Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 1 of 17
`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 1 of 17
`
`EXHIBIT 4
`EXHIBIT 4
`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 2 of 17
`111111
`1111111111111111111111111111111111111111111111111111111111111
`US008141154B2
`
`c12) United States Patent
`Gruzman et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,141,154 B2
`Mar.20,2012
`
`(54) SYSTEM AND METHOD FOR INSPECTING
`DYNAMICALLY GENERATED EXECUTABLE
`CODE
`
`(75)
`
`Inventors: David Gruzman, Ramat Gan (IL);
`Yuval Ben-Itzhak, Tel Aviv (IL)
`
`(73) Assignee: Finjan, Inc. (IL)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 12/814,584
`
`(22) Filed:
`
`Jun.14,2010
`
`(65)
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`Prior Publication Data
`
`US 2010/0251373 Al
`
`Sep. 30, 2010
`
`Int. Cl.
`G06F 11100
`(2006.01)
`U.S. Cl. ............... 726/22; 726/23; 726/24; 713/188
`Field of Classification Search ........................ None
`See application file for complete search history.
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,359,659 A
`10/1994 Rosenthal ....................... 726/24
`5,974,549 A
`10/1999 Golan ............................. 726/23
`1111999 Ji
`.................................... 726113
`5,983,348 A
`6,092,194 A
`7/2000 Touboul .......................... 726/24
`6,167,520 A
`12/2000 Touboul .......................... 726/23
`6,272,641 B1
`8/2001 Ji
`.................................... 726/24
`6,934,857 B1
`8/2005 Bartleson eta!. ................. 726/5
`6,965,968 B1
`1112005 Touboul ........................ 7111118
`7,203,934 B2
`4/2007 Souloglou et al ............. 717/146
`7,287,279 B2
`10/2007 Bertman eta!. ................ 726/23
`7,313,822 B2
`12/2007 Ben-Itzhak ..................... 726/24
`7,739,682 B1 *
`6/2010 Badenell ....................... 717/174
`7,836,504 B2 *
`1112010 Ray eta!. ........................ 726/24
`
`200110005889 A1 *
`2002/0116635 A1
`2004/0133796 A1
`2004/0153644 A1
`2004/0158729 A1
`2005/0108562 A1
`2005/0149749 A1 *
`2006/0015940 A1
`2006/0161981 A1
`2007/0016948 A1
`* cited by examiner
`
`6/2001 Albrecht ....................... 713/201
`8/2002 Sheymov ........................ 726/24
`7/2004 Cohen eta!. .................... 726/24
`8/2004 McCorkendale eta!. .... 713/156
`8/2004 Szor .............................. 713/190
`5/2005 Khazan eta!. .................. 726/23
`7/2005 Van Brabant ................. 713/200
`112006 Zamir et a!.
`.................... 726/22
`7/2006 Sheth eta!. ..................... 726/22
`112007 Dubrovsky eta!. ............. 726/22
`
`Primary Examiner- Ponnoreay Pich
`(74) Attorney, Agent, or Firm- Dawn-Marie Bey; King &
`Spalding LLP
`
`ABSTRACT
`(57)
`A method for protecting a client computer from dynamically
`generated malicious content, including receiving at a gateway
`computer content being sent to a client computer for process(cid:173)
`ing, the content including a call to an original function, and
`the call including an input, modifying the content at the
`gateway computer, including replacing the call to the original
`function with a corresponding call to a substitute function, the
`substitute function being operational to send the input to a
`security computer for inspection, transmitting the modified
`content from the gateway computer to the client computer,
`processing the modified content at the client computer, trans(cid:173)
`mitting the input to the security computer for inspection when
`the substitute function is invoked, determining at the security
`computer whether it is safe for the client computer to invoke
`the original function with the input, transmitting an indicator
`of whether it is safe for the client computer to invoke the
`original function with the input, from the security computer to
`the client computer, and invoking the original function at the
`client computer with the input, only if the indicator received
`from the security computer indicates that such invocation is
`safe. A system and a computer-readable storage medium are
`also described and claimed.
