Patent Publication Number: US-10791124-B2

Title: Method and terminal device for encrypting message

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2016-0150565, filed on Nov. 11, 2016 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Field 
     Methods and Apparatuses consistent with one or more exemplary embodiments relate to a method and device for securing data of a message, and more particularly, to methods for encrypting and decrypting a message on a user terminal itself independently of an application. 
     Description of the Related Art 
     As portable terminal devices such as smartphones, tablet PCs, and the like are becoming widespread, users of the portable terminal devices may increasingly use various applications. In particular, users may chat with a third party through various chatting applications instead of using text messages such as existing Short Message Service (SMS)/Multimedia Messaging Service (MMS). 
     However, chatting through such chatting applications is vulnerable in terms of security. 
     Therefore, companies running chatting applications are making efforts to maintain security through message encryption. However, users select a chatting application based on ease of use, design, awareness, and the like while understanding that security is an important consideration. 
     Further, the security of the chatting applications is managed by a private company or a business operator that runs the corresponding application. Therefore, there is a possibility that the corresponding private company or the business operator may abuse the security of the application. In a case in which the security of the application is hacked, it is very likely that important information such as personal information will be leaked or exposed. 
     Therefore, regardless of the type of chatting application, it is important to develop a technology that allows the user to use the chatting application with peace of mind. As a result, there is a need for a technology capable of performing an encryption independently of the chatting applications. 
     Various technologies to solve the above-mentioned problems have been proposed in a related art. One technology for encrypting a message is to provide an application programming interface (API) at a platform stage. Here, a platform may be a software platform such as ANDROID and TIZEN. 
     However, the method of providing the API at the platform stage has disadvantages in that encryption may not be performed when the corresponding API is not used and encryption may not be performed independently of the application. 
     Meanwhile, in order to perform the encryption in the chatting application, a chatting party must be determined. A related art technology finds the chatting party by performing an optical character recognition (OCR) reading after screen capturing a chatting window of the chatting application. 
     However, the method for performing the OCR reading has problems in that it does not suggest a solution when the chatting party is a person with the same name, it has a possibility that an error will occur during the OCR reading, and the chatting party is not recognized in the case of a group chatting. 
     SUMMARY 
     One or more exemplary embodiments may overcome the above disadvantages and other disadvantages not described above. Also, an exemplary embodiment is not required to overcome the disadvantages described above, and an exemplary embodiment may not overcome any of the problems described above. 
     Aspects of one or more exemplary embodiments provide a method and device for accurately encrypting a chatting message independently of a server without a separate operation of a user even in a case in which a chatting application is changed. 
     Aspects of one or more exemplary embodiments also provide a control method and device for encrypting a message independently of a chatting application regardless of the type of chatting application. 
     According to an aspect of an exemplary embodiment, there is provided a method for encrypting a message of a user terminal device, the method including: receiving a first message via a displayed message input window; displaying the received first message; encrypting the first message using a key index and an encryption key corresponding to a chatting window for the first message based on an instruction for transmitting the first message to another chatting party being received; and transmitting the encrypted first message to the other chatting party. 
     The method may further include: receiving a second message encrypted using the key index and the encryption key from the other chatting party; decrypting the encrypted second message by using the key index and the encryption key based on the encrypted second message being received from the other chatting party; and displaying the decrypted second message. 
     The method may further include determining whether a chatting history for the chatting window exists, wherein the encrypting the first message may include encrypting the first message by adding the key index and the encryption key previously determined for the chatting window to the first message, based on determining that the chatting history exists. 
     The encrypting the first message may further include encrypting the first message by generating the key index and the encryption key corresponding to the chatting window and adding the generated key index and the generated encryption key to the first message, based on determining that the chatting history does not exist. 
     The encrypting the first message may further include exchanging a session key between at least two terminal devices and generating the key index corresponding to the session key based on determining that the chatting history does not exist. 
     The session key may be generated by a Diffie-Hellman key agreement protocol. 
     The encrypted first message may be a stream cipher. 
     According to an aspect of another exemplary embodiment, there is provided a user terminal device encrypting and decrypting a message, the user terminal device including: an input configured to receive an input of a first message; a communicator configured to perform communication with an external terminal; a display configured to display the input first message; and a processor configured to encrypt the first message using a key index and an encryption key corresponding to a chatting window fir the first message based on an instruction for transmitting the first message to another chatting party being received, and to control the communicator to transmit the encrypted first message to the other chatting party. 
