--- date: 2023-10-04 13:12 description: Introduction, Phases 1-5 of Bomb Lab for CSCI 2400 Lab - 2 tags: gdb, reverse-engineering, c++, csci2400, assembly --- # Bomb Lab Phases 1-5 ## Introduction Lab 2 for CSCI 2400 - Computer Systems. I like using objdump to disassemble the code and see a broad overview of what is happening. `objdump -d bomb > dis.txt` ## Phase 1 ``` jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_1' -ex 'break explode_bomb' -ex 'run' ./bomb GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1 Copyright (C) 2022 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: . Find the GDB manual and other documentation resources online at: . For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from ./bomb... Breakpoint 1 at 0x15c7 Breakpoint 2 at 0x1d4a Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Welcome to my fiendish little bomb. You have 6 phases with which to blow yourself up. Have a nice day! test string Breakpoint 1, 0x00005555555555c7 in phase_1 () (gdb) dias phase_1 Undefined command: "dias". Try "help". (gdb) disas phase_1 Dump of assembler code for function phase_1: => 0x00005555555555c7 <+0>: endbr64 0x00005555555555cb <+4>: sub $0x8,%rsp 0x00005555555555cf <+8>: lea 0x1b7a(%rip),%rsi # 0x555555557150 0x00005555555555d6 <+15>: call 0x555555555b31 0x00005555555555db <+20>: test %eax,%eax 0x00005555555555dd <+22>: jne 0x5555555555e4 0x00005555555555df <+24>: add $0x8,%rsp 0x00005555555555e3 <+28>: ret 0x00005555555555e4 <+29>: call 0x555555555d4a 0x00005555555555e9 <+34>: jmp 0x5555555555df End of assembler dump. (gdb) print 0x555555557150 $1 = 93824992244048 (gdb) x/1s 0x555555557150 0x555555557150: "Controlling complexity is the essence of computer programming." (gdb) ``` ## Phase 2 ``` Phase 1 defused. How about the next one? 1 2 3 4 5 6 Breakpoint 1, 0x00005555555555eb in phase_2 () (gdb) disas Dump of assembler code for function phase_2: => 0x00005555555555eb <+0>: endbr64 0x00005555555555ef <+4>: push %rbp 0x00005555555555f0 <+5>: push %rbx 0x00005555555555f1 <+6>: sub $0x28,%rsp 0x00005555555555f5 <+10>: mov %rsp,%rsi 0x00005555555555f8 <+13>: call 0x555555555d97 0x00005555555555fd <+18>: cmpl $0x0,(%rsp) 0x0000555555555601 <+22>: js 0x55555555560d 0x0000555555555603 <+24>: mov %rsp,%rbp 0x0000555555555606 <+27>: mov $0x1,%ebx 0x000055555555560b <+32>: jmp 0x555555555620 0x000055555555560d <+34>: call 0x555555555d4a 0x0000555555555612 <+39>: jmp 0x555555555603 0x0000555555555614 <+41>: add $0x1,%ebx 0x0000555555555617 <+44>: add $0x4,%rbp 0x000055555555561b <+48>: cmp $0x6,%ebx 0x000055555555561e <+51>: je 0x555555555631 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 0x000055555555562a <+63>: call 0x555555555d4a 0x000055555555562f <+68>: jmp 0x555555555614 0x0000555555555631 <+70>: add $0x28,%rsp 0x0000555555555635 <+74>: pop %rbx 0x0000555555555636 <+75>: pop %rbp 0x0000555555555637 <+76>: ret End of assembler dump. (gdb) ``` ``` 0x00005555555555fd <+18>: cmpl $0x0,(%rsp) 0x0000555555555601 <+22>: js 0x55555555560d ... 