`
`12 Claims, 5 Drawing Sheets
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`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 3 of 17
`
`U.S. Patent
`
`Mar.20,2012
`
`Sheet 1 of 5
`
`US 8,141,154 B2
`
`I CONTENT PROCESSOR I
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`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 4 of 17
`
`U.S. Patent
`
`Mar.20,2012
`
`Sheet 2 of 5
`
`US 8,141,154 B2
`
`I
`INPUT. CUENT ID
`F====~\=====~====~v'l SECURITY RECEIVER
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`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 5 of 17
`
`U.S. Patent
`
`Mar.20,2012
`
`Sheet 3 of 5
`
`US 8,141,154 B2
`
`r- GATEWAYCOMPUTER --+j
`
`1-- CLIENT COMPUTER -.J
`
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`
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`
`SET INDICATOR • TRUE
`
`SET INDICATOR c FALSE
`
`FIG. 3
`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 6 of 17
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`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 7 of 17
`
`U.S. Patent
`
`Mar.20,2012
`
`Sheet 5 of 5
`
`US 8,141,154 B2
`
`r- CLIENT COMPUTER
`
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`
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`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 8 of 17
`
`US 8,141,154 B2
`
`1
`SYSTEM AND METHOD FOR INSPECTING
`DYNAMICALLY GENERATED EXECUTABLE
`CODE
`
`FIELD OF THE INVENTION
`
`The present invention relates to computer security, and
`more particularly to protection against malicious code such as
`computer viruses.
`
`BACKGROUND OF THE INVENTION
`
`Computer viruses have been rampant for over two decades
`now. Computer viruses generally come in the form of execut(cid:173)
`able code that performs adverse operations, such as modifY(cid:173)
`ing a computer's operating system or file system, damaging a
`computer's hardware or hardware interfaces, or automati(cid:173)
`cally transmitting data from one computer to another. Gener(cid:173)
`ally, computer viruses are generated by hackers willfully, in
`order to exploit computer vulnerabilities. However, viruses
`can also arise by accident due to bugs in software applica(cid:173)
`tions.
`Originally computer viruses were transmitted as execut(cid:173)
`able code inserted into files. As each new virus was discov(cid:173)
`ered, a signature of the virus was collected by anti-virus
`companies and used from then on to detect the virus and
`protect computers against it. Users began routinely scanning
`their file systems using anti-virus software, which regularly
`updated its signature database as each new virus was discov(cid:173)
`ered.
`Such anti -virus protection is referred to as "reactive", since
`it can only protect in reaction to viruses that have already been
`discovered.
`With the advent of the Internet and the ability to run execut(cid:173)
`able code such as scripts within Internet browsers, a new type
`of virus formed; namely, a virus that enters a computer over
`the Internet and not through the computer's file system. Such
`Internet viruses can be embedded within web pages and other
`web content, and begin executing within an Internet browser
`as soon as they enter a computer. Routine file scans are not
`able to detect such viruses, and as a result more sophisticated
`anti-virus tools had to be developed.
`Two generic types of anti-virus applications that are cur(cid:173)
`rently available to protect against such Internet viruses are (i)
`gateway security applications, and (ii) desktop security appli(cid:173)
`cations. Gateway security applications shield web content
`before the content is delivered to its intended destination
`computer. Gateway security applications scan web content,
`and block the content from reaching the destination computer
`if the content is deemed by the security application to be
`potentially malicious. In distinction, desktop security appli(cid:173)
`cations shield against web content after the content reaches its
`intended destination computer.
`Moreover, in addition to reactive anti-virus applications,
`that are based on databases of known virus signatures,
`recently "proactive" antivirus applications have been devel(cid:173)
`oped. Proactive anti-virus protection uses a methodology
`known as "behavioral analysis" to analyze computer content
`for the presence of viruses. Behavior analysis is used to auto(cid:173)
`matically scan and parse executable content, in order to detect
`which computer operations the content may perform. As
`such, behavioral analysis can block viruses that have not been
`previously detected and which do not have a signature on
`record, hence the name "proactive".