     The communicator may be configured to receive an encrypted second message from the other chatting party and to provide the encrypted second message to the processor, and the processor may be configured to decrypt the encrypted second message by using the key index and the encryption key based on the encrypted second message being received from the communicator. 
     The processor may be configured to determine whether a chatting history for the chatting window exists, and to encrypt the first message by adding the key index and the encryption key previously determined for the chatting window to the first message based on determining that the chatting history exists. 
     The processor may be configured to encrypt the first message by generating the key index and the encryption key corresponding to the chatting window and adding the generated key index and the generated encryption key to the first message, based on determining that the chatting history does not exist. 
     The processor may be configured to exchange a session key between at least two terminal devices and to generate the key index corresponding to the session key, based on determining that the chatting history does not exist. 
     The session key may be generated by a Diffie-Hellman key agreement protocol. 
     The encrypted first message may be a stream cipher. 
     According to an aspect of another exemplary embodiment, there is provided a non-transitory computer readable recording medium including a program executing a method for encrypting and decrypting a message of a user terminal device, wherein the method includes: receiving a first message via a displayed message input window; displaying the received first message; encrypting the first message using a key index and an encryption key corresponding to a chatting window for the first message based on an instruction for transmitting the first message to another chatting party being received; and transmitting the encrypted first message to the other chatting party. 
     As described above, according to aspects of one or more exemplary embodiments, when a transmission terminal device and a reception terminal device perform chatting through a chatting application, a chatting message may be encrypted and decrypted independently of a server without a separate operation of a user even in the case in which the chatting application is changed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The above and/or other aspects will be more apparent by describing certain exemplary embodiments with reference to the accompanying drawings, in which: 
         FIG. 1  is a system diagram illustrating a transmission and reception system according to an exemplary embodiment; 
         FIG. 2  is a block diagram illustrating a configuration of a terminal device according to an exemplary embodiment; 
         FIG. 3  is a block diagram illustrating a detailed configuration of a terminal device according to an exemplary embodiment; 
         FIG. 4  is an illustrative diagram illustrating an execution screen of a chatting application according to an exemplary embodiment; 
         FIG. 5  is a flowchart illustrating a process of transmitting a message in a message transmission and reception system according to an exemplary embodiment; 
         FIG. 6  is a flowchart illustrating an encryption process according to an exemplary embodiment; 
         FIG. 7  is a flowchart illustrating a decrypting process according to an exemplary embodiment; and 
         FIG. 8  is a flowchart illustrating a process of encrypting a message of a transmission terminal device according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. In the drawings, parts irrelevant or insignificant to the description of exemplary embodiments are not illustrated and like reference numerals denote like elements. 
     Although the terms used in the following description are selected, as much as possible, from general terms that are widely used at present while taking into consideration the functions obtained in accordance with exemplary embodiments, these terms may be replaced with other terms based on intentions of those skilled in the art, customs, emergence of new technologies, or the like. Accordingly, it should be noted that the terms used herein should be construed based on practical meanings thereof and the whole content of this specification, rather than being simply construed based on names of the terms. 
     Exemplary embodiments may be diversely modified. Accordingly, specific exemplary embodiments are illustrated in the drawings and are described in detail in the detailed description. However, it is to be understood that the present disclosure is not limited to a specific exemplary embodiment, but includes all modifications, equivalents, combinations between disclosed exemplary embodiments, and substitutions without departing from the scope and spirit of the present disclosure. In describing exemplary embodiments, when it is determined that a specific description of known technologies would obscure the gist of the present disclosure, a detailed description thereof will be omitted. 
     Terms such as first, second, etc., can be used to describe various components, but the components should not be limited to the terms. The terms are only used to distinguish one component from the others. 
     As used herein, the singular forms are intended to include plural forms as well, unless the context clearly indicates otherwise. In the present application, the terms “include” or “consist of” intend to designate the presence of features, numbers, steps, operations, components, elements, or a combination thereof that are written in the specification, but do not exclude the presence or possibility of addition of one or more other features, numbers, steps, operations, components, elements, or a combination thereof. 
     In an exemplary embodiment, a “module” or a “unit” performs at least one function or operation, and may be implemented in hardware, software, or a combination of hardware and software. In addition, a plurality of “modules” or a plurality of “units” may be integrated into at least one module and may be implemented in at least one processor, except for a “module” or a “unit” that must be implemented in specific hardware. 