0x000055555555560d <+34>: call 0x555555555d4a ``` The program first compares if the first number is not 0. If the number is not 0, then the `cmpl` instruction returns a negative value. The `js` instruction stands for jump if sign -> causing a jump to the specified address if the sign bit is set. This would result in the explode_bomb function being called. ``` 0x0000555555555603 <+24>: mov %rsp,%rbp 0x0000555555555606 <+27>: mov $0x1,%ebx ``` `%rsp` in x86-64 asm, is the stack pointer i.e. it points to the top of the current stack frame. Since the program just read six numbers, the top of the stack (`%rsp`) contains the address of the first number. By executing `mov %rsp,%rbp` we are setting the base pointer (`%rbp`) to point to this address. Now, for the second instruction `mov $0x1,%ebx`, we are initalising the `%ebx` register with the value 1. Based on the assembly code, you can see that this is being used as a counter/index for the loop. ``` 0x000055555555560b <+32>: jmp 0x555555555620 ``` The program now jumps to ``` 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 ``` Here, the value from `%ebx` is copied to the `%eax` register. For this iteration, the value should be 1. Then, the value at the memory location pointed by `%rbp` is added to the value in `%eax`. For now, 0 is added (the first number that we read). `cmp %eax,0x4(%rbp)` - The instruction compares the value in %eax to the value at the memory address `%rbp + 4`. Since Integers in this context are stored using a word of memory of 4 bytes, this indicates it checks against the second number in the sequence. `je 0x555555555614 ` - The program will jump to `phase_2+41` if the previous `cmp` instruction determined the values as equal. ``` 0x0000555555555614 <+41>: add $0x1,%ebx 0x0000555555555617 <+44>: add $0x4,%rbp 0x000055555555561b <+48>: cmp $0x6,%ebx 0x000055555555561e <+51>: je 0x555555555631 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 ``` Here, we can see that the program increments `%ebx` by 1, adds a 4 byte offset to `%rbp` (the number we will be matching now), and checks if `%ebx` is equal to 6. If it is, it breaks the loop and jumps to `` succesfully finishing this stage. Now, given that we know the first two numbers in the sequence are `0 1`, we can calculate the other numbers by following the pattern of adding the counter and the value of the previous number. Thus, * 3rd number = 1 (previous value) + 2 = 3 * 4th number = 3 (prev value) + 3 = 6 * 5th number = 6 (prev value) + 4 = 10 * 6th number = 10 (prev value) + 5 = 15 ``` ... Phase 1 defused. How about the next one? 0 1 3 6 10 15 Breakpoint 1, 0x00005555555555eb in phase_2 () (gdb) continue Continuing. That's number 2. Keep going! ``` ## Phase 3 Let us look at the disassembled code first ``` 0000000000001638 : 1638: f3 0f 1e fa endbr64 163c: 48 83 ec 18 sub $0x18,%rsp 1640: 48 8d 4c 24 07 lea 0x7(%rsp),%rcx 1645: 48 8d 54 24 0c lea 0xc(%rsp),%rdx 164a: 4c 8d 44 24 08 lea 0x8(%rsp),%r8 164f: 48 8d 35 60 1b 00 00 lea 0x1b60(%rip),%rsi # 31b6 <_IO_stdin_used+0x1b6> 1656: b8 00 00 00 00 mov $0x0,%eax 165b: e8 80 fc ff ff call 12e0 <__isoc99_sscanf@plt> 1660: 83 f8 02 cmp $0x2,%eax 1663: 7e 20 jle 1685 1665: 83 7c 24 0c 07 cmpl $0x7,0xc(%rsp) 166a: 0f 87 0d 01 00 00 ja 177d 1670: 8b 44 24 0c mov 0xc(%rsp),%eax 1674: 48 8d 15 55 1b 00 00 lea 0x1b55(%rip),%rdx # 31d0 <_IO_stdin_used+0x1d0> 167b: 48 63 04 82 movslq (%rdx,%rax,4),%rax 167f: 48 01 d0 add %rdx,%rax 1682: 3e ff e0 notrack jmp *%rax 1685: e8 c0 06 00 00 call 1d4a 168a: eb d9 jmp 1665 168c: b8 63 00 00 00 mov $0x63,%eax 1691: 81 7c 24 08 3d 