Patent Publication Number: US-11399016-B2

Title: System and method for identifying exchanges of encrypted communication traffic

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to the monitoring of communication traffic generated by users of computer applications. 
     BACKGROUND OF THE DISCLOSURE 
     Many computer applications use encrypted protocols, such that the communication traffic exchanged by these applications is encrypted. Examples of such applications include WhatsApp, Skype, Line, and Dropbox. Examples of encrypted protocols include the Secure Sockets Layer (SSL) protocol and the Transport Layer Security (TLS) protocol. 
     US Patent Application Publication 2016/0285978, whose disclosure is incorporated herein by reference, describes a monitoring system that monitors traffic flows exchanged over a communication network. The system characterizes the flows in terms of their temporal traffic features, and uses this characterization to identify communication devices that participate in the same communication session. By identifying the communication devices that serve as endpoints in the same session, the system establishes correlations between the users of these communication devices. The monitoring system characterizes the flows using traffic features such as flow start time, flow end time, inter-burst time and burst size, and/or statistical properties of such features. The system typically generates compressed-form representations (“signatures”) for the traffic flows based on the temporal traffic features, and finds matching flows by finding similarities between signatures. 
     SUMMARY OF THE DISCLOSURE 
     There is provided, in accordance with some embodiments of the present disclosure, a system including a data storage and a processor. The processor is configured to posit, by analyzing encrypted communication traffic passed over multiple connections without decrypting the traffic, that at least one file was transferred over one connection of the connections. The processor is further configured to group packets belonging to the connection into at least one sequence, in response to the positing. The processor is further configured to compute an estimated size of the file, based on respective sizes of those of the packets belonging to the sequence, and to store the estimated size in the data storage. 
     In some embodiments, the processor is configured to posit that the file was transferred over the connection in response to identifying, in one of the packets, an identifier of a server known to service file exchanges. 
     In some embodiments, the identifier is an Internet Protocol (IP) address. 
     In some embodiments, the processor is configured to posit that the file was transferred over the connection in response to an indication in one of the packets or in another packet that the connection was made by an application used for file transfers. 
     In some embodiments, the indication includes a specification of a protocol used by a class of applications used for file transfers. 
     In some embodiments, the processor is configured to group the packets by demarcating between the sequence and others of the packets that were communicated in the same direction as was the sequence. 
     In some embodiments, the processor is configured to demarcate between the sequence and the others of the packets based on a time gap between the sequence and a closest one of the others of the packets. 
     In some embodiments, the processor is configured to demarcate between the sequence and the others of the packets based on a decrease in throughput at an end of the sequence. 
     In some embodiments, the processor is configured to compute the estimated size of the file by: 
     computing a sum of the respective sizes, and 
     computing the estimated size of the file by dividing the computed sum by a predefined packet-size inflation divisor that is greater than one. 
     In some embodiments, the predefined packet-size inflation divisor is expressed as a probability distribution, such that the processor is configured to compute the estimated size as another probability distribution. 
     In some embodiments, 
     the sequence was downloaded by a client, and 
     the processor is further configured to, prior to computing the estimated size of the file:
         identify another sequence of other packets downloaded by the client,   compute another sum of respective sizes of the other packets,   posit that the other sequence carried another file having a known size, and   in response to the positing, compute the packet-size inflation divisor by dividing the other sum by the known size.       

     In some embodiments, the processor is further configured to posit that the other file was downloaded by multiple other clients, and the processor is configured to posit that the other sequence carried the other file in response to a number of the other clients. 
     In some embodiments, the processor is further configured to communicate the other file to the client, such as to cause the other sequence to be downloaded by the client. 
     In some embodiments, the sequence was exchanged between a server and a client, and the processor is further configured to, prior to computing the estimated size of the file: 
     infer one or more parameters from one or more of the connections belonging to the client, and 
     select the packet-size inflation divisor from multiple predefined inflation divisors, based on a predefined association between the packet-size inflation divisor and the parameters. 
     In some embodiments, at least one of the parameters is selected from the group of parameters consisting of: a type of the client, an operating system running on the client, and an encryption protocol used by the client. 
     In some embodiments, 
     the sequence was uploaded by a client, and 
     the processor is further configured to, prior to computing the estimated size of the file:
         identify, with respective levels of confidence, instances in which respective other files were communicated from the client to respective other clients, and   based on the identified instances and on a predefined distribution of another packet-size inflation divisor for downloads, compute the packet-size inflation divisor.       

     There is further provided, in accordance with some embodiments of the present disclosure, a method including, by analyzing encrypted communication traffic passed over multiple connections without decrypting the traffic, positing that at least one file was transferred over one connection of the connections. The method further includes, in response to the positing, grouping packets belonging to the connection into at least one sequence. The method further includes computing an estimated size of the file, based on respective sizes of those of the packets belonging to the sequence, and storing the estimated size in a database. 
     There is further provided, in accordance with some embodiments of the present disclosure, a computer software product including a tangible non-transitory computer-readable medium in which program instructions are stored. The instructions, when read by a processor, cause the processor to posit, by analyzing encrypted communication traffic passed over multiple connections without decrypting the traffic, that at least one file was transferred over one connection of the connections. The instructions further cause the processor to group packets belonging to the connection into at least one sequence, in response to the positing. The instructions further cause the processor to compute an estimated size of the file, based on respective sizes of those of the packets belonging to the sequence, and to store the estimated size in a database. 
     There is further provided, in accordance with some embodiments of the present disclosure, a system including a peripheral device and a processor. The processor is configured to compute a measure of similarity between (i) a first estimated size of first encrypted file content transferred over a network and (ii) a second estimated size of second encrypted file content transferred over the network. The processor is further configured to posit, based on the measure of similarity, that the first encrypted file content and the second encrypted file content represent the same file. The processor is further configured to generate an output to the peripheral device in response to the positing. 
     In some embodiments, the peripheral device is selected from the group of devices consisting of: a display, and a data storage. 
     In some embodiments, the first estimated size and the second estimated size are expressed as respective probability distributions. 
     In some embodiments, 
     the first encrypted file content was uploaded by a first user at a first time, 
     the second encrypted file content was downloaded by a second user at a second time subsequent to the first time, 
     the processor is further configured to compute a difference between the second time and the first time, and 
     the processor is configured to generate the output responsively to the difference being less than a predefined threshold. 
     In some embodiments, the output indicates that the first user communicated the file to the second user. 
     In some embodiments, the processor is configured to generate the output by increasing a relatedness score between the first user and the second user. 
     In some embodiments, 
     the first encrypted file content was downloaded by a first user at a first time, 
     the second encrypted file content was downloaded by a second user at a second time subsequent to the first time, 
     the processor is configured to generate the output in response to a metadata link having been communicated by the first user between the first time and the second time, and 
     the output indicates that the metadata link pointed to the file and was communicated to the second user. 
     In some embodiments, the output indicates that a first user communicated the file to a second user, and the processor is configured to generate the output in response to a relatedness score between the first user and the second user. 
     In some embodiments, the processor is further configured to identify a frequency with which files having the first estimated size are communicated over the network, and the processor is configured to posit that the first encrypted file content and the second encrypted file content represent the same file with a likelihood that decreases with the frequency. 
     In some embodiments, 
     the first encrypted file content was downloaded by a first user at a first time, 
     the second encrypted file content was downloaded by a second user at a second time, 
     the processor is further configured to compute a difference between the first time and the second time, and 
     the processor is configured to generate the output responsively to the difference being less than a predefined threshold. 
     In some embodiments, the processor is further configured to: 
     receive a query specifying a second-file-content transfer of the second encrypted file content, and 
     identify a first-file-content transfer of the first encrypted file content in response to the query, 
     the processor is configured to compute the measure of similarity in response to identifying the first-file-content transfer, and 
     the output includes parameters of the first-file-content transfer. 
     In some embodiments, 
     the second-file-content transfer was performed using a class of applications, and 
     the processor is configured to identify the first-file-content transfer of the first encrypted file content by:
         retrieving, from a database, multiple other-file-content transfers of other encrypted file content, which were performed using the class of applications, and   identifying the first-file-content transfer from among the other-file-content transfers.       

