the plain text corresponding to a message transmitted between users. The SureSpot application, a mobile instant messenger, can send and receive messages including media data by encrypting them with AES-256-GCM. In addition, the SureSpot application can store the IV in the message.sss file, and when deleting the conversation, the IV can also be deleted. Here, message.sss is a file that stores encrypted data and may mainly store messages and media data exchanged between users.

When sending and receiving media files, the SureSpot application encrypts data and transmits it to the server, and transmits the URL containing the data to the other party. When you delete a message in the SureSpot application, all data in message.sss is deleted, but the cache file may not be deleted. The message deletion detection unit 310 may perform an operation of detecting whether a message is deleted from the encrypted file generated by the SureSopt application.

In an embodiment, the message deletion detection unit 310 may determine the deletion of the message when disconnection of continuity with respect to a message identifier (ID) identifying a message between users is found in the encrypted file. For example, a unique ID exists as a message identifier in Message.sss, which may be assigned a value that starts from 1 and increases by 1, and may not decrease even when a message is deleted. The message deletion detection unit 310 may determine that the message has been deleted if there is an ID missing in the middle by examining the message identifier in the encrypted file. That is, the omission of ID may mean discontinuity of continuity.

In an embodiment, the message deletion detection unit 310 may store transmission/reception times of each of the messages before and after adjacent to the disconnection point of continuity in association with deletion of the corresponding message. For example, when the message ID 50 is omitted, a corresponding point corresponds to a disconnection point of continuity, and a message having IDs 49 and 51 may be selected as a before/after message. The message deletion detection unit 310 may determine a message before/after a point where the continuity of the message identifier is disconnected, and may extract and store the transmission/reception time of each of the corresponding messages.

When the deletion of the message is detected, the deletion message estimating unit 330 may estimate the deletion message based on a timestamp in a cache file corresponding to the encrypted file. In the SureSpot application, when a media file is deleted from within the application, the cache file may remain without being deleted. The deletion message estimating unit 330 may search the database 150 to determine a cache file corresponding to the encrypted file, and estimate the deleted location using a timestamp recorded in the corresponding cache file to determine the cache data stored in the corresponding location. Can be estimated as a deletion message.

In one embodiment, the deletion message estimating unit 330 may estimate a plurality of candidate cache data existing between the transmission/reception time as a deletion message by comparing the transmission/reception time and the timestamp. That is, the deletion message estimating unit 330 matches the transmission/reception time of the message before/after the point where the message was deleted by the message deletion detection unit 310 with the timestamp of the cache file, and exists at a position estimated as the deleted point. The cached data can be estimated as deleted message candidates. There may be a plurality of cache data estimated as a deletion message candidate, and whether it corresponds to an actual deleted message may be determined through an operation in a later step.

The counter recovery unit 350 may recover a counter based on plain text and cipher text. Since the SureSpot application deletes the IVs when deleting the conversation contents, the IV restoration needs to be performed first in order to recover the deleted messages. The SureSpot application can use AES-256-GCM to encrypt data, and AES-256-GCM is a mode in which the encryption module combines the CTR mode and the authentication module, and the actual encryption process can be the same as the CTR mode.

Accordingly, the counter recovery unit 350 may correspond to IV recovery by recovering the counter in the CTR mode, and may be expressed as Equation 1 below.

[Equation 1]

That is, the counter recovery unit 350 may use a plaintext and a corresponding ciphertext to recover the counter.

In an embodiment, the counter recovery unit 350 determines a first block of ciphertext, determining a second block of plaintext corresponding to the first block, and recovers the counter based on the first and second blocks. You can perform the steps. For counter recovery, one block of plaintext corresponding to one block of ciphertext may be required, and the plaintext corresponding to the ciphertext can be determined by using the fact that a specific area of the file header is fixed.

More specifically, the counter recovery unit 350 may determine a specific area of the ciphertext to be used for counter recovery as the first block, and when the first block of the ciphertext is determined, determine a second block of the plaintext corresponding to the first block. I can. In this case, fixed data according to the media type can be used for matching between blocks. The counter recovery unit 350 may recover the counter using Equation 1 on the basis of the cipher text first block and the corresponding plain text second block.