`Assignee's U.S. Pat. No. 6,092,194 entitled SYSTEM
`AND METHOD FOR PROTECTING A COMPUTER AND
`A NETWORK FROM HOSTILE DOWNLOAD ABLES, the
`
`2
`contents of which are hereby incorporated by reference,
`describes gateway level behavioral analysis. Such behavioral
`analysis scans and parses content received at a gateway and
`generates a security profile for the content. A security profile
`is a general list or delineation of suspicious, or potentially
`malicious, operations that executable content may perform.
`The derived security profile is then compared with a security
`policy for the computer being protected, to determine
`whether or not the content's security profile violates the com-
`10 puter's security policy. A security policy is a general set of
`simple or complex rules, that may be applied logically in
`series or in parallel, which determine whether or not a specific
`operation is permitted or forbidden to be performed by the
`content on the computer being protected. Security policies are
`15 generally configurable, and set by an administrator of the
`computer that is being protected.
`Assignee's U.S. Pat. No. 6,167,520 entitled SYSTEM
`AND METHOD FOR PROTECTING A CLIENT DURING
`RUNTIME FROM HOSTILE DOWNLOADABLES, the
`20 contents of which are hereby incorporated by reference,
`describes desktop level behavioral analysis. Desktop level
`behavioral analysis is generally implemented during run(cid:173)
`time, while a computer's web browser is processing web
`content received over the Internet. As the content is being
`25 processed, desktop security applications monitor calls made
`to critical systems of the computer, such as the operating
`system, the file system and the network system. Desktop
`security applications use hooks to intercept calls made to
`operating system functions, and allow or block the calls as
`30 appropriate, based on the computer's security policy.
`Each of the various anti-virus technologies, gateway vs.
`desktop, reactive vs. proactive, has its pros and cons. Reactive
`anti-virus protection is computationally simple and fast; pro(cid:173)
`active virus protection is computationally intensive and
`35 slower. Reactive anti-virus protection cannot protect against
`new "first-time" viruses, and carmot protect a user if his
`signature file is out of date; proactive anti -virus protection can
`protect against new "first-time" viruses and do not require
`regular downloading of updated signature files. Gateway
`40 level protection keeps computer viruses at a greater distance
`from a local network of computers; desktop level protection is
`more accurate. Desktop level protection is generally available
`in the consumer market for hackers to obtain, and is suscep(cid:173)
`tible to reverse engineering; gateway level protection is not
`45 generally available to hackers.
`Reference is now made to FIG. 1, which is a simplified
`block diagram of prior art systems for blocking malicious
`content, as described hereinabove. The topmost system
`shown in FIG. 1 illustrates a gateway level security applica-
`50 tion. The middle system shown in FIG. 1 illustrates a desktop
`level security application, and the bottom system shown in
`FIG. 1 illustrates a combined gateway+desktop level security
`application.
`The topmost system shown in FIG. 1 includes a gateway
`55 computer 105 that receives content from the Internet, the
`content intended for delivery to a client computer 110. Gate(cid:173)
`way computer 105 receives the content over a communication
`channel120, and gateway computer communicates with cli(cid:173)
`ent computer 110 over a communication charmel125. Gate-
`60 way computer 105 includes a gateway receiver 135 and a
`gateway transmitter 140. Client computer 110 includes a
`client receiver 145. Client computer generally also has a
`client transmitter, which is not shown.
`Client computer 110 includes a content processor 170,
`65 such as a conventional web browser, which processes Internet
`content and renders it for interactive viewing on a display
`monitor. Such Internet content may be in the form of execut-
`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 9 of 17
`
`US 8,141,154 B2
`
`4
`tion, since the malicious JavaScript is not present in the con(cid:173)
`tent prior to run-time. A content inspector will only detect the
`presence of a call to Document. write() with input text that is
`yet unknown. If such a content inspector were to block all
`calls to Document. write() indiscriminately, then many harm(cid:173)
`less scripts will be blocked, since most of the time calls to
`Document.write( ) are made for dynamic display purposes
`only.