     In an exemplary embodiment, a case in which any one part is “connected” with another part includes a case in which the parts are “directly connected” with each other and a case in which the parts are “electrically connected” or “indirectly connected” with each other with other elements interposed therebetween. In addition, a case in which any one part is “connected” with another part includes a physical connection as well as a wireless connection. In addition, unless explicitly described to the contrary, “comprising” any components will be understood to imply the inclusion of other elements rather than the exclusion of any other elements. 
     The terminology used herein is for the purpose of describing particular exemplary embodiments and is not intended to limit the exemplary embodiments. It is understood that expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. Additionally, as used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
       FIG. 1  is a system diagram illustrating a transmission and reception system according to an exemplary embodiment. 
     A transmission and reception system according to an exemplary embodiment may include a transmission terminal device  100 , a reception terminal device  200 , and an external server  300 . 
     The transmission terminal device  100  and the reception terminal device  200  each transmit and receive a message through a chatting application. However, the transmission terminal device  100  may also receive a message and the reception terminal device  200  may also transmit a message. An operation of the terminal devices will be described in detail with reference to  FIG. 2 . 
     The external server  300  serves to transport the messages transmitted from the respective terminal devices  100  and  200 . Further, the external server  300  may exchange a session key used when the transmission terminal device  100  and the reception terminal device  200  transmit and receive a message. 
     A session refers to a period from when a chatting participant establishes a channel for chatting to transmit and receive information to when the chatting participant terminates the channel in which the information is transmitted and received. The session key may be used in only one established session and is removed from the external server  300  when the session is terminated. The other chatting party may encrypt and decrypt a message through the session key. Here, the session key may not be able to derive an encryption key of the participant of the chatting and may not be able to be derived from a previous session key. 
     Meanwhile, a protocol for establishing the session key may be referred to as a session key establishment protocol, which is classified into a key agreement protocol and a key transport protocol. The key agreement protocol is a scheme in which two chatting participants generate and exchange the respective secrete values to generate a share key. The key transport protocol is a scheme in which one participant of the chatting participants arbitrarily generates the share key to transport it to another participant. 
     An exemplary embodiment uses a scheme of exchanging the session key through the key agreement protocol, but the session key may also be exchanged by using the key transport protocol in another exemplary embodiment. 
     In a case in which the exchange of the session key is completed, the transmission terminal device  100  and the reception terminal device  200  may generate a key index and an encryption key by using the session key, and may store the generated key index and encryption key. Here, the key index may be used to determine the other party of chatting and the encryption key may be used to encrypt or decrypt messages. 
     Specifically, the key index is a value for finding, obtaining, or determining the encryption key. The key index may also be published to other users, other than chatting participants. However, the other users may not be able to determine the encryption key from the key index. Further, a chatting participant may be able to uniquely determine a value of the encryption key using the key index. 
     Meanwhile, as described above, the session key is deleted when the session between the transmission terminal device  100  and the reception terminal device  200  is terminated. However, the key index is not deleted even in the case in which the session between the transmission terminal device  100  and the reception terminal device  200  is terminated. That is, the key index is a value stored inside the transmission terminal device  100  and the reception terminal device  200 . Therefore, in a case in which the transmission terminal device  100  and the reception terminal device  200  additionally transmit and receive the message after the termination of the session, the key index and the encryption key that are previously stored may be used. 
     After the key index and the encryption key are generated and stored, the user may input the message into a text input window  420  of the chatting application. Here, the input message may be encrypted by the key index and the encryption key and may be transmitted to the reception terminal device  200  through the external server  300 . 
     Further, the transmission terminal device  100  and the reception terminal device  200  may decrypt the encrypted message before displaying the message. That is, the transmission terminal device  100  and the reception terminal device  200  may encrypt and decrypt the message independently of the external server  300 . 
       FIG. 2  is a block diagram illustrating a configuration of a terminal device according to an exemplary embodiment. 
     The terminal device  100  of  FIG. 2  may include an input  110 , a communicator  120 , a display  130 , and a processor  140 . However, other components may also be added or included. 
     The terminal device  100  according to an exemplary embodiment may be a smartphone. However, the terminal device  100  is not limited thereto, but may also be any other terminal device capable of transmitting the message through the chatting application. For example, the terminal device  100  according to an exemplary embodiment may be a tablet personal computer (PC), a mobile phone, a desktop PC, a laptop PC, a netbook computer, a workstation, a personal digital assistant (PDA), and a portable multimedia player (PMP) other than the smartphone, etc. 
