Google CTF: Beginner Quest: GATEKEEPER (Reverse Engineering)

what's up guys John Hammond here still checking out some of the Google capture-the-flag beginner quest challenges from the previous competition this past weekend Before we jump in a one another shout out to live overflow I know I've done this for literally every single video that I've had release in this series but I just really really want to tip my hat to him he's a super cool guy was willing to hang out with me and try for our hands at this Google CTF together and he did some rock-solid cool awesome work on some of the legitimate challenges during the game I did this baby stuff in the the noob category in the beginners quest but he is a genius and you should totally check him out on YouTube alright let's get into the game and see where we left off beginner's quest I'm kind of on this top track here getting the yellow next one is a challenge is called gatekeeper so reverse engineering challenge supposedly and I actually did do a little bit of reverse engineering in this challenge not a whole lot but enough to I guess to be considered a reverse engineering challenge in my eyes so let's save this file get a command line open or we can work with it and get into the directory this is another zip file as we've seen with just about everything let's call it gatekeeper dot zip so we can unzip it without a problem and we have this file gatekeeper which is a binary so let's mark it executable and let's try and play with it foap I have an L there when I run gatekeeper there's this neat animation thing here geek keeper access your PC from everywhere login information missing okay so I need to pass it a username and password I don't know any username and password though so I says remember and it does this verifying thing takes some time it looks like but whatever okay access tonight incorrect username that's some neat print out effect but it doesn't really help me so let's do the low-hanging fruit let's just run strings on this guy I can list through this and we see some of the regular ones gate keeper access to dialogue information thing et cetera and I see some odd ones one warm I think and clicks d4m t0g I those things look peculiar and those things look weird I don't know where or how they go though so one thing I tried is I actually opened up hopper and if you haven't used hopper before it's a pretty awesome disassembler that is essentially free the demo will run for 30 minutes but it's also not that expensive to buy I think it was only like $90 by the time I bought it I don't know if they've up their part their prices or anything but so we can see the strings and the labels etc points over here and again these strings are over in the tabs on the on the side here I see that one warm string and leet-speak and they're all being referenced in Maine so I went to Maine to view the disassembly and Hanan and all those op codes but if I wanted to I could alt enter and see the sea like pseudocode and it doesn't show me these things all that easily except that it does tell me it actually pretty easily if we don't supply the arguments it'll tell us okay here's the usage otherwise it'll do some text animation on verifying looks like these are the functions that are trying to call and it checks okay if string compare one warm it looks like that's the username we're trying to look at you can see that here being denoted as cats that's the test and it will print out the flag if we get the right password supposedly I don't know what this loop is doing though string compare is Alex d4 am i tog whatever so let's try and try and try those strings to see if they are something that we actually want it's from geek keeper that was a username supposedly and we'll try this as the password just like that we'll go through that animation verifying verifying verifying that takes however long it needs to take however access denied incorrect password okay so our username seems to be correct but our password is wrong so looking at this for more than a couple seconds you can probably start to see this is something backwards this says I got mad skills and leet speak backwards so we can reverse this if we really wanted to let's do that in Python Idol reversed it so I got mad skills and that is the password that we need to use and it can give this to us in it'll give us the flag like once once we got that typed and we're good correct there's the flag and okay literally for some reason I don't know why that password is the flag but let's let's take note of that nano flag context CTF curly-braces surrounding that cool we're good and you can write it get flag script for that really easily if you wanted to because you're only doing is passing that in and cutting up the last line I I did see this just kind of at first glance when I realized that string was backwards but if you wanted to do a little bit more like true reversing I suppose you can I ran L trace and stuff on this and this is pretty annoying right because it says password let's give it something it just pumps out all these functions because of the sleep and the flush that's doing I want to try to create these animations but it does do some like loops here you saw that happen and we can probably assume it's going to run a string compare so if you wanted to grep for that STR compare you're totally good remember L trace and s trace are going to be piping or sending all that output to their standard error stream because it they want to be able to show that as well as the computer program like the binary that you're trying to run showing that originally uncentered output so the standard errors we're all they're debugging information is gonna go so if we wanted to actually be able to grip through that stuff we'd have to pipe to or redirect to the standard error stream to standard output so ampersand one so we can see it on Senate output and then grep through that so now once I hit enter it's not going to get all those put character and flush and sleeve commands but eventually it'll hit the string compare and eventually it will hit another string compare we're testing if the password we supplied backwards is the same as what we'd seen it as the string so okay it's clearly doing something to manipulate the string backwards and that's probably what we saw way up here earlier when that was flashing by and there was a segment that did an interesting loop it looked like so that's some of the techniques you could use to figure out okay it's reversing the string but whatever we've got the flag is it still in my clipboard whatever what's yeah we can wrap that in CTF and submit and we're good let's mark that as complete and we have completed another one of the beginners quest challenges in google CTF so thank you guys for watching hope you're enjoying these videos and one another shout out to live overflow check him out on YouTube thanks again for him and thanks again for being willing to actually explore some of this caption flag competition with me so hey if you did like the video please press that button to like the video if you'd like to leave me a comment let me know what you think or what else I could do better what else you'd like to see if you're willing to subscribe and if you really really want to help me out I would love it if you were to click on one the advertisements during the video that helps me put food on the table so thanks again see you soon

Channel: John Hammond

Views: 31,450

Keywords: hack, hacker, hacking, computer hacking, computer security, cyber, cyber security, google, google security, google hacking, google code, google competition, google coding, google capture the flag, google ctf, beginner, beginner quest, learn hacking, learn programming, computer, computer science, liveoverflow, live overflow, liveoverflowctf, ctf, capture the flag, linux, binary, binary exploitation, reverse engineering, firmware, password, username, strings

Id: bshuAGkgY3M

Length: 8min 3sec (483 seconds)

Published: Sun Jul 01 2018