     In some embodiments, the processor is further configured to: 
     identify multiple other-file-content transfers of other encrypted file content in response to the query, and 
     posit that the file was transferred in each of the other-file-content transfers, and 
     the processor is configured to generate the output by outputting a timeline of the other-file-content transfers and the first-file-content transfer. 
     In some embodiments, at least some of the other-file-content transfers were performed using different respective applications. 
     There is further provided, in accordance with some embodiments of the present disclosure, a method including computing a measure of similarity between (i) a first estimated size of first encrypted file content transferred over a network and (ii) a second estimated size of second encrypted file content transferred over the network. The method further includes, based on the measure of similarity, positing that the first encrypted file content and the second encrypted file content represent the same file, and in response to the positing, generating an output. 
     There is further provided, in accordance with some embodiments of the present disclosure, a computer software product including a tangible non-transitory computer-readable medium in which program instructions are stored. The instructions, when read by a processor, cause the processor to compute a measure of similarity between (i) a first estimated size of first encrypted file content transferred over a network and (ii) a second estimated size of second encrypted file content transferred over the network. The instructions further cause the processor to posit, based on the measure of similarity, that the first encrypted file content and the second encrypted file content represent the same file, and to generate an output in response to the positing. 
     The present disclosure will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a system for monitoring communication exchanged over a network, in accordance with some embodiments of the present disclosure; 
         FIG. 2  is a schematic illustration of a series of file exchanges that may be identified in accordance with some embodiments of the present disclosure; 
         FIG. 3  is a schematic illustration of a method for identifying file transfers, in accordance with some embodiments of the present disclosure; 
         FIG. 4  is a flow diagram for an algorithm for maintaining a file-transfer database, in accordance with some embodiments of the present disclosure; 
         FIG. 5  is a flow diagram for an algorithm for maintaining a relationship database, in accordance with some embodiments of the present disclosure; and 
         FIGS. 6-7  are flow diagrams for algorithms for handling queries, in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Glossary 
     In the context of the present application, including the claims, a file is said to be “exchanged” if the file is passed from a first user to a second user, regardless of whether the second user passes any communication to the first user. If the exchange is performed via a server, the file is said to be transferred to the server by the first user, and then transferred to the second user by the server. 
     In the context of the present application, including the claims, an “original distributor” of any given file is the first user to communicate the file to another user. In some cases, the original distributor is the first user to communicate the file to another user using a particular application, even if the file was previously communicated using a different application. For example, in the event that User A created a file and then passed the file to User B over Skype, and User B subsequently passed the file to User C over WhatsApp, both User A and User B may be referred to as original distributors of the file: User B as the original distributor over WhatsApp, and User A as the “global” original distributor. 
     In the context of the present application, including the claims, a processor is said to “posit” a particular hypothesis if the processor computes a relatively significant likelihood, or level of confidence, that the hypothesis is true. In general, a “relatively significant” likelihood is any likelihood that exceeds a particular predefined threshold. (The threshold may be defined as an absolute number, or as a particular percentile or other statistical measure of a distribution of likelihoods.) In the context of the present application, including the claims, multiple mutually-exclusive hypotheses may be simultaneously posited with different respective likelihoods. 
     For example, using the techniques described herein, a processor may identify several candidate original distributors of a particular file, i.e., several users who have a sufficiently high likelihood of being the original distributor of the file. In this case, it may be said with reference to each of these users that the processor “posits” that the user is the original distributor of the file. 
     In the present application, including the claims, words such as “identify” and “ascertain” may be used interchangeably with “posit.” 
     In the context of the present application, including the claims, the process of “calculating” or “estimating” a particular quantity may include, within its scope, calculating a range or probability distribution for the quantity. 
     In the context of the present application, including the claims, the terms “user,” “client,” and “device” may be used interchangeably in certain contexts. For example, with reference to a particular upload of a file, it may be said that the user uploaded the file, or that the device or client used by the user uploaded the file. 
     In the context of the present application, including the claims, one file transfer is said to “correspond” to another file transfer if the two transfers are of the same file. A specific type of correspondence is “strong correspondence.” In the context of the present application, including the claims, two file transfers are said to “strongly correspond” to one another if the two transfers belong to the same instance of communication of the file. As an example, in a scenario in which a first user communicates a file to a second user via a server, the upload of the file by the first user and the download of the file by the second user are said to strongly correspond to one another. As another example, in a scenario in which one user communicates a file to multiple other users in a single instance of communication (e.g., by communicating the file to a group including the multiple other users), the respective downloads of the file by the other users are said to strongly correspond to each another (as well as to the upload of the file). 
     Overview 
     Embodiments of the present disclosure provide techniques for identifying sequences of encrypted packets that carry files between clients and application servers, and for estimating the sizes of these files. Advantageously, these techniques require only passive monitoring of the communication that is exchanged with the application servers, and do not require any decryption of the encrypted packets. 
     To perform these techniques, a traffic-monitoring system first searches the traffic for connections that appear to carry file content. Subsequently to identifying such a connection, the system estimates the number of files that were transferred over the connection. To perform this estimation, the system may interpret a change in the direction of packet flow as indicating the end of a file. Thus, for example, one or more uploaded packets situated between two sequences of downloaded packets may indicate that the two sequences of downloaded packets carry different respective files. Alternatively or additionally, the system may interpret a duration between successive packets that exceeds a predefined threshold as an indicator of the end of one file and the beginning of another. Alternatively or additionally, the system may interpret a drop in throughput as indicating the end of a file. Alternatively or additionally, the system, using machine-learned techniques, may identify patterns in the timing, sizes, and/or directions of the packets that indicate the end of one file and the beginning of another. Alternatively or additionally, the system, using machine-learned techniques, may identify, on a parallel connection between the two parties (such as a connection that carries control messages), a message that indicates the end of one file and/or the beginning of another. 
     Next, the system estimates the respective sizes of one or more of the files that were transferred over the connection. To perform this estimation, the system first “peels away” as many lower-level protocol headers (e.g., Transmission Control Protocol (TCP) headers, IP headers, and SSL headers) as possible from each of the packets that carries part of the file, and identifies the size that is specified (in an unencrypted manner) in the lowest-level payload that remains. Next, the system tallies the specified sizes. Finally, the system reduces the packet-size tally to account for an estimated overhead due to the encryption of the packets. 
     More specifically, as the present inventors have observed, an encrypted packet that carries file content is generally larger than the file content. The amount of “inflation” of the packet is generally a function of various factors, such as the type of device that uploads or downloads the file, the operating system running on the device, and the encryption protocol that is used to encrypt the packet. For example, when a particular device uploads a file having an unencrypted size of S bytes, the packets carrying the file may have a total size of S*(1+x) bytes, where x is an inflation percentage for uploads. Similarly, when the device downloads a file having an unencrypted size of S bytes, the packets carrying the file may have a total size of S*(1+y) bytes. (x and y are generally different from one another, and may change from time to time.) Hence, to estimate the size of a given file, the system first looks up or estimates x or y, and then divides the packet-size tally by 1+x or 1+y. (The quantities 1+x and 1+y are referred to below as “packet-size inflation divisors.”) 
     In some cases, the system may learn the inflation percentages for a particular device of interest, by exchanging files of known sizes with the device of interest. (This exchange may be performed silently, such that the user of the device of interest is unaware of the file exchanges.) Subsequently, given an encrypted file transfer performed by the device of interest, the system may compute the estimated size of the file using the relevant learned inflation percentage. 
     Alternatively or additionally, the system may learn the inflation percentages for various different device types, operating systems, and encryption protocols of interest. Subsequently, given a file-carrying sequence of packets exchanged with a particular device, the system may ascertain, from the sequence of packets (and/or from other packets exchanged with the device), the device type, the operating system run by the device, and/or the encryption protocol that was used to generate the sequence. The system may then look up the inflation percentage corresponding to these parameters. 
     In some embodiments, subsequently to identifying multiple file-carrying packet sequences as described above, the system may posit that two or more of the sequences correspond to each other by virtue of carrying the same file. This positing may be based on the times at which the sequences were communicated, the estimated sizes of the files carried by the sequences, and/or knowledge of prior communication between the relevant devices or users. 