In an embodiment, the counter recovery unit 350 may determine the first and second blocks by using a fixed value that is different according to the type of message and is fixed to a specific area of the transmission data during a message transmission process between users. In general, when large data is transmitted from a communication device such as a mobile messenger, data can be compressed and transmitted in an appropriate size and quality to save data and improve transmission speed. Accordingly, a specific area of the file header may be fixed or changed to a predictable value, and the counter recovery unit 350 may obtain data of about 1 block of plaintext corresponding to 1 block of ciphertext by using this.

For example, in the case of the SureSpot application, since media is compressed and transmitted in a specific format when transmitting, a fixed value may exist. Referring to FIG. 5, an embodiment of fixed data according to media can be confirmed. In FIG. 5, when mimeType is Image, fixed data may be obtained from Bytes Offset 0x00 to 0xA3, and if mimeType is audio/mp4, fixed data may be obtained from Bytes Offset 0x00 to 0x1A. In addition, fixed data when mimeType is Image may have a larger data size than fixed data when audio/mp4 is used.

In an embodiment, the counter recovery unit 350 may determine the sizes of the first and second blocks as 16 bytes, respectively. That is, the size of one block of cipher text used for counter recovery and one block of plain text corresponding thereto may correspond to 16 bytes.

The deleted message recovery unit 370 may recover the deleted message based on the cache file using a counter and a symmetric key used for encryption. The deleted message recovery process may be performed in the reverse order of the message encryption process. That is, the deletion message recovery unit 370 may decrypt the cache data estimated as the deletion message to obtain the deleted original data, and in the decryption process, the counter recovered by the counter recovery unit 350 and the symmetrical used for encryption The key can be used.

In an embodiment, the deletion message recovery unit 370 may perform decryption on the estimated deleted message by applying the symmetric key to a decryption algorithm corresponding to the encryption algorithm. In this case, the decryption algorithm may proceed in the reverse order of the encryption algorithm, and the estimated deletion message may correspond to candidate cache data existing in the cache file. Also, the symmetric key used for encryption may be the same as the key obtained from secrets.sss in the case of the SureSpot application.

The SureSpot application, a mobile instant messenger, can encrypt and store all of the user's sensitive data. At this time, there is no data required separately, and all data can be encrypted based on the login password registered when creating the ID. Encrypted data can be classified into three broad categories, and for example, data in Username.ssi, Secrets.sss, and Message.sss can be included. Each data can store a user-only private key and public key, a symmetric key for encrypting conversations, and messages and media data exchanged through chat.

The message recovery device 130 may be implemented by including an independent module that performs an operation of decrypting an encrypted file storing the symmetric key in order to obtain a symmetric key for use in message recovery. Meanwhile, the message recovery device 130 may obtain a corresponding symmetric key from a decryption device that decrypts a file to which end-to-end encryption has been applied, and for this purpose, it may be combined and implemented to interwork with the decryption device.

In one embodiment, the encryption algorithm may include AES-256-GCM, and the decryption algorithm may include AES-256-CTR mode. AES-265-GCM is a mode in which the encryption module CTR mode and the authentication module are integrated into one, and the encryption process can be the same as the CTR mode. The deleted message recovery unit 370 may recover the deleted message by decrypting using the AES-256-CTR mode based on the encrypted file and the cache file generated in the process of encrypting using the AES-256-GCM.

The control unit 390 controls the overall operation of the message recovery device 130, and a control flow between the message deletion detection unit 310, the deletion message estimation unit 330, the counter recovery unit 350, and the deleted message recovery unit 370 Or you can manage the data flow.

4 is a flowchart illustrating a process of recovering a deleted message using a cache file performed by the message recovery apparatus of FIG. 1.

4, the message recovery device 130 encrypts the plain text corresponding to the message transmitted between users through the message deletion detection unit 310 to determine whether to delete the message from the encrypted file storing the encrypted text in the process of generating the encrypted text. Can be detected (step S410). When a message deletion is detected by the message deletion detection unit 310, the message recovery apparatus 130 may estimate the deletion message based on a timestamp in the cache file corresponding to the encrypted file through the deletion message estimating unit 330. (Step S430).