     Meanwhile, the terminal device  100  may include either of the transmission terminal device  100  and the reception terminal device  200 . However, the present description will be with reference to the transmission terminal device  100  for convenience, except for in certain particular cases. 
     The input  110  receives an input of a user. The input  110  according to an exemplary embodiment may include a touch input. However, the input  110  is not limited to such a component, but may include various forms such as a keyboard, a microphone, and the like. In the present specification, the input  110  is described as being limited to the touch input, but is not limited thereto in one or more other exemplary embodiments. 
     The message input by the input  110  may be a text message. However, the message input by the input  110  is not limited to the text message, but may also include various forms such as photograph files, voice files, video files, and the like in one or more other exemplary embodiments. 
     The communicator  120  transmits the message encrypted by the processor  140  to the external server  300 . The encrypted message is transmitted to the reception terminal device  200  through the external server  300 . Meanwhile, although an exemplary embodiment described above describes the case in which the transmission terminal device  100  and the reception terminal device  200  transmit and receive the message through the external server  300 , this case is merely one illustrative example and the message may be directly transmitted and received without using the transmission terminal device  100  and the reception terminal device  200 . Further, the communicator  120  may transmit the session key for encrypting and decrypting the message during the session period to the reception terminal device  200 . 
     The display  130  may output image data. In particular, the display  130  may display an execution screen of the chatting application. As illustrated in  FIG. 4 , the execution screen  400  of the chatting application may include a chatting window  410  displaying the message and a message input window  420  through which the user inputs a variety of information (e.g., photographs, videos, and the like). 
     In particular, the display  130  displays the message received from the reception terminal device  200  on the execution screen  400  of the chatting application. Specifically, when an encrypted message of the other chatting party is transmitted to the processor  140  through the communicator  120 , the processor  140  may control the display  130  to decrypt the encrypted message and to display the decrypted message. 
     The processor  140  controls an overall operation of the transmission terminal device  100 . In particular, when a user instruction for transmitting the message input to the message input window  420  through the input  110  to the other party of chatting is input, the processor  140  may encrypt the message using the key index and the encryption key corresponding to the chatting window  410  of the message, and may control the communicator  120  to transmit the encrypted message to the reception terminal device  200 . 
     Here, the processor  140  may control the communicator  120  to receive the encrypted message from the reception terminal device  200 . In addition, the processor  140  may decrypt the encrypted message by using the key index and the encryption key. 
     In particular, the processor  140  may determine whether a chatting history of the chatting window exists. In a case in which the chatting history exists, the processor  140  may add the key index and the encryption key pre-existing in the chatting window to the message to encrypt it. 
     In a case in which the chatting history does not exist, the processor  140  may generate the key index and the encryption key corresponding to the chatting window and may encrypt the message by using the generated key index and encryption key. 
     The processor  140  may exchange the session key with the reception terminal device to generate the key index and the encryption key. Here, the session key may be generated by a Diffie-Hellman key agreement protocol. 
     The processor  140  may be implemented in various schemes. For example, the processor  140  may be implemented in at least one of an application specific integrated circuit (ASIC), an embedded processor, a microprocessor, a hardware control logic, a hardware finite state machine (FSM), and a digital signal processor (DSP). 
       FIG. 3  is a block diagram illustrating a detailed configuration of a terminal device according to an exemplary embodiment. 
     Referring to  FIG. 3 , the transmission terminal device  100  may further include a memory  150  and a sensor  160  other than the input  110 , the communicator  120 , the display  130 , and the processor  140 . However, the transmission terminal device  100  is not limited to the above-mentioned configuration, but may be added with additional functions and additional components for executing a function thereof in one or more other exemplary embodiments. 
     The communicator  120  may include at least one of a Wi-Fi chip  121 , a Bluetooth chip  122 , a wireless communication chip  123 , and a near field communications (NFC) chip  124 . In particular, each of the Wi-Fi chip  121  and the Bluetooth chip  122  may perform communication in a Wi-Fi scheme and a Bluetooth scheme, respectively. In a case in which the Wi-Fi chip  121  or the Bluetooth chip  122  is used, the communicator  120  may first transmit and receive a variety of access information such as a service set identifier (SSID), a session key, and the like, connect communication using the variety of information, and then transmit and receive a variety of information. The wireless communication chip  123  may be a chip that performs communication according to various communication standards such as IEEE, ZigBee, 3rd generation (3G), 3rd generation partnership project (3GPP), Long Term Evolution (LTE), and the like. The NFC chip  124  may be a chip that is operated in an NFC scheme that uses a frequency band of 13.56 MHz among various RF-ID frequency bands such as 135 kHz, 13.56 MHz, 433 MHz, 860 to 960 MHz, 2.45 GHz, and the like. 
