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rhme-2016 write-up Jumpy

Jumpy (Reverse - 100 pts)

This challenge is a classic reverse engineering challenge and the aim is to find the password by reversing the jumpy.bin file provided

As our aim was not to do it like others by using the excellent Radare2 or IDA Pro ...

We have started by checking AVR Simulator and we have found the excellent simavr In fact this simulator can be also used as as fantastic trace/debugger tool if we change one line in the makefile Just enable this line in Makefile

CFLAGS	+= -DCONFIG_SIMAVR_TRACE=1

First step was to convert Jumpy.bin to Jumpy.hex in order to be loaded by simavr which is done by using 010 Editor by loading the Jumpy.bin and doing Export Hex... to Intel 16-Bit Hex Code (*.hex) => Jumpy.hex

The next step was to modify simavr\examples\board_simduino to simavr\examples\board_hydra in order to add pty simulation for UART RX&TX with also the support of AVR GDB (to allow step by step with GDB including also the trace ...)

Shell 1

$ export SIMAVR_UART_XTERM=1
$ ./simavr_hydra.elf -g -d -t Jumpy.hex > trace_abcdefghi012345.txt
uart_pty_connect_19200: /tmp/simavr-uart0 now points to /dev/pts/4
cat < /dev/pts/4
echo 'hello' > /dev/pts/4
picocom -b 19200 /dev/pts/4

Shell 2

cat < /dev/pts/4

Shell 3

echo 'abcdefghk012345' > /dev/pts/4

Shell 4

avr-gdb -ex 'target remote localhost:1234' 
cont

Shell 2 output

Input: 
Better luck next time!

Shell 1 output

inf loop exit
Press Ctrl C to quit

Now we have a superb trace_abcdefghi012345.txt (of more than 30MB)

  • Open the trace and search cpi
  • We can clearly see the checks on those lines which shall be reversed to find the computation and the right expected characters for the password Extract of the interesting cpi & reverse of the password
avr_run_one: 034e: cpi r24[0d], 0x0d
avr_run_one: 0392: cpi r24[d3], 0xd3 => 0x0135[0x5F/_] + 0x0136[0x74/t]				=> Char 8	 9	
avr_run_one: 06e2: cpi r24[10], 0xc0 0x15 => 0x0136[0x74/t] x 0x0137[0x30/0]	=> Char 9	 10
avr_run_one: 0646: cpi r24[22], 0xb7 0x13 => 0x012e[0x67/g] x 0x012f[0x31/1]	=> Char 1		2	
avr_run_one: 0694: cpi r24[ac], 0x82 0x17 => 0x0130[0x76/v] x 0x0131[0x33/3]	=> Char 3		4	
avr_run_one: 04d4: cpi r24[c9], 0x92 => 0x0131[0x33/3] + 0x0132[0x5F/_]				=> Char 4		5	
avr_run_one: 0570: cpi r24[d8], 0x0c 0x2b => 0x0134[0x74/t] x 0x0135[0x5F/_]	=> Char 7		8	
avr_run_one: 0726: cpi r24[cd], 0xa5 => 0x0133[0x31/1] + 0x0134[0x74/t]				=> Char 6		7	
avr_run_one: 0442: cpi r24[61], 0x8f => 0x0137[0x30/0] + 0x0138[0x5F/_] 			=> Char 10	11
avr_run_one: 05b4: cpi r24[c5], 0xa7 => 0x012f[0x31/1] + 0x0130[0x76/v]				=> Char 2		3	
avr_run_one: 0522: cpi r24[92], 0x73 0x28 => 0x0138[0x5F/_] x 0x0139[0x6D/m]	=> Char 11	12
avr_run_one: 03fc: cpi r24[97], 0x97 0x02 => 0x013a[0x33/3] x 0xD (0x013b)		=> Char 13	14
avr_run_one: 0490: cpi r24[3e], 0x2f 0x12 => 0x0133[0x31/1] x 0x0132[0x5F/_]	=> Char 6		5	
avr_run_one: 05f8: cpi r24[65], 0xa0 => 0x0139[0x6D/m] + 0x013a[0x33/3]				=> Char 12	13
avr_run_one: 075e: cpi r24[00], 0xff 0x3f => 0x013e 0x013f

Password is: g1v3_1t_t0_m3

Shell 1

./simavr_hydra.elf Jumpy.hex

Shell 2

cat < /dev/pts/4

Shell 3

echo -e 'g1v3_1t_t0_m3' > /dev/pts/4

Shell 2 Output

Input: 
FLAG:

Doing that with the real board output the real flag