     For example, using the techniques described above, the system may ascertain that User A downloaded first file content, having a first estimated size, from a particular server and that, at a previous time, User B uploaded second file content, having a second estimated size, to the server. If the two estimated sizes are sufficiently close to one another, the system may posit that the first file content and second file content represent the same file. In response thereto—particularly if User A&#39;s download began shortly after the end of User B&#39;s upload, and/or if the two users previously communicated with one another—the system may posit that User B communicated the file to User A via the server. Furthermore, if the system also posits that one or more other users downloaded the same file at approximately the same time as User A, the system may posit that the other users belong to the same communication group or distribution list as does User A. 
     Alternatively or additionally, based on corresponding uploads and downloads, the system may calculate a more accurate estimate of the inflation percentage for a particular device. For example, the system may identify multiple file uploads performed by the device, and may further identify various file downloads that appear to correspond, respectively, to these uploads (i.e., that appear to carry the files that are carried by the uploads, respectively). By comparing the upload packet-size tallies to the download packet-size tallies, the system may estimate the upload inflation percentage for the device. This estimate may then be refined over time, based on further observations of file exchanges. 
     Alternatively or additionally, based on an identified upload, by a particular user, that is not preceded by any downloads of a similarly-sized file, the system may posit that the particular user is the original distributor of the file. This information may be particularly useful in the event that the file contains illegal or malicious content. 
     System Description 
     Reference is initially made to  FIG. 1 , which is a schematic illustration of a system  20  for monitoring encrypted communication exchanged over a network  22 , such as the Internet, in accordance with some embodiments of the present disclosure. 
       FIG. 1  depicts a plurality of users  24  using computer applications to transfer files over network  22 . For example, users  24  may exchange files using email applications, file transfer protocol (FTP) applications, messaging applications, gaming applications, or chat room applications. As another example, one user may upload a file to a website, such as an online storage or file-transfer site, and another user may then download the file from the website. The applications used for transferring files may run on any suitable devices (or “clients”), such as personal computers or mobile devices  34 . 
     A plurality of servers  26  service the computer applications, such that the files are transferred via servers  26 . (Any given server may service more than one application.) In some cases, a file may be exchanged between two users via more than one server. For example, the emailing of a file from a first user to a second user may comprise (i) the uploading of the file from the first user&#39;s device to the first user&#39;s email server, (ii) the communication of the file from the first user&#39;s email server to the second user&#39;s email server, and (iii) the downloading of the file from the second user&#39;s email server to the second user&#39;s device. 
     Typically, system  20  passively monitors the communication over network  22 , in that the system does not intermediate the exchange of communication traffic between users  24  and servers  26 , but rather, receives copies of the traffic from at least one network tap  32 . Network tap  32  may be situated, for example, near an Internet Service Provider (ISP)  23 , and ISP  23  may mirror the traffic to the network tap. 
     Communication traffic over network  22  is exchanged in units of packets. Although the content of these packets may be encrypted, a packet may include certain unencrypted metadata, such as a specification of the communication protocols (e.g., the TCP protocol) used to construct the packet, a source IP address and/or port, a destination IP address and/or port, and the size of the packet at various layers. As described in detail below, system  20  uses this information to identify connections in which files are transferred, and to estimate the sizes of these files. 
     Although the traffic generally does not explicitly indicate the identities of the users, the system may use external sources of information to discover these identities. The system may thus, subsequently to identifying the transfer of a file, identify the user who performed the transfer. 
     For example, the system may receive, from a cellular service provider, a first mapping between IP addresses and mobile phone numbers (derived, for example, from General Packet Radio Service Tunneling Protocol (GTP) data), along with a second mapping between mobile phone numbers and users&#39; names. Using these mappings, the system may use a specified source or destination IP address to identify the user who sent or received a particular file. Alternatively, for example, to identify the user who is using a particular IP address, the system may identify unencrypted communication in which the user&#39;s email address is associated with the IP address, and then search the Internet for a user profile in which the user&#39;s email address is associated with the user&#39;s name. 
     In some cases, a device may use a network address translator (NAT), which allows multiple devices to use a single IP address. In such cases, the system may not be able to infer the user&#39;s identity from the IP address as described above. However, the system may nonetheless infer the user&#39;s identify from unencrypted identifying information contained in another connection belonging to the same device as does the connection in which the file was transferred. To ascertain that the two connections belong to the same device, the system may, for example, use any of the techniques described in US Patent Application Publication 2017/0222922, whose disclosure is incorporated herein by reference. For example, the system may group the two connections together based on common usage characteristics exhibited by the two connections. 
     System  20  comprises a network interface  28 , such as a network interface controller (NIC), a processor  30 , and one or more peripheral devices, such as a display  36 , a data storage  31  (e.g., a hard drive or a flash memory), and/or one or more input devices (e.g., a keyboard or a mouse) to facilitate interaction with the system. In some embodiments, all of the aforementioned components belong to a single server  37 . In other embodiments, the components are distributed over multiple servers  37 . Each of the peripheral devices may be internal or external to the server to which it belongs. 
     In general, the packets from network tap  32  are received by processor  30  via network interface  28 . Processor  30  processes the packets, as described herein, such as to identify file transfers and/or to perform any other relevant function described herein. Further to processing a collection of packets, the processor may generate any suitable output to one or more of the peripheral devices. For example, the processor may generate a visual output to display  36 . Alternatively or additionally, the processor may store information (e.g., in any one of the databases described hereinbelow) in storage  31 . 
     In general, processor  30  may be embodied as a single processor, or as a cooperatively networked or clustered set of processors. For example, a first processor may receive communication traffic via network interface  28  and, based on the traffic, generate and store a network-traffic report, which is described below with reference to  FIG. 3 . Subsequently, or while the network-traffic report is continually updated by the first processor, a second processor may identify file transfers from the network-traffic report, and store information relating to the file transfers in data storage  31 . The second processor may further process this information to identify corresponding file transfers; alternatively, this function may be performed by yet another processor. 
     In some embodiments, the functionality of processor  30 , as described herein, is implemented solely in hardware, e.g., using one or more Application-Specific Integrated Circuits (ASICs) or Field-Programmable Gate Arrays (FPGAs). In other embodiments, the functionality of processor  30  is implemented at least partly in software. For example, in some embodiments, processor  30  is embodied as a programmed digital computing device comprising at least a central processing unit (CPU) and random access memory (RAM). Program code, including software programs, and/or data are loaded into the RAM for execution and processing by the CPU. The program code and/or data may be downloaded to the processor in electronic form, over a network, for example. Alternatively or additionally, the program code and/or data may be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. Such program code and/or data, when provided to the processor, produce a machine or special-purpose computer, configured to perform the tasks described herein. 
     Identifying File Exchanges and Original Distributors 
     Reference is now made to  FIG. 2 , which is a schematic illustration of a series of file exchanges that may be identified in accordance with some embodiments of the present disclosure. 
       FIG. 2  depicts a first user  24   a  communicating a file to a second user  24   b  via a server  26 . In particular, first user  24   a  uploads the file to the server, and the server subsequently communicates the file to the device of second user  24   b , such that the second user&#39;s device downloads the file from the server. 
     As further shown in  FIG. 2 , subsequently to downloading the file, second user  24   b  may forward the file to one or more or other users. For example, second user  24   b  may upload the file to the server, and the server may then communicate the file to the other users. Alternatively, instead of uploading the file itself, the second user&#39;s device may send, to the server (or to a cooperatively networked server that handles non-file-bearing traffic), a metadata link to the file. The server may then verify that the file, which was already received from the first user, is cached locally. If yes, the server may communicate the file to the other users. As yet another alternative, the second user, using a different application from that which was used by first user  24   a , may upload the file to another server, and the other server may then communicate the file to the other users. Eventually, after any number of such exchanges, the file is downloaded by a third user  24   c.    
     As described in detail below, by analyzing the encrypted traffic that is generated in such a scenario, system  20  ( FIG. 1 ) may identify information of interest, and may generate any suitable output (e.g., a visual output displayed on display  36  ( FIG. 1 )) indicating this information. For example, the system may first identify that the second user&#39;s device downloaded a file, using the techniques described below in the subsection entitled “Identifying a file transfer.” The system may further identify the second user, based on an IP address and/or other information contained in the traffic and/or any relevant external data sources, such as a GTP data source. Subsequently, the system may estimate the size of the file, using the techniques described below in the subsection entitled “Estimating the file sizes.” The processor may further identify that the first user uploaded a similarly-sized file shortly before the second user&#39;s download. Based on this information and/or other factors described below, the processor may posit that the first user communicated the file to the second user. 