     The processor  140  may include a key management module  141 , an encryption module  142 , and a decryption module  143 . 
     The key management module  141  may manage the session key, the key index, and the encryption key for performing the encryption and the decryption. Specifically, in a case in which the chatting history with the other party of chatting exists, the key management module  141  may receive the key index and the encryption key that are previously stored from the memory  150  and transmit the key index and the encryption key to the encryption module  142  and the decryption module  143 . On other hand, in a case in which the chatting history does not exist, the key management module  141  may newly generate a key index and an encryption key and transmit the generated key index and encryption key to the encryption module  142 , the decryption module  143 , and the memory  150 . 
     As described above, the key management module  141  may generate the session key by using the Diffie-Hellman key agreement protocol. 
     The Diffie-Hellman key agreement protocol is one of representative methods for exchanging the session key. Specifically, the Diffie-Hellman key agreement protocol is an encryption algorithm using a discrete logarithm difficulty. 
     Specifically, when it is assumed that the encryption key of the transmission terminal device  100  is Xa and the encryption key of the reception terminal device  200  is Xb, the transmission terminal device  100  and the reception terminal device  200  may obtain a public key Ya of the transmission terminal device  100  and a public key Yb of the reception terminal device  200  by using Mathematical Expression 1:
 
 Ya=g{circumflex over ( )}Xa  mod  P  
 
 Yb=g{circumflex over ( )}Xb  mod  P   [Mathematical Expression 1]
 
     Here, g and P, which are Diffie-Hellman parameters, correspond to arbitrary values agreed between the transmission terminal device  100  and the reception terminal device  200 . 
     The transmission terminal device  100  and the reception terminal device  200  may exchange the respective public keys Ya and Yb and then share the session key through Mathematical Expression 2:
 
 SK=Yb{circumflex over ( )}Xa  mod  P =( g{circumflex over ( )}Xb ){circumflex over ( )} Xa  mod  P=g {circumflex over ( )}( XaXb ) mod  P  
 
 SK=Ya{circumflex over ( )}Xb  mod  P =( g{circumflex over ( )}Xa ){circumflex over ( )} Xb  mod  P=g {circumflex over ( )}( XaXb ) mod  P   [Mathematical Expression 2]
 
     That is, the transmission terminal device  100  generates the same session key as the session key of the reception terminal device  200  by using a private key Xa of the transmission terminal device  100  and the share key Yb transmitted by the reception terminal device  200 , and the reception terminal device  200  generates the same session key as the session key of the transmission terminal device  100  by using a private key Xb of the reception terminal device  200  and the share key Ya transmitted by the transmission terminal device  100 . 
     In this case, the private keys Xa and Xb of the transmission terminal device  100  and the reception terminal device  200  are to be secured, although the Diffie-Hellman parameters g and P may be exposed to a third party. 
     Further, the key management module  141  may generate the key index by using a cryptographic hash function. The hash function refers to a function that maps data of an arbitrary length to fixed data. 
     The cryptographic hash function is a kind of hash function and refers to a function having a property in which it is difficult to find a relationship with an original input value from a hash value. The cryptographic hash function may have preimage resistance, second preimage resistance, and collision resistance. 
     The preimage resistance refers to a property that for a given hash value, it is difficult to find an input value generating the hash value. The second preimage resistance refers to a property that for the input value, it is difficult to change an input while not changing the hash value of the input. The collision resistance refers to a property that it is computationally difficult to find two input values generating the same hash value. Meanwhile, the cryptographic hash function includes MD5 and SHA series hash functions, but is not limited thereto. 
     For example, the key index may be determined by an output value of an arbitrary hash function. In this case, the input value of the hash function that determines the key index may be an arbitrary value capable of finding a value of the encryption key. 
     According to an exemplary embodiment, the value of the encryption key may be the input value of the hash function. According to another exemplary embodiment, the time at which the key sharing ends may be the input value of the hash function. However, one or more other exemplary embodiments are not limited thereto, and the key index may be obtained from any related art technology which may be easily deduced by those skilled in the art. 