`U.S. Pat. Nos. 5,983,348 and 6,272,641, both to Ji, describe
`10 reactive client level content inspection, that modifies down(cid:173)
`loaded executable code within a desktop level anti-virus
`application. However, such inspection can only protect
`against static malicious content, and cannot protect against
`dynamically generated malicious content.
`Desktop level run-time behavioral analysis has a chance of
`shielding a client computer against dynamically generated
`malicious code, since such code will ultimately make a call to
`an operating system function. However, desktop anti-virus
`protection has a disadvantage of being widely available to the
`20 hacker community, which is always eager to find vulnerabili(cid:173)
`ties. In addition, desktop anti -virus protection has a disadvan(cid:173)
`tage of requiring installation of client software.
`As such, there is a need for a new form of behavioral
`analysis, which can shield computers from dynamically gen-
`25 erated malicious code without running on the computer itself
`that is being shielded.
`
`15
`
`SUMMARY OF THE DESCRIPTION
`
`3
`able code, JavaScript, VBScript, Java applets, ActiveX con(cid:173)
`trols, which are supported by web browsers.
`Gateway computer 105 includes a content inspector 17 4
`which may be reactive or proactive, or a combination of
`reactive and proactive. Incoming content is analyzed by con(cid:173)
`tent inspector 17 4 before being transmitted to client computer
`110. If incoming content is deemed to be malicious, then
`gateway computer 105 preferably prevents the content from
`reaching client computer 110. Alternatively, gateway com(cid:173)
`puter 105 may modifY the content so as to render it harmless,
`and subsequently transmit the modified content to client com(cid:173)
`puter 110.
`Content inspector 17 4 can be used to inspect incoming
`content, on its way to client computer 110 as its destination,
`and also to inspect outgoing content, being sent from client
`computer 110 as its origin.
`The middle system shown in FIG. 1 includes a gateway
`computer 105 and a client computer 110, the client computer
`110 including a content inspector 176. Content inspector 176
`may be a conventional Signature-based anti-virus applica(cid:173)
`tion, or a run-time behavioral based application that monitors
`run-time calls invoked by content processor 170 to operating
`system, file system and network system functions.
`The bottom system shown in FIG. 1 includes both a content
`inspector 174 at gateway computer 105, and a content inspec(cid:173)
`tor 176 at client computer 110. Such a system can support
`conventional gateway level protection, desktop level protec(cid:173)
`tion, reactive anti-virus protection and proactive anti-virus
`protection.
`As the hacker vs. anti-virus protection battle continues to
`wage, a newer type of virus has sprung forward; namely,
`dynamically generated viruses. These viruses are themselves
`generated only at run-time, thus thwarting conventional reac(cid:173)
`tive analysis and conventional gateway level proactive behav- 35
`ioral analysis. These viruses take advantage of features of
`dynamic HTML generation, such as executable code or
`scripts that are embedded within HTML pages, to generate
`themselves on the fly at runtime.
`For example, consider the following portion of a standard 40
`HTMLpage:
`
`30
`
`The present invention concerns systems and methods for
`implementing new behavioral analysis technology. The new
`behavioral analysis technology affords protection against
`dynamically generated malicious code, in addition to conven(cid:173)
`tiona! computer viruses that are statically generated.
`The present invention operates through a security com(cid:173)
`puter that is preferably remote from a client computer that is
`being shielded while processing network content. During
`run-time, while processing the network content, but before
`the client computer invokes a function call that may poten(cid:173)
`tially dynamically generate malicious code, the client com(cid:173)
`puter passes the input to the function to the security computer
`for inspection, and suspends processing the network content
`pending a reply back from the security computer. Since the
`input to the function is being passed at run-time, it has already
`45 been dynamically generated and is thus readily inspected by
`a content inspector. Referring to the example above, were the
`input to be passed to the security computer prior to run-time,
`it would take the form of indeterminate text; whereas the
`input passed during run-time takes the determinate form
`50 <SCRIPT>malicious JavaScript</SCRIPT>,
`which can readily be inspected. Upon receipt of a reply from
`the security computer, the client computer resumes process(cid:173)
`ing the network content, and knows whether to by-pass the
`function call invocation.