     In response to positing that the first user communicated the file to the second user, the system may increase a relatedness score for the pair of users, as described below in the subsection entitled “Example algorithms.” This score quantifies the extent to which the pair of users are assumed to communicate with one another. The relatedness score may also be increased in response to identifying other types of communication or interaction between the users, such as any of the types of communication (e.g., the exchange of text messages) described in US Patent Application Publication 2018/0316638, whose disclosure is incorporated herein by reference. Alternatively or additionally, the relatedness score may be increased in response to the users living or working at the same address or near one another. 
     Similarly, using the same techniques for identifying file transfers and estimating file sizes, the system may posit that another user (not shown in  FIG. 2 ) downloaded the file at approximately the same time as second user  24   b , indicating that the other user and second user  24   b  might belong to the same group or distribution list. In response thereto, the system may increase the relatedness score for second user  24   b  and the other user, alternatively or additionally to increasing the relatedness score for first user  24   a  and the other user. 
     In addition to calculating relatedness scores for various pairs of users, the system may posit that a particular user is the original distributor of a particular file of interest. In other words, the system may identify the particular user as a candidate original distributor of the file. In response thereto, the system may generate an output indicating the likelihood that this user is the original distributor of the file. 
     For example, if the file downloaded by third user  24   c  is deemed to be of interest (e.g., if the file is found to contain illegal or malicious content), the system may track the successive exchanges of the file going backward in time, until first user  24   a  is identified as a candidate original distributor of the file. In other words, the system may identify one or more users who may have forwarded the file to the third user, then one or more users who may have forwarded the file to these latter users, and so forth, until first user  24   a  (in addition to any other candidate original distributors) is reached. 
     Alternatively or additionally to explicitly identifying candidate original distributors of a file, the system may output a timeline of the file transfers, as further described below with reference to  FIG. 7 . A user may then identify any candidate original distributors by referring to the timeline. 
     It is emphasized that the system may posit an original distributor or output a timeline as described above even if the series of file transfers occurs across multiple different applications. For example, even if the first user communicates the file to the second user using Skype, while the third user receives the file over WhatsApp, the system may nonetheless identify the first user as a candidate original distributor of the file. 
     In some cases, the actual original distributor of the file may be indeterminable from the traffic, such as where the original distributor used Virtual Private Network (VPN) tunneling or another tunneling protocol, or where the original distributor used an ISP that is outside the coverage of system  20 . Even in such cases, however, other techniques may be used to link one or more of the candidate original distributors, as identified by the system, to the actual original distributor. 
     Identifying File Transfers 
     Reference is now made to  FIG. 3 , which is a schematic illustration of a method for identifying file transfers, in accordance with some embodiments of the present disclosure. 
       FIG. 3  depicts an example network-traffic report  38 , which specifies various properties of at least some of the encrypted packets that are received by system  20  via network tap  32 . Report  38  may be constructed by processor  30  using any suitable software, such as the Wireshark open source software. It is noted that report  38  typically combines traffic generated by multiple different applications, such as the top five, ten, fifty, or hundred applications used to transfer files between users. 
     As shown in  FIG. 3 , for each packet that is included in report  38 , the report typically specifies the time at which the packet was received by the network tap, the source Internet Protocol (IP) address (SrcIP) and destination IP address (DstIP) specified in the packet (and/or the domain names to which the SrcIP and DstIP correspond), and the size of the packet. (In  FIG. 3 , the time is indicated in the Unix time format, while the size is indicated in bytes.) Typically, the report includes additional properties of each packet, such as the source and destination port numbers, and the protocol(s) used to construct the packet. 
     By way of example,  FIG. 3  shows a first sequence of packets S 1  in which a file is uploaded to a server having IP address 212.199.140.162, and a second sequence of packets S 2  in which a file is downloaded from the server. Although a file-carrying sequence of packets typically includes tens, hundreds, or thousands or packets,  FIG. 3 , for ease of illustration, shows only the first two packets and the last packet of each sequence. Also for ease of illustration,  FIG. 3  does not show any other packets interspersed with the packets belonging to sequences S 1  and S 2 . 
     Typically, prior to processing report  38  so as to identify sequences of packets that carry files, the processor preprocesses the report. For example, the processor may reorder packets that were received out of order by tap  32  due to TCP reconstruction or retransmission. (This may be done, for example, based on the TCP sequence numbers specified in the packets.) The processor may also filter out packets whose sizes are below a particular threshold, or are otherwise known a priori not to carry file content. Examples of such packets include keepalive and negotiation packets. 
     In some embodiments, the processor also separates the report into two separate sub-reports: one sub-report including only uploaded packets, and the other sub-report including only downloaded packets. The processor then processes each sub-report separately. (Thus, the processor may first identify first sequence S 1  by processing the sub-report for uploads, and then identify sequence S 2  by processing the sub-report for downloads.) In other embodiments, the processor does not separate the uploaded packets from the downloaded packets. In yet other embodiments, the processor performs the processing techniques described below for the full, unseparated report  38 , and also for the separate sub-reports. Advantageously, this may facilitate a more accurate identification of file transfers. 
     Following the preprocessing of report  38 , the processor scans the report (or the separated sub-reports) for possible file transfers. In performing this scan, the processor identifies each connection for which there is an indication that the connection was used, or at least may have been used, for a file transfer. In response to identifying this indication, the processor posits that the connection carries at least one file, even if the processor cannot identify the particular application used to make (or “open”) the connection. 
     For example, the processor may posit that a connection carries at least one file in response to identifying, in a packet belonging to the connection, an identifier, such as an IP address or a certificate, of a server known to service file exchanges. Thus, for example, the packets in sequences S 1  and S 2  may be identified as file-carrying packets based on the fact that the IP address 212.199.140.162, which is known to service file content, is specified in these packets. The identifiers of servers that handle file content—and, optionally, the applications serviced by these servers—may be learned from Domain Name System (DNS) requests, Hypertext Transfer Protocol Secure (HTTPS) certificate negotiations, and/or any other relevant type of traffic. 
     For example, the processor may identify an identifier of a messaging-application server that is known by the processor to exclusively handle file content. Alternatively, the processor may identify an identifier of a server, such as a file-storage server or an email server, that is known by the processor to service a particular application or class of applications used exclusively or non-exclusively for file transfers. 
     In some cases, the processor posits that a connection carries at least one file in response to an indication that the connection was made by an application used for file transfers. This indication may be contained in one of the packets belonging to the connection, or in another packet belonging to another connection sharing the same source and destination IP addresses with the connection in question. The indication may include an identifier of a server, as described immediately above. Alternatively, the indication may include a layer  7  (i.e., an application-layer) specification of an application protocol known to be used by a class of applications used exclusively or non-exclusively for file transfers. For example, a specification of the Simple Mail Transfer Protocol (SMTP), a version of the Post Office Protocol (POP), or another clear email exchange protocol may indicate that the connection was made by an email application. 
     In some cases, identifying the aforementioned indication or server-identifier may not be sufficient for positing that the connection carries at least one file, due to the relatively low likelihood of this hypothesis. For example, identifying that an email application was used to make the connection may not be sufficient for positing that the connection carries at least one file, given that most emails do not carry files. Hence, the processor typically learns, using any suitable machine learning techniques, other properties of connections that indicate that a file transfer may have been performed. Subsequently, in response to identifying one or more of these properties (and in some cases, even without identifying the aforementioned indication or server-identifier), the processor may posit that the connection carries at least one file. 
     For example, the processor may posit that the connection carries at least one file in response to statistical properties of the packets belonging to the connection, such as a ratio of uploaded packets to downloaded packets. Alternatively or additionally, the processor may identify a relatively high throughput (i.e., a throughput exceeding a predefined threshold) as an indication of a possible file transfer. 
     Alternatively or additionally, the processor may posit that the connection carries at least one file in response to identifying, in another connection, an exchange of packets indicating that the former connection carries at least one file. For example, when some messaging applications are used to exchange a file, two connections are opened: a first connection to a first server, over which the file is transferred, and a second connection to a second server, over which related messages, such as a message describing the file or acknowledging the transfer, are exchanged. Hence, subsequently to identifying that a first connection was made by a messaging application used for file transfers, the processor may identify a second connection made by the same application and occurring at approximately the same time as, and sharing the same device IP address as, the first connection. The processor may further identify an exchange of packets in the second connection that appears to be related to a file transfer. In response thereto, the processor may posit that the first connection carries at least one file. 