     The encryption module  142  may receive the session key, the key index, and the encryption key from the key management module  141  to encrypt the message. 
     Specifically, the encryption module  142  may encrypt the message through a stream cipher scheme. The stream cipher encrypts the message in units of bits, bytes, or words. In general, the stream cipher may be generated by combining the message and a key stream by an exclusive-or (XOR) operation in units of bits. For the stream cipher, algorithms such as a stream running mode of a block cipher, RC4, Chacha, and the like may be used. 
     However, this is merely an example, and it is understood that one or more other exemplary embodiments are not limited thereto. Therefore, the message encryption is not limited to the stream cipher method, and a block cipher method using algorithms such as DES, AES, Blowfish, RC5, and the like may also be used. 
     The decryption module  143  decrypts the encrypted message received from the transmission terminal device  100  or the reception terminal device  200 , so as to correspond to the encryption module  142 . The decryption module  143  may receive the session key, the key index, and the encryption key from the memory  150  for decryption. 
     The memory  150  may store the session key, the key index, and the encryption key generated by the processor  140 . The memory  150  may receive and store the session key, the key index, and the encryption key from the processor  140 . Further, when the processor  140  requests, the memory  150  may transmit session key, the key index, and the encryption key to the processor  140 . Further, the memory  150  may store an encryption algorithm program used in the encryption module  142 . 
     The sensor  160  may be implemented by various sensing sensors for sensing a user input. For example, the sensor  160  may include a touch sensor. The touch sensor may be coupled to the display  130  to implement a touch screen. However, this is merely an example, and it is understood that one or more other exemplary embodiments are not limited thereto. 
       FIG. 4  is an illustrative diagram illustrating an execution screen of a chatting application according to an exemplary embodiment. 
     The execution screen  400  may include the chatting window  410  and the input window  420 . 
     The chatting window  410  includes a message  411  received from the reception terminal device  200  and a message  412  transmitted by the transmission terminal device  100 . The received message  411  and the transmitted message  412  are messages that are encrypted and decrypted by the processor  140 . The received message  411  and the transmitted message  412  may include the key index. Here, the processor  140  may determine the reception terminal device  200  to which the message is to be transmitted using the key index included in the received message  411  and the transmitted message  412 . 
     When there are a plurality of chatting windows, the respective chatting windows  410  may have different key indexes and encryption keys. Meanwhile, in a case in which the chatting window  410  is a group chatting window  410  in which a plurality of other parties of chatting participate, the plurality of other parties of chatting may have the same key index and encryption key. 
     The message input window  420  displays a message that the user is inputting. The message displayed on the message input window  420  is a message that may not be encrypted and decrypted. 
       FIG. 5  is a flowchart illustrating a process of transmitting a message in a message transmission and reception system according to an exemplary embodiment. 
     In a case in which the transmission terminal device  100  and the reception terminal device  200  first start chatting, both of the terminals may exchange the session key by using a Diffie-Hellman key exchange protocol (operation S 501 ). 
     In a case in which the sharing of the session key is completed, the transmission terminal device  100  and the reception terminal device  200  each generate the key index. In this case, the key index may be generated by using the cryptographic hash function. If the message is input to the message input window  420  of the transmission terminal device  100  (operation S 521 ) and a transmission button is input (operation S 522 ), the processor  140  may encrypt the message input through the input  110  by using the encryption key. In this case, the processor  140  may add the key index to a header of the encrypted message. 
     The processor  140  may decrypt the encrypted message by using the encryption key (operation S 524 ) and control the display  130  to display the decrypted message on the chatting window  410  (operation S 525 ). 
     Meanwhile, if the processor  140  encrypts the message input through the input  110  by using the encryption key (operation S 523 ), the processor  140  may control the communicator  120  to transmit the encrypted message to the reception terminal device  200  through the external server  300  (operation S 512 ). 
     Meanwhile, the reception terminal device  200  receives the encrypted message (operation S 531 ). In this case, the processor of the reception terminal device  200  may determine an encryption key used for decryption from the key index of the received message. 
     The processor of the reception terminal device  200  may decrypt the encrypted message by using the encryption key (operation S 532 ), and control the display  130  to display the message on the chatting window (operation S 533 ). 