`To enable the client computer to pass function inputs to the
`security computer and suspend processing of content pending
`replies from the security computer, the present invention
`operates by replacing original function calls with substitute
`function calls within the content, at a gateway computer, prior
`60 to the content being received at the client computer.
`The present invention also provides protection against
`arbitrarily many recursive levels of dynamic generation of
`malicious code, whereby such code is generated via a series
`of successive function calls, one within the next.
`By operating through the medium of a security computer,
`the present invention overcomes the disadvantages of desktop
`anti-virus applications, which are available to the hacker
`
`<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
`Transitional/ /EN">
`<HTML>
`<SCRIPT LANGUAGE~"JavaScript">
`document.write("<h1>text that is generated at run-time</h1>");
`</SCRIPT>
`<BODY>
`</BODY>
`</HTML>
`
`The text within the <SCRIPT> tags is JavaScript, and
`includes a call to the standard function document.write( ),
`which generates dynamic HTML. In the example above, the 55
`function document.write() is used to generate HTML header
`text, with a text string that is generated at run-time. If the text
`string generated at run-time is of the form
`<SCRIPT>malicious JavaScript</SCRIPT>
`then the document.write( ) function will insert malicious
`JavaScript into the HTML page that is currently being ren(cid:173)
`dered by a web browser. In turn, when the web browser
`processes the inserted text, it will perform malicious opera(cid:173)
`tions to the client computer.
`Such dynamically generated malicious code cannot be 65
`detected by conventional reactive content inspection and con(cid:173)
`ventional gateway level behavioral analysis content inspec-
`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 10 of 17
`
`US 8,141,154 B2
`
`5
`community for exploit. Security applications embodying the
`present invention are concealed securely within managed
`computers.
`There is thus provided in accordance with a preferred
`embodiment of the present invention a method for protecting
`a client computer from dynamically generated malicious con(cid:173)
`tent, including receiving at a gateway computer content being
`sent to a client computer for processing, the content including
`a call to an original function, and the call including an input,
`modifYing the content at the gateway computer, including 10
`replacing the call to the original function with a correspond(cid:173)
`ing call to a substitute function, the substitute function being
`operational to send the input to a security computer for
`inspection, transmitting the modified content from the gate(cid:173)
`way computer to the client computer, processing the modified 15
`content at the client computer, transmitting the input to the
`security computer for inspection when the substitute function
`is invoked, determining at the security computer whether it is
`safe for the client computer to invoke the original function
`with the input, transmitting an indicator of whether it is safe 20
`for the client computer to invoke the original function with the
`input, from the security computer to the client computer, and
`invoking the original function at the client computer with the
`input, only if the indicator received from the security com(cid:173)
`puter indicates that such invocation is safe.
`There is further provided in accordance with a preferred
`embodiment of the present invention a system for protecting
`a client computer from dynamically generated malicious con(cid:173)
`tent, including a gateway computer, including a gateway
`receiver for receiving content being sent to a client computer 30
`for processing, the content including a call to an original
`function, and the call including an input, a content modifier
`for modifying the received content by replacing the call to the
`original function with a corresponding call to a substitute
`function, the substitute function being operational to send the 35
`input to a security computer for inspection, and a gateway
`transmitter for transmitting the modified content from the
`gateway computer to the client computer, a security com(cid:173)
`puter, including a security receiver for receiving the input
`from the client computer, an input inspector for determining 40
`whether it is safe for the client computer to invoke the original
`function with the input, and a security transmitter for trans(cid:173)
`mitting an indicator of the determining to the client computer,
`and a client computer communicating with the gateway com(cid:173)
`puter and with the security computer, including a client 45
`receiver for receiving the modified content from the gateway
`computer, and for receiving the indicator from the security
`computer, a content processor for processing the modified
`content, and for invoking the original function only if the
`indicator indicates that such invocation is safe; and a client 50
`transmitter for transmitting the input to the security computer
`for inspection, when the substitute function is invoked.