     For each connection posited to carry a file, the processor groups the packets belonging to the connection into one or more packet sequences, each of which is posited to carry a different respective file. For each sequence, the processor calculates an estimated size of the file assumed to be carried by the sequence. Subsequently, relevant properties of the sequence are stored in a file-transfer database, as further described below with reference to  FIG. 4 . (In some cases, such as cases in which the processor is not certain that the sequence carried a file, the processor may store the properties of the sequence only if the size of the sequence exceeds a predefined threshold.) 
     To group the packets belonging to the connection, the processor typically looks for features of the packets indicating the beginning or end of a file transfer. Based on these features, the processor demarcates each sequence of packets that is assumed to be carrying a respective file. 
     For example, if both uploaded and downloaded packets are processed together, the processor may identify an uploaded packet that immediately follows a large number of downloaded packets. In response thereto, the processor may posit that the last of the downloaded packets is the last packet of a downloaded file-carrying sequence. Similarly, the processor may identify the end of an uploaded file-carrying sequence by identifying a downloaded packet that immediately follows a large number of uploaded packets. 
     One example of a packet that signifies the end of a file transfer is an “end of file” (EOF) acknowledgement message. Using machine-learning techniques, the processor may learn to identify this type of message, and hence, to identify the end of a file transfer. (Based on the identified EOF acknowledgement messages, the processor may learn the statistical properties of the time gaps between the transfer of different respective files, which may be used to demarcate file-transfer sequences as described immediately below.) 
     Alternatively or additionally, the processor may demarcate a sequence based on a time gap that precedes the sequence, and/or another time gap that follows the sequence. For example, if the duration between two consecutive packets exceeds a predefined threshold, the processor may assume that the first of these packets is the last packet in an earlier sequence carrying a first file, while the second of these packets is the first packet in a later sequence carrying a second file. The processor may define the threshold based on the statistical properties of the time gaps between the packets. For example, the threshold may be set to the mean time gap between consecutive packets plus a certain number of standard deviations. 
     For example, with reference to  FIG. 3 , the processor may demarcate between sequence S 1  and a subsequent sequence of uploaded packets belonging to the same connection as sequence S 1 , by terminating sequence S 1  at the first packet that precedes the closest succeeding packet by more than the threshold time gap. Similarly, the processor may demarcate between sequence S 1  and a preceding sequence of uploaded packets belonging to the same connection as sequence S 1 , by beginning sequence S 1  at the first packet that follows the closest preceding packet by more than the threshold time gap. 
     In many cases, towards the end of a file transfer, the throughput of the transfer decreases, in that the size of the packets decreases and/or the duration between successive packets increases relative to earlier portions of the file transfer. Subsequently, at the beginning of the next file transfer, the throughput increases to its previous level. Hence, alternatively or additionally to using the techniques described above, the processor may demarcate a sequence based on a decrease in throughput preceding the sequence, and/or another decrease in throughput exhibited at the end of the sequence. 
     For example, to demarcate sequence S 1  from a subsequent file upload belonging to the same connection, the processor may identify several consecutive smaller packets, which are both preceded and followed by larger packets. In response to identifying this decrease in throughput, the processor may terminate sequence S 1  at the last of the smaller packets, and begin the next sequence at the first subsequent larger packet. Alternatively, the processor may identify a downward trend in the size of the packets, followed by an upward trend in the size. In response thereto, the processor may terminate sequence S 1  at the transition between the downward trend and the upward trend, i.e., at the local minimum in throughput. 
     Alternatively to using the throughput as a demarcator, the processor may use the throughput to set the threshold for the time gap, which in turn may be used to demarcate the sequence as described above. In particular, the threshold may be relatively high if the throughput is generally stable, but lower if the throughput exhibits a downward trend. 
     Alternatively or additionally, the processor may identify in the sizes and/or directions of the packets, and/or in the durations between the packets, a pattern that indicates the beginning or end of a file transfer. The processor may use any suitable machine-learning techniques to learn the patterns that indicate such transitions. 
     For example, as the present inventors have observed, a file-carrying sequence may include (i) an “N-packet pattern,” which includes multiple consecutive sub-sequences of N packets each, where each of the sub-sequences includes N−1 packets of size SZ 1  followed by a smaller residual packet of size SZ 2 , followed by (ii) a final sub-sequence that differs from the previous sub-sequences with respect to the number or size of the packets. Thus, in response to identifying a first N-packet pattern, followed by one or more middle packets, followed by a second N-packet pattern, the processor may terminate one sequence at the end of the middle packets (which are assumed to be the final sub-sequence of the sequence), and begin another sequence at the beginning of the second N-packet pattern. (The value of N for the second N-packet pattern is not necessarily the same as the value of N for the first N-packet pattern.) 
     For example, sequence S 2  may include multiple sub-sequences of (for example) nine packets each, followed by a final packet of 3925 bytes. (Each of the sub-sequences may include, for example, eight packets of 8030 bytes each, followed by a ninth packet of 1561 bytes.) In response to observing this pattern, the processor may terminate sequence S 2  at the final packet of 3925 bytes, thus demarcating sequence S 2  from a subsequent file download over the same connection. 
     In some cases, the processor does not identify any features that appear to mark the beginning or end of a file transfer. In such cases, the processor may group all of the uploaded or downloaded packets in the connection into a single sequence. 
     In some cases, the processor may group the packets in a connection into multiple, mutually exclusive sets of sequences, i.e., the processor may group the packets in multiple different ways. In such cases, the processor may compute a level of confidence for each grouping, and then select the grouping with the highest level of confidence. The sequences per this grouping may then be stored in the aforementioned file-transfer database. Alternatively, the processor may store the sequences per each grouping, and then consider each grouping when processing any subsequent queries. 
     Estimating the File Sizes 
     Subsequently to extracting a sequence of packets from a connection as described above, the processor estimates the size of the file assumed to be carried by the sequence. To perform this estimation, the processor first “peels” the sequence of packets to the highest level possible, as described above in the Overview. The size of the remaining, unpeeled content of each packet (referred to below, for convenience, simply as “the size of the packet”) is then treated as the size of the file content carried by the packet. 
     Next, the processor sums the respective sizes of the packets in the sequence. In some embodiments, the processor then returns this sum as the estimate of the size of the file. In other embodiments, to estimate the size of the file, the processor divides the aforementioned sum by a predefined packet-size inflation divisor that is greater than one. (Equivalently, it may be said that the processor multiplies the sum by a factor, equivalent to the inverse of the aforementioned divisor, that is less than one.) In such embodiments, prior to estimating the size of a given file, the processor—either before or after extracting the sequence of packets—computes the relevant packet-size inflation divisor. 
     In some embodiments, the processor computes the divisor for a particular downloading device by identifying another sequence of packets downloaded by the device, computing the sum SUM_ 0  of the respective sizes of the packets in this sequence, positing that this sequence carried another file having a known size SZ_ 0 , and then computing the divisor by dividing SUM_ 0  by SZ_ 0 . 
     For example, the processor may first, using the IP address of the downloading device as described above, look up the phone number or email address of the user of the downloading device. Subsequently, using the phone number or email address, the processor may communicate a test file, which has a known size, to the downloading device, so as to cause the downloading device to download a sequence of packets that carries the test file. The processor may then identify this sequence in report  38 , and compute the total size of the sequence. The processor may then compute the divisor by dividing this size by the known size of the test file. 
     For example, it will be assumed that sequence S 2  is downloaded by a device D 2  having the IP address 109.65.90.65. It will further be assumed that the sum of the packet sizes in sequence S 2  is SUM 2 . In such a case, to estimate the size of the file carried by sequence S 2 , the processor may first communicate a test file having a size of SZ_ 0  to device D 2 . The processor may then identify the sequence of packets carrying the test file, and compute the total size of this sequence as SUM_ 0 . Subsequently, the processor may calculate the packet-size inflation divisor for downloads performed by device D 2  as SUM_ 0 /SZ_ 0 . The processor may then estimate the size of the file carried by sequence S 2  as SUM 2 /(SUM_ 0 /SZ_ 0 ). 