       FIG. 6  is a flowchart illustrating an encryption process according to an exemplary embodiment. Referring to  FIG. 6 , a sender selects the chatting window  410  with a receiver to chat in the chatting application of the terminal device  100  (operation S 610 ). 
     The processor  140  determines whether or not a chatting history exists in the chatting window selected by the user (operation S 620 ). If a previous chatting history does not exist in the chatting window (No in operation S 620 ), the processor  140  exchanges the session key with the reception terminal device  200  and generates the key index and the encryption key from the session key. The generated key index and encryption key may be stored in the memory  150 . 
     If the chatting history exists (Yes in operation S 630 ), the processor  140  searches for the key index included in the header of the previous message existing in the chatting window (operation S 640 ). That is, if the chatting with the reception terminal device  200  in which a previous chatting record exists is performed, each of the transmission terminal device  100  and the reception terminal device  200  performs the encryption and the decryption by using the stored index key and encryption key. 
     The processor  140  encrypts the message by using the key index and the encryption key (operation S 650 ). In this case, the processor  140  may encrypt the message in a stream cipher scheme by using the encryption key. As another example, the processor  140  may encrypt the message by using a block cipher scheme. 
     Further, the processor  140  may add the key index to the header of the message encrypted by the stream cipher scheme. 
     The processor  140  controls the communicator  120  to transmit the message to the reception terminal device  200  through the external server  300  (operation S 660 ). 
       FIG. 7  is a flowchart illustrating a decoding process according to an exemplary embodiment. 
     Referring to  FIG. 7 , the processor of the reception terminal device  200  receives the encrypted message through the external server  300  (operation S 710 ). 
     If the chatting history does not exist, that is, the message received by the reception terminal device  200  is a message that is received for a first time from the transmission terminal device  100 , the reception terminal device  200  may exchange the session key with the transmission terminal device  100 . After the exchange of the session key, the reception terminal device  200  may generate and store the key index and the encryption key. 
     If the chatting history exists, the processor of the reception terminal device  200  receiving the encrypted message may search for the encryption key corresponding to the key index included in the header of the received message. 
     The processor may decrypt the encrypted message by using the encryption key corresponding to the key index (operation S 720 ). In this case, the processor may perform the decryption in the same scheme as the scheme in which the processor  140  of the transmission terminal device  100  performs the encryption. 
     The processor  140  transmits the decrypted message to the display  130  (operation S 730 ), and the transmission and reception of the message end. 
       FIG. 8  is a flowchart illustrating a process of encrypting a message of a transmission terminal device according to an exemplary embodiment. 
       FIG. 8  illustrates an exemplary embodiment in which the user selects the above-mentioned encryption function to increase a convenience of use. 
     Specifically, if the transmission terminal device  100  according to an exemplary embodiment executes the chatting application, the transmission terminal device  100  may be configured to select whether to set an encryption security function. Further, the transmission terminal device  100  may also be configured to select a chatting window from a chatting list of a chatting history (operation S 812 ) as well as to select a user from a user list of the chatting application (operation S 811 ). 
     The processor  140  checks whether the security function is operated or selected (operation S 820 ). Here, if the security function is activated (Yes in operation S 820 ), the processor  140  checks whether or not the chatting history exists and begins to encrypt the message. If the security function is non-activated (No in operation S 820 ), the processor  140  may request or inquire from the user as to whether to set the security function (operation S 830 ). If the user does not want to set the security function (No in operation S 830 ), the processor  140  may transmit the message to the reception terminal device  200  without performing the encryption. 
     If the user sets the security function (Yes in operation S 830 ) and the chatting history exists (Yes in operation S 840 ), the processor  140  selects the encryption key corresponding to the key index (operation S 870 ) and encrypts the message (operation S 880 ). The processor  140  may transmit the encrypted message to the reception terminal device (operation S 890 ). 
     If the chatting history does not exist (No in operation S 840 ), the processor  140  controls the memory  150  to exchange the session key with the reception terminal device  200 , generate the key index and the encryption key, and store the generated key index and encryption key (operation S 860 ). Next, the processor  140  selects the encryption key (operation S 870 ) and encrypts the message (operation S 880 ). Here, the key index may be added to the header of the encrypted message. The processor  140  may transmit the encrypted message to the reception terminal device (operation S 890 ). 
     Hereinabove, although one or more exemplary embodiments have been described with reference to the drawings, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the inventive concept(s) as set forth in the accompanying claims.