`There is yet further provided in accordance with a preferred
`embodiment of the present invention a computer-readable
`storage medium storing program code for causing at least one 55
`computing device to receive content including a call to an
`original function, and the call including an input, replace the
`call to the original function with a corresponding call to a
`substitute function, the substitute function being operational
`to send the input for inspection, thereby generating modified 60
`content, process the modified content, transmit the input for
`inspection, when the substitute function is invoked while
`processing the modified content, and suspend processing of
`the modified content, determine whether it is safe to invoke
`the original function with the input, transmit an indicator of 65
`whether it is safe for a computer to invoke the original func(cid:173)
`tion with the input, and resume processing of the modified
`
`6
`content after receiving the indicator, and invoke the original
`function with the input only if the indicator indicates that such
`invocation is safe.
`There is additionally provided in accordance with a pre(cid:173)
`ferred embodiment of the present invention a method for
`protecting a client computer from dynamically generated
`malicious content, including receiving content being sent to a
`client computer for processing, the content including a call to
`an original function, and the call including an input, modify(cid:173)
`ing the content, including replacing the call to the original
`function with a corresponding call to a substitute function, the
`substitute function being operational to send the input to a
`security computer for inspection, and transmitting the modi(cid:173)
`fied content to the client computer for processing.
`There is moreover provided in accordance with a preferred
`embodiment of the present invention a system for protecting
`a client computer from dynamically generated malicious con(cid:173)
`tent, including a receiver for receiving content being sent to a
`client computer for processing, the content including a call to
`an original function, and the call including an input, a content
`modifier for modifying the received content by replacing the
`call to the original function with a corresponding call to a
`substitute function, the substitute function being operational
`to send the input to a security computer for inspection, and a
`25 transmitter for transmitting the modified content to the client
`computer.
`There is further provided in accordance with a preferred
`embodiment of the present invention a computer-readable
`storage medium storing program code for causing a comput(cid:173)
`ing device to receive content including a call to an original
`function, and the call including an input, and replace the call
`to the original function with a corresponding call to a substi(cid:173)
`tute function, the substitute function being operational to send
`the input for inspection.
`There is yet further provided in accordance with a preferred
`embodiment of the present invention a method for protecting
`a client computer from dynamically generated malicious con(cid:173)
`tent, including receiving content being sent to a client com(cid:173)
`puter for processing, the content including a call to an original
`function, and the call including an input, modifying the con(cid:173)
`tent, including replacing the call to the original function with
`a corresponding call to a substitute function, the substitute
`function being operational to send the input for inspection,
`transmitting the modified content to the client computer for
`processing, receiving the input from the client computer,
`determining whether it is safe for the client computer to
`invoke the original function with the input, and transmitting
`to the client computer an indicator of whether it is safe for the
`client computer to invoke the original function with the input.
`There is additionally provided in accordance with a pre(cid:173)
`ferred embodiment of the present invention a system for
`protecting a client computer from dynamically generated
`malicious content, including a receiver (i) for receiving con(cid:173)
`tent being sent to a client computer for processing, the content
`including a call to an original function, and the call including
`an input, and (ii) for receiving the input from the client com-
`puter, a content modifier for modifying the received content
`by replacing the call to the original function with a corre(cid:173)
`sponding call to a substitute function, the substitute function
`being operational to send the input for inspection, an input
`inspector for determining whether it is safe for the client
`computer to invoke the original function with the input, and a
`transmitter (i) for transmitting the modified content to the
`client computer, and (ii) for transmitting an indicator of the
`determining to the client computer.
`There is moreover provided in accordance with a preferred
`embodiment of the present invention a computer-readable
`
`

`

`Case 3:17-cv-05659-WHA Document 391-6 Filed 03/14/19 Page 11 of 17
`
`US 8,141,154 B2
`
`7
`storage medium storing program code for causing a comput(cid:173)
`ing device to receive content including a call to an original
`function, and the call including an input, replace the call to the
`original function with a corresponding call to a substitute
`function, the substitute function being operational to send the
`input for inspection, and determine whether it is safe for a
`computer to invoke the original function with the input.
`There is further provided in accord