     Alternatively, the processor may (e.g., using an external device, such as a mobile phone, that is controlled by the processor) download a particular file that appears to be viral. The processor may then ascertain the size SZ_ 0  of the apparently-viral file. Subsequently, by monitoring the encrypted traffic over network  22  and computing an estimated file size for each identified download, the processor may posit that the apparently-viral file was indeed downloaded by multiple other devices. (For example, each sequence of downloaded packets having a total size sufficiently close to SZ_ 0  may be posited to carry the apparently-viral file.) The processor may further ascertain that device D 2  downloaded a sequence of packets having a total size SUM_ 0  that is similar to SZ_ 0 . In response thereto, and in response to the number of other devices that were posited to have downloaded the file, the processor may posit that device D 2  downloaded the file. (In other words, the processor may posit that device D 2  downloaded the file with a likelihood that is an increasing function of the number of other devices that were posited to have downloaded the file.) Hence, the processor may calculate the divisor for downloads performed by device D 2  as SUM_ 0 /SZ_ 0 . 
     Similarly, the processor may calculate the divisor for a particular uploading device by requesting that the uploading device communicate any test file to the processor. Subsequently to receiving the test file, the processor may ascertain the size SZ_ 0  of the test file, identify the corresponding sequence of packets uploaded by the uploading device and calculate the total size SUM_ 0  of this sequence, and then calculate the packet-size inflation divisor as the quotient of SUM_ 0  and SZ_ 0 . Thus, for example, assuming that sequence S 1  is uploaded by a device D 1  having the IP address 10.0.20.128 and that the sum of the packets in sequence S 1  is SUM 1 , the processor may (i) request that device D 1  communicate any test file to the processor, (ii) ascertain the size SZ_ 0  of the test file, (iii) compute the total size SUM_ 0  of the uploaded sequence carrying the test file, (iv) calculate the packet-size inflation divisor for uploads performed by device D 1  as SUM_ 0 /SZ_ 0 , and hence (v) estimate the size of the file carried by sequence S 1  as SUM 1 /(SUM_ 0 /SZ_ 0 ). 
     Alternatively, the processor may first learn the divisors for various different sets of file-transfer properties. Each divisor may then be stored in association with the set of properties to which the divisor corresponds. Subsequently, given a sequence of packets assumed to transfer a file, the processor may infer one or more properties of the assumed file transfer from the connection to which the sequence belongs and/or from other connections belonging to the same client. The processor may then select the appropriate divisor from the multiple predefined divisors that were stored, based on an association between this divisor and the properties. Examples of relevant file-transfer properties include the type of client performing the transfer, the operating system running on the client, and the encryption protocol used by the client to perform the transfer. (In some embodiments, the operating system is identified from Hypertext Transfer Protocol (HTTP) headers that include a user agent.) 
     For example, with reference again to  FIG. 3 , the processor may initially construct a lookup table that specifies the divisor for uploaded files for various device types, operating systems, and encryption protocols. Subsequently, the processor may deduce (e.g., using the P0f TCP/IP stack fingerprinting tool) that sequence S 1  was uploaded from a Samsung Galaxy S5 device running the Android 5.1.1 operating system, using the Transport Layer Security (TLS) protocol. The processor may then find the appropriate divisor for sequence S 1  by looking up these parameters in the lookup table. Subsequently, the processor may estimate the size of the file carried by sequence S 1 , by dividing the total size of the sequence by the divisor. 
     Alternatively, the processor may compute the divisor for a particular device and for a particular file-transfer directionality, based on observing a large number of file exchanges in which the device performed a file-transfer with the file-transfer directionality. In particular, the processor may assume a particular distribution of the divisor (for the opposite directionality) for the other devices with which the file exchanges were performed, and, based on the distribution, compute the divisor for the device. 
     For example, to compute the divisor for sequence S 1 , the processor may identify other sequences of packets that were uploaded to the server by device D 1  and carry respective other uploaded files, and compute the respective total sizes of these other uploaded sequences. For each of these other uploaded sequences, the processor may identify, with a particular level of confidence, a strongly corresponding downloaded sequence that was downloaded from the server by another client, and compute the size thereof (The downloaded sequences may be identified based on the proximity in time of the sequence to the uploaded sequence, based on similarity in size (ignoring packet size inflation, or using rough estimates for the inflation divisors), based on a relatively high relatedness score between the two devices, and/or based on any of the other factors mentioned above.) The processor may then compare the sizes of the uploaded sequences to the sizes of the downloaded sequences. Based on this comparison, and based on an assumed distribution of the divisor for downloads, the processor may compute the packet-size inflation divisor for uploads performed by device D 1 . 
     For example, by observing a large number of uploads and downloads of files of known sizes for various device types, operating systems, and encryption protocols, the processor may ascertain that the packet-size inflation divisor for both uploads and downloads is distributed (e.g., uniformly or normally) between 1.002 and 1.441. Assuming this distribution, if the sizes of the sequences uploaded by device D 1  are generally greater than the sizes of the corresponding downloaded sequences, the processor may compute, for the uploading device, a packet-size inflation divisor that is closer to 1.441 than to 1.002. Conversely, if the sizes of the uploaded sequences are generally less than the sizes of the corresponding downloaded sequences, the processor may compute, for device D 1 , a packet-size inflation divisor that is closer to 1.002 than to 1.441. 
     In some embodiments, as assumed below, the processor computes the estimated file size as a probability distribution. For example, the processor may calculate the packet-size inflation divisor as a probability distribution, and then calculate an estimated file-size distribution by dividing the aforementioned sum of packet sizes by this probability distribution. 
     As noted above, in some cases, the processor is not certain that the sequence actually carries a file. In such cases, subsequently to estimating the file size, the processor may compare the estimated file size to a predefined threshold. In response to the estimated file size exceeding the predefined threshold, the processor may assume that the sequence indeed carries a file. In response thereto, the processor stores the estimated file size, along with other relevant properties of the sequence, in a database, as further described below with reference to  FIG. 4 . In the event that the processor knows a priori that the sequence carries a file, e.g., by virtue of the processor having identified the IP address of a messaging-application server known to service file exchanges, the processor may store the properties of the sequence even without first comparing the estimated file size to the predefined threshold. 
     Examples of relevant properties of a sequence that may be stored include the time at which the sequence was communicated and the direction of the sequence (i.e., whether the sequence was uploaded or downloaded). Other examples include properties of the device that performed the transfer, such as the device type, the operating system running on the device, the encryption protocol used by the device, or an identifier, such as an IP address, of the device. Alternatively or additionally, the processor may store identifying information for the user who performed the file transfer, and/or information relating to the application via which the file was transferred. Examples of such information include the IP address or certificate of the server to or from which the file was transferred, and the name of the application or the class of applications (e.g., “email”) used for performing the transfer. Alternatively or additionally, the processor may store the level of confidence with which the processor posited that the sequence carries a file, and/or the level of confidence with which the processor estimated the file size. 
     In some embodiments, the processor, using machine-learned techniques, identifies, in report  38 , instances in which a file was forwarded by the sending of a metadata link. The relevant parameters of each such instance are stored in the aforementioned database. 
     Finding Corresponding File Transfers 
     As described above with reference to  FIG. 2 , the processor is configured to find corresponding pairs of file transfers. For example, given a download of a file performed by a second user, the processor may find a strongly corresponding upload performed by a first user, which, together with the download performed by the second user, constitutes an exchange of the file between the two users. Alternatively or additionally, the processor may find other downloads of the file that occurred at approximately the same time as the given download, and/or other uploads or downloads of the file that occurred at previous times. 
     In general, the likelihood for each candidate corresponding file transfer is a function of the similarity between the respective estimated file sizes of the transfers. Thus, to evaluate the likelihood that a given pair of given file transfers correspond to one another, the processor computes a measure of similarity between the two estimates. For example, the processor may compute the absolute difference between the estimates, and/or compute a ratio between the estimates. The processor then computes the likelihood of correspondence as a function of this similarity measure. 
