Bot Communication

~$ cd ..

We were given an IP and a port on which a bot was running, and the goal was to interact with it. As we wrote this write up the service was unfortunately down, and we didn’t keep all exact answers returned by the service

By trying to reach the service running on 99.80.109.60:8080 using a simple HTTP GET request, we received something like:

Unknown command: TW96aWxsYS81LjA=

By base64-decoding it, we found that our user agent was taken as a command, hence we tried to guess the commands the bot could understand. We first tried plaintext commands, such as:

$ curl -H "User-Agent: help" http://99.80.109.60:8080
	Unkown command: aGVscA==
$ curl -H "User-Agent: flag" http://99.80.109.60:8080
	Unknown command: ZmxhZw==
$ curl -H "User-Agent: login" http://99.80.109.60:8080
	Unknown command: bG9naW4=

but we always received an error message with our command, base64 encoded. One of us suggested to send something giving a valid command, once base64-decoded, such as:

$ curl -H "User-Agent: $(echo help|base64 -d)" http://99.80.109.60:8080
	Unknown command: help

Nope, wrong path. What we actually had to do was to send our commands, base64 encoded, such as:

$ curl -H "User-Agent: $(echo HELP | base64)" http://99.80.109.60:8080

and finally the bot returned us a nice message, giving us the valid commands: HELP, ECHO, TOKEN, and KEY

HELP and ECHO were not useful, but KEY returned the string C0FFEE as a response and TOKEN gave us the following gibberish:

$ curl -H "User-Agent: $(echo KEY | base64)" http://99.80.109.60:8080
	C0FFEE
$ curl -H "User-Agent: $(echo TOKEN | base64)" http://99.80.109.60:8080
	c=&$'R$u!ptsvq&srq""#|vv"s#{"%rruq#}%Q# vupt~ppwTup "T;

In fact, there was a null byte at the beginning of the token we could not see. To get the exact answer, we piped the output into xxd:

00000000: 0063 053d 2624 2752 2475 2170 7208 7473  .c.=&$'R$u!pr.ts
00000010: 7671 2600 7f73 7271 2207 2223 7c76 7600  vq&..srq"."#|vv.
00000020: 7f22 7323 7b04 2225 7272 7008 7571 237d  ."s#{."%rrp.uq#}
00000030: 2551 2320 7675 7001 747e 7070 7754 7f75  %Q# vup.t~ppwT.u
00000040: 7020 2254 3b0a                           p "T;.

Knowing the the flag format was CSC{<string>}`` and that the first byte of the key was also a capital C, it was then clear that we had to xor the token and the key:

token = '0063053d262427522475217072087473767126007f737271220722237c7676007f2273237b042225727270087571237d2551232076757001747e707077547f75702022543b'.decode('hex')
key = 'C0FFEE'
print ''.join(chr(ord(key[i % len(key)])^ord(token[i])) for i in xrange(len(token)))
	CSC{cadbb3d5182534e09574a7de93509d6f84dc773837f8faef303128554d935ead}