In addition, the message recovery device 130 may recover a counter based on the plain text and the encrypted text through the counter recovery unit 350 (step S450). The message recovery device 130 may recover the deleted message based on the cache file using a counter and a symmetric key used for encryption through the deleted message recovery unit 370 (step S470).

6 is a flowchart illustrating a process of recovering a deleted message using a cache file according to an embodiment of the present invention.

Referring to FIG. 6, the message recovery device 130 may check whether a deleted message exists in the encrypted file Message.sss (conversation content DB). In general, when a message is deleted, the sequence numbers of the messages are not continuous. If the corresponding part exists, it can be assumed that the message has been deleted. The message recovery device 130 may measure and store a message transmission/reception time existing before and after the estimated message.

Further, the message recovery device 130 may check a timestamp based on the stored transmission/reception time and find cache data existing therebetween. Corresponding cache data may be candidates for deleted messages. Thereafter, the message recovery device 130 may recover the IV based on the previously acquired fixed data, and may recover the deleted conversation contents by decrypting the encrypted message.

Although the above has been described with reference to preferred embodiments of the present invention, those skilled in the art will variously modify and change the present invention within the scope not departing from the spirit and scope of the present invention described in the following claims. You will understand that you can do it.

100: Deleted message recovery device system using cache files
110: user terminal 130: message recovery device
150: database
210: processor 230: memory
250: user input/output unit 270: network input/output unit
310: message deletion detection unit 330: deletion message estimation unit
350: counter recovery unit 370: deleted message recovery unit
390: control unit

Claims (10)

A message deletion detector configured to detect whether a message is deleted from an encrypted file storing the encrypted text in the process of generating an encrypted text by encrypting a plain text corresponding to a message transmitted between users;
A deletion message estimating unit for estimating a deletion message based on a timestamp in a cache file corresponding to the encrypted file when a message deletion is detected;
A counter recovery unit for recovering a counter based on the plain text and the cipher text; And
A deleted message recovery apparatus using a cache file including a deleted message recovery unit for recovering the deleted message based on the cache file using the counter and the symmetric key used for encryption.
The method of claim 1, wherein the message deletion detection unit
And determining deletion of the message when a discontinuity in continuity with respect to a message identifier (ID) identifying a message between users is found in the encrypted file.
The method of claim 2, wherein the message deletion detection unit
A deleted message recovery apparatus using a cache file, characterized in that for storing transmission/reception times of each of the preceding/post messages adjacent to the disconnection point of the continuity in association with deletion of the corresponding message.
The method of claim 3, wherein the deletion message estimation unit
And comparing the transmission/reception time with the timestamp and estimating a plurality of candidate cache data existing between the transmission/reception time as the deletion message.
The method of claim 1, wherein the counter recovery unit
Determining a first block of the ciphertext;
Determining a second block of the plaintext corresponding to the first block; And
A deleted message recovery device using a cache file, characterized in that performing the step of recovering the counter based on the first and second blocks.
The method of claim 5, wherein the counter recovery unit
A device for recovering deleted messages using a cache file, wherein the first and second blocks are determined using a fixed value that is different according to a message type and fixed to a specific area of transmission data during a message transmission process between users.
The method of claim 5, wherein the counter recovery unit
A device for recovering a deleted message using a cache file, characterized in that the sizes of the first and second blocks are each determined to be 16 bytes.
The method of claim 1, wherein the deletion message recovery unit
And decrypting the estimated deleted message by applying the symmetric key to a decryption algorithm corresponding to an encryption algorithm.
The method of claim 8,
The encryption algorithm includes AES-256-GCM, and the decryption algorithm includes AES-256-CTR mode.
In the recovery method performed in a deleted message recovery device using a cache file,
Detecting whether a message is deleted from an encrypted file storing the encrypted text in the process of generating an encrypted text by encrypting a plain text corresponding to a message transmitted between users;
Estimating a deletion message based on a timestamp in a cache file corresponding to the encrypted file when a message deletion is detected;
Recovering a counter based on the plaintext and the ciphertext; And
And recovering the deleted message based on the cache file by using the counter and the symmetric key used for encryption.

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