     Typically, the likelihood of correspondence is also a function of the “uniqueness” of the estimated file sizes; the more unique the file sizes are, the greater the likelihood of correspondence. (Typically, larger file sizes are more unique than, i.e., have smaller frequencies than, smaller file sizes.) Thus, given the estimated file size for one of the transfers, the processor may identify the frequency with which files having the estimated file size are communicated over the network. For example, the processor may look up the estimated file size in a histogram, which the processor may construct, in advance, from data from a large number of monitored file transfers. The processor may then posit the correspondence with a likelihood that is a decreasing function of the frequency. 
     In some cases, the likelihood of correspondence—and especially the likelihood of strong correspondence—may also be a function of the proximity in time between the two transfers. 
     Thus, for example, the processor may posit, with a relatively high likelihood, that sequence S 1  and sequence S 2  carry the same file, in response to the estimated file size for sequence S 1  being relatively large, and also relatively close to the estimated file size for sequence S 2 . If the receipt time of the first packet of sequence S 2  is within a relatively short duration of the receipt time of the last packet of sequence S 1 , the processor may further posit, with a relatively high likelihood, that the uploader of sequence S 1  communicated the file to the downloader of sequence S 2 . 
     Alternatively or additionally to the factors described above, the likelihood of strong correspondence between an upload and a download may be a function of the relatedness score for the performer of the upload and the performer of the download. The processor may thus implement a reinforcement feedback loop, whereby positing a file exchange between two devices raises the likelihood of positing another file exchange between the devices in the future. 
     As noted above, in some embodiments, the processor identifies the sending of metadata links. As further described below, such instances may be treated as candidate corresponding file transfers for a given download, in response to (i) the metadata link having been sent prior to the given download, and (ii) the user who sent the link having downloaded a similarly-sized file within a predefined period of time prior to the sending of the link. 
     Example Algorithms 
     Processor  30  typically executes several different algorithms in parallel to each other. One of these algorithms, which is run continuously in the background as the monitored traffic is received, identifies file transfers in the traffic and stores the properties of these file transfers in a file-transfer database. Another algorithm, which is also run continuously in the background, identifies strongly corresponding file transfers in the file-transfer database and maintains a relationship database based on the identified strongly corresponding file transfers. Yet another algorithm handles queries from users of the system, using information in both the file-transfer database and the relationship database. These algorithms are hereby described with reference to  FIGS. 4-7 , respectively. 
     First, reference is made to  FIG. 4 , which is a flow diagram for an algorithm  40  for maintaining a file-transfer database, in accordance with some embodiments of the present disclosure. 
     Per algorithm  40 , the processor repeatedly checks, at a first checking step  41 , whether the communication traffic received from tap  32  ( FIG. 1 ) includes any connections that have not yet been processed by the processor. In response to identifying at least one unprocessed connection, the processor selects the next unprocessed connection at a connection-selecting step  42 . Subsequently, at a connection-examining step  44 , the processor ascertains whether the selected connection possibly carries at least one file. If not, the processor ignores the connection, and returns to first checking step  41 . Otherwise, the processor continues to process the connection. 
     In continuing to process the connection, the processor peels the packets in the connection, at a peeling step  46 . Next, at a grouping step  48 , the processor groups the packets in the connection into one or more packet sequences assumed to carry respective files, as described above with reference to  FIG. 3 . 
     Subsequently, at a sequence-selecting step  50 , the processor selects one of the separated packet sequences. Next, at a size-calculating step  52 , the processor calculates the size of the selected sequence by summing the number of bytes in the sequence. This size may then be adjusted (in particular, reduced) at a size-adjusting step  54 , to account for any unpeeled headers. 
     Next, at a divisor-selecting step  56 , the processor selects the appropriate probability distribution for the packet-size inflation divisor, as described above with reference to  FIG. 3 . Subsequently, using the selected probability distribution, the processor, at a file-size-computing step  58 , computes a probability distribution for the size of the file assumed to be carried by the selected sequence of packets. (Next, the processor may compare a statistical measure, such as the mean, of this probability distribution to a predefined threshold, in order to verify that the sequence has a significant likelihood of carrying a file. If the threshold is not passed, the processor may skip to second checking step  62 , described below.) 
     Subsequently, the processor stores information pertaining to the file transfer in the file-transfer database, at an information-storing step  60 . Such information may include, for example, the time of the transfer, the source and destination IP addresses for the transfer, the computed file-size probability distribution, any identifiers of the file-transferring device or of the user thereof (e.g., a Mobile Station International Subscriber Directory Number (MSISDN) of the device or the name of the user), and/or information relating to the application or class of applications via which the file was transferred. 
     Subsequently to storing the file-transfer information, the processor checks, at a second checking step  62 , whether any unprocessed packet sequences in the connection remain. If yes, the processor returns to sequence-selecting step  50 , and selects the next unprocessed packet sequence. Otherwise, the processor returns to first checking step  41 . 
     As noted above, while processing each connection, the processor may further identify the transfer of one or more metadata links. Information relating to each identified metadata-link transfer may be stored in the file-transfer database at information-storing step  60 . Such information may include, for example, any of the example information items listed above, except for a file-size probability distribution, which is not applicable to a metadata-link transfer. 
     Reference is now made to  FIG. 5 , which is a flow diagram for an algorithm  64  for maintaining a relationship database, in accordance with some embodiments of the present disclosure. 
     Per algorithm  64 , the processor repeatedly checks, at a third checking step  66 , whether the file-transfer database contains any downloads that have not yet been processed by the processor. In response to identifying at least one unprocessed download, the processor selects the next unprocessed download from the file-transfer database, at a download-selecting step  68 . Subsequently, the processor, at a first frequency-lookup step  70 , looks up the frequency of the most likely file size of the selected download, which may be the mean of the file-size probability distribution stored for the download. 
     Subsequently, at a candidate-upload-retrieving step  72 , the processor retrieves any candidate strongly corresponding uploads from the file-transfer database. This retrieval is typically based on the difference in time (“Δ(TIME)”) and the difference in file size (“Δ(SIZE)”) between the uploads and the selected download. (Δ(SIZE) may be quantified using any suitable statistical distance measure for probability distributions.) For example, the processor may require that, for any candidate, Δ(TIME) be less than a first predefined threshold and Δ(SIZE) be less than a second predefined threshold. In addition, the processor requires that each candidate be performed using the same application, or class of applications, as was the selected download. For example, if the selected download was performed using WhatsApp, the processor retrieves uploads performed using WhatsApp. If the specified download was performed using email, the processor retrieves uploads performed using any email application. 
     Subsequently to retrieving the candidate strongly corresponding uploads, the processor, at a candidate-upload-selecting step  74 , selects one of the candidates. Next, at a score-retrieving step  76 , the processor retrieves, from the relationship database, the current relatedness score between the uploader of the selected candidate strongly corresponding upload and the downloader of the selected download (provided this score exists in the relationship database). Subsequently, the processor, at a first likelihood-computing step  78 , computes a likelihood of strong correspondence between the selected candidate and the selected download. This computation is based on (a) Δ(TIME), (b) Δ(SIZE), (c) the retrieved relatedness score, and (d) the frequency of the most likely file size of the selected download. (In particular, the likelihood of strong correspondence is generally a decreasing function of (a), (b), and (d), and is an increasing function of (c).) 
     Subsequently, the processor checks, at a fourth checking step  80 , whether any unprocessed candidate strongly corresponding uploads remain. If yes, the processor returns to candidate-upload-selecting step  74 , and selects the next unprocessed candidate upload. 
     In response to ascertaining, at fourth checking step  80 , that no unprocessed candidates remain, the processor, at a score-adjusting step  82 , adjusts the relatedness scores in the relationship database, based on the likelihoods of correspondence. For example, for any likelihood exceeding a predefined threshold, the processor may increase the corresponding score. (The “corresponding score” referred to in the sentence above is the score for the pair of users consisting of (a) the user who performed the selected download and (b) the user who performed the candidate strongly corresponding upload.) 
     Subsequently to adjusting the relatedness scores, the processor returns to third checking step  66 . 
     Reference is now made to  FIG. 6 , which is a flow diagram for an algorithm  84  for handling a query, in accordance with some embodiments of the present disclosure. 
     Algorithm  84  begins with a first query-receiving step  86 , at which the processor receives, from a user of the system, a query requesting a list of candidate strongly corresponding file transfers for a specified download. 
     In some cases, the specified download is retrieved, by the user, from the file-transfer database prior to the submission of the query. Alternatively, the specified download may be identified by the user without using the file-transfer database, in which case the download might not be stored in the file-transfer database. 
     Subsequently to receiving the query, the processor, at a second frequency-lookup step  88 , looks up the frequency of the most likely file size of the specified download, provided that the most likely file size of the specified download is stored in the file-transfer database or is indicated explicitly or implicitly in the query. If the most likely file size is not available, second frequency-lookup step  88  is omitted. 
     Subsequently, the processor, at a first candidate-transfer-retrieving step  90 , retrieves candidate strongly corresponding file transfers from the file-transfer database. In particular, the processor retrieves each file transfer (i.e., each upload and download) for which Δ(TIME) and Δ(SIZE), relative to the specified download, are sufficiently small, and which was performed using the same application, or class of applications, as was the specified download. The processor designates each retrieved file transfer as a candidate strongly corresponding file transfer. 
     In some embodiments, the processor further retrieves each metadata-link transfer, performed using the same application or class of applications as was used for the specified download, for which Δ(TIME) is sufficiently small, provided that the metadata-link transfer is preceded, within a predefined period of time, by one or more downloads, performed by the same user using the same application or class of applications, for which Δ(SIZE), relative to the specified download, is sufficiently small. The processor further defines one or more candidate strongly corresponding file transfers, each of these candidates consisting of the metadata-link transfer in combination with a different respective one of the preceding downloads. In other words, the processor treats the metadata-link transfer as if it were an upload of each of the previously-downloaded files. 
     For example, it will be assumed that a metadata-link transfer was performed by a particular user immediately prior to the specified download (i.e., prior to the specified download with a sufficiently small Δ(TIME)), and that three downloads whose sizes are within Δ(SIZE) of the size of the specified download were performed, by the particular user, prior to the metadata-link transfer within a predefined period of time. In such a case, the processor may define three candidate strongly corresponding file transfers, each consisting of the metadata-link transfer in combination with a different respective one of the three downloads. 
     Following first candidate-transfer-retrieving step  90 , the processor selects one of the candidate strongly corresponding file transfers at a first candidate-transfer-selecting step  92 . Subsequently, at a relatedness-score-retrieving step  94 , the processor retrieves, from the relationship database, the relatedness score between the performer of the selected candidate strongly corresponding file transfer and the downloader of the specified download. (If no such score is stored in the relationship database, the processor assumes a default minimum value for the score.) Next, at a second likelihood-computing step  96 , the processor computes the likelihood of strong correspondence between the specified download and the selected candidate strongly corresponding file transfer. This computation is based on Δ(TIME), Δ(SIZE), the relatedness score, and the frequency of the most likely file size (if available). (For a candidate consisting of a metadata-link transfer in combination with a preceding download, Δ(SIZE) is the difference between the size of the specified download and the size of the preceding download.) 
     Following second likelihood-computing step  96 , the processor checks, at a fifth checking step  98 , whether any unprocessed candidate strongly corresponding file transfers remain. If yes, the processor returns to first candidate-transfer-selecting step  92 , and selects the next candidate. 
     Following the processing of all of the candidate strongly corresponding file transfers, the processor proceeds to a first top-candidate-outputting step  100 . At first top-candidate-outputting step  100 , the processor outputs those candidate strongly corresponding uploads having the highest likelihoods of strong correspondence from among all of the candidate strongly corresponding uploads, along with those candidate strongly corresponding downloads having the highest likelihoods of strong correspondence from among all of the candidate strongly corresponding downloads. (In this context and in the context of  FIG. 7 , “uploads” may include metadata-link transfers in combination with preceding downloads.) Each candidate is output along with its likelihood of strong correspondence. 
     The number “N” of top uploads that are output, along with the number “M” of top downloads that are output, may be predefined or specified in the query. Alternatively, these numbers may be calculated on the fly in response to the likelihoods of strong correspondence. For example, the processor may set N such that the likelihood of each of the top uploads is greater than a threshold likelihood, which may be defined as an absolute number or set to a particular percentile of the likelihoods for the candidate uploads. Similarly, the processor may set M such that the likelihood of each of the top downloads is greater than another threshold likelihood. 
     Reference is now made to  FIG. 7 , which is a flow diagram for another algorithm  102  for handling a query, in accordance with some embodiments of the present disclosure. 
     Algorithm  102  begins with a second query-receiving step  104 , at which the processor receives a query from a user of the system. This query is similar to the query of  FIG. 6 , except that this query requests any candidate corresponding file transfers—rather than only strongly corresponding file transfers—for the specified download. Optionally, the user may request that the candidate corresponding file transfers be presented in a timeline, such as to facilitate tracking the distribution of the downloaded file and identifying the original distributor of the file. 
     Subsequently to receiving the query, the processor performs second frequency-lookup step  88 , as described above with reference to  FIG. 6 . Next, at a second candidate-transfer-retrieving step  106 , the processor retrieves candidate corresponding file transfers from the file-transfer database, based on Δ(TIME) and Δ(SIZE). (In this case, the candidates are not restricted to the same application or class of applications used to perform the specified download.) 
     As described above with reference to  FIG. 6 , in some embodiments, the processor designates transfers of metadata links, in combination with preceding downloads, as candidate corresponding file transfers. To reduce the number of false candidates, however, the processor may apply stricter retrieval criteria for transfers of metadata links than for file transfers. For example, the processor may require that Δ(TIME) (which, in the case of a metadata-link transfer in combination with a preceding download, is the difference between the time of the specified download and the time of the metadata-link transfer) be less for metadata-link transfers than for file transfers. Alternatively or additionally, the processor may require that Δ(SIZE) (which, in the case of a metadata-link transfer in combination with a preceding download, is the difference between the size of the specified download and the size of the preceding download) be less for metadata-link transfers than for file transfers. 
     Subsequently, at a second candidate-transfer-selecting step  108 , the processor selects one of the candidate corresponding file transfers. Next, the processor computes the likelihood of correspondence for the selected candidate at either a third likelihood-computing step  112  or a fourth likelihood-computing step  114 . In third likelihood-computing step  112 , the computation is based on Δ(SIZE) and the frequency of the most likely file size (if available). In fourth likelihood-computing step  114 , on the other hand, the computation is also based on Δ(TIME). At a query-evaluating step  110 , the processor decides which of the two steps to perform: if the query requests a timeline of corresponding file transfers, the processor performs third likelihood-computing step  112 ; otherwise, the processor performs fourth likelihood-computing step  114 . 
     (Since a timeline already conveys the respective chronological distances between the specified download and the candidate corresponding file transfers, it is assumed that the user does not want the likelihoods to take Δ(TIME) into account. Hence, third likelihood-computing step  112  does not take Δ(TIME) into account. Nevertheless, to reduce the number of unrelated file transfers included in the timeline, second candidate-transfer-retrieving step  106  takes Δ(TIME) into account.) 
     Following third likelihood-computing step  112  or fourth likelihood-computing step  114 , the processor checks, at a sixth checking step  116 , whether any unprocessed candidates remain. If yes, the processor returns to second candidate-transfer-selecting step  108  and selects the next candidate. Otherwise, the processor provides an output to the user, e.g., by displaying the output on display  36  ( FIG. 1 ). In particular, if a timeline was requested, the processor, at a timeline-outputting step  118 , outputs a timeline of the candidate corresponding file transfers (typically, all of the candidates retrieved at second candidate-transfer-retrieving step  106 ) with their respective likelihoods of correspondence. Otherwise, the processor, at a second top-candidate-outputting step  120 , outputs those candidate corresponding uploads having the highest likelihoods of correspondence from among all of the candidate corresponding uploads, along with those candidate corresponding downloads having the highest likelihoods of strong correspondence from among all of the candidate corresponding downloads. Each candidate is output with its likelihood of correspondence. The number “N” of top uploads that are output, along with the number “M” of top downloads that are output, may be determined as described above with reference to  FIG. 6 . 
     In some cases, the user may request only corresponding downloads or only corresponding uploads. In such cases, the processor may execute algorithm  102  as described above, but retrieve only candidate corresponding downloads or only candidate corresponding uploads at second candidate-transfer-retrieving step  106 . 
     It is noted that alternatively to using Δ(SIZE) in each of the algorithms in  FIGS. 5-7 , any other suitable measure of similarity between the sizes, such as a ratio between the sizes, may be used. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of embodiments of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.