02 00 cmpl $0x23d,0x8(%rsp) 1698: 00 1699: 0f 84 e8 00 00 00 je 1787 169f: e8 a6 06 00 00 call 1d4a 16a4: b8 63 00 00 00 mov $0x63,%eax 16a9: e9 d9 00 00 00 jmp 1787 16ae: b8 61 00 00 00 mov $0x61,%eax 16b3: 81 7c 24 08 27 01 00 cmpl $0x127,0x8(%rsp) 16ba: 00 16bb: 0f 84 c6 00 00 00 je 1787 16c1: e8 84 06 00 00 call 1d4a 16c6: b8 61 00 00 00 mov $0x61,%eax 16cb: e9 b7 00 00 00 jmp 1787 16d0: b8 78 00 00 00 mov $0x78,%eax 16d5: 81 7c 24 08 e7 02 00 cmpl $0x2e7,0x8(%rsp) 16dc: 00 16dd: 0f 84 a4 00 00 00 je 1787 16e3: e8 62 06 00 00 call 1d4a 16e8: b8 78 00 00 00 mov $0x78,%eax 16ed: e9 95 00 00 00 jmp 1787 16f2: b8 64 00 00 00 mov $0x64,%eax 16f7: 81 7c 24 08 80 02 00 cmpl $0x280,0x8(%rsp) 16fe: 00 16ff: 0f 84 82 00 00 00 je 1787 1705: e8 40 06 00 00 call 1d4a 170a: b8 64 00 00 00 mov $0x64,%eax 170f: eb 76 jmp 1787 1711: b8 6d 00 00 00 mov $0x6d,%eax 1716: 81 7c 24 08 ff 02 00 cmpl $0x2ff,0x8(%rsp) 171d: 00 171e: 74 67 je 1787 1720: e8 25 06 00 00 call 1d4a 1725: b8 6d 00 00 00 mov $0x6d,%eax 172a: eb 5b jmp 1787 172c: b8 71 00 00 00 mov $0x71,%eax 1731: 81 7c 24 08 75 03 00 cmpl $0x375,0x8(%rsp) 1738: 00 1739: 74 4c je 1787 173b: e8 0a 06 00 00 call 1d4a 1740: b8 71 00 00 00 mov $0x71,%eax 1745: eb 40 jmp 1787 1747: b8 79 00 00 00 mov $0x79,%eax 174c: 81 7c 24 08 94 02 00 cmpl $0x294,0x8(%rsp) 1753: 00 1754: 74 31 je 1787 1756: e8 ef 05 00 00 call 1d4a 175b: b8 79 00 00 00 mov $0x79,%eax 1760: eb 25 jmp 1787 1762: b8 79 00 00 00 mov $0x79,%eax 1767: 81 7c 24 08 88 02 00 cmpl $0x288,0x8(%rsp) 176e: 00 176f: 74 16 je 1787 1771: e8 d4 05 00 00 call 1d4a 1776: b8 79 00 00 00 mov $0x79,%eax 177b: eb 0a jmp 1787 177d: e8 c8 05 00 00 call 1d4a 1782: b8 68 00 00 00 mov $0x68,%eax 1787: 38 44 24 07 cmp %al,0x7(%rsp) 178b: 75 05 jne 1792 178d: 48 83 c4 18 add $0x18,%rsp 1791: c3 ret 1792: e8 b3 05 00 00 call 1d4a 1797: eb f4 jmp 178d ``` ``` ... 165b: e8 80 fc ff ff call 12e0 <__isoc99_sscanf@plt> ... ``` We can see that `scanf` is being called which means we need to figure out what datatype(s) the program is expecting. Because I do not want to enter the solutions to phases 1 and 2 again and again, I am goig to pass a file which has these solutions. ``` jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_3' -ex 'break explode_bomb' -ex 'run' -args ./bomb sol.txt GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1 Copyright (C) 2022 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: . Find the GDB manual and other documentation resources online at: . For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from ./bomb... Breakpoint 1 at 0x1638 Breakpoint 2 at 0x1d4a Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb sol.txt [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Welcome to my fiendish little bomb. You have 6 phases with which to blow yourself up. Have a nice day! Phase 1 defused. How about the next one? That's number 2. Keep going! random string Breakpoint 1, 0x0000555555555638 in phase_3 () (gdb) disas Dump of assembler code for function phase_3: => 0x0000555555555638 <+0>: endbr64 0x000055555555563c <+4>: sub $0x18,%rsp 0x0000555555555640 <+8>: lea 0x7(%rsp),%rcx 0x0000555555555645 <+13>: lea 0xc(%rsp),%rdx 0x000055555555564a <+18>: lea 0x8(%rsp),%r8 0x000055555555564f <+23>: lea 0x1b60(%rip),%rsi # 0x5555555571b6 0x0000555555555656 <+30>: mov $0x0,%eax 0x000055555555565b <+35>: call 0x5555555552e0 <__isoc99_sscanf@plt> 0x0000555555555660 <+40>: cmp $0x2,%eax 0x0000555555555663 <+43>: jle 0x555555555685 0x0000555555555665 <+45>: cmpl $0x7,0xc(%rsp) 0x000055555555566a <+50>: ja 0x55555555577d 0x0000555555555670 <+56>: mov 0xc(%rsp),%eax 0x0000555555555674 <+60>: lea 0x1b55(%rip),%rdx # 0x5555555571d0 0x000055555555567b <+67>: movslq (%rdx,%rax,4),%rax 0x000055555555567f <+71>: add %rdx,%rax 0x0000555555555682 <+74>: notrack jmp *%rax 0x0000555555555685 <+77>: call 0x555555555d4a 0x000055555555568a <+82>: jmp 0x555555555665 0x000055555555568c <+84>: mov $0x63,%eax 0x0000555555555691 <+89>: cmpl $0x23d,0x8(%rsp) 0x0000555555555699 <+97>: je 0x555555555787 0x000055555555569f <+103>: call 0x555555555d4a 0x00005555555556a4 <+108>: mov $0x63,%eax 0x00005555555556a9 <+113>: jmp 0x555555555787 --Type for more, q to quit, c to continue without paging-- ``` `gdb` has thankfully marked the address which is being passed to `scanf`. We can access the value: ``` (gdb) x/1s 0x5555555571b6 0x5555555571b6: "%d %c %d" (gdb) ``` BINGO! The program expects an integer, character, and another integer. Onwards. ``` 0x0000555555555660 <+40>: cmp $0x2,%eax 0x0000555555555663 <+43>: jle 0x555555555685 ... 0x0000555555555685 <+77>: call 0x555555555d4a ``` The program checks whether `scanf` returns a value <= 2, if it does then it calls the `explode_bomb` function. *Note: `scanf` returns the number of fields that were succesfully converted and assigned* ``` 0x0000555555555665 <+45>: cmpl $0x7,0xc(%rsp) 0x000055555555566a <+50>: ja 0x55555555577d ... 0x000055555555577d <+325>: call 0x555555555d4a ``` Similarly, the program checks and ensures the returned value is not > 7. ``` 0x0000555555555670 <+56>: mov 0xc(%rsp),%eax 0x0000555555555674 <+60>: lea 0x1b55(%rip),%rdx # 0x5555555571d0 0x000055555555567b <+67>: movslq (%rdx,%rax,4),%rax 0x000055555555567f <+71>: add %rdx,%rax 0x0000555555555682 <+74>: notrack jmp *%rax 0x0000555555555685 <+77>: call 0x555555555d4a ``` * `0x0000555555555670 <+56>: mov 0xc(%rsp),%eax` - Moves value located at `0xc` (12 in Decimal) bytes above the stack pointer to `%eax` register. * `0x0000555555555674 <+60>: lea 0x1b55(%rip),%rdx # 0x5555555571d0` - This instruction calculates an effective address by adding `0x1b55` to the current instruction pointer (`%rip`). The result is stored in the `%rdx` register. * `0x000055555555567b <+67>: movslq (%rdx,%rax,4),%rax` * `movslq` stands for "move with sign-extension from a 32-bit value to a 64-bit value." (if the 32-bit value is negative, the 64-bit result will have all its upper 32 bits set to 1; otherwise, they'll be set to 0). * `(%rdx,%rax,4)` - First start with the value in the %rdx register, then add to it the value in the %rax register multiplied by 4. * `%rax` - Destination Register * `0x000055555555567f <+71>: add %rdx,%rax` - Adds base address in `%rdx` to the offset in `%rax` * `0x0000555555555682 <+74>: notrack jmp *%rax` - Jumps to the address stored in `%rax` * `0x0000555555555685 <+77>: call 0x555555555d4a ` - If we are unable to jump to the specified instruction, call `explode_bomb` Let us try to run the program again with a valid input for the first number and see what the program is computing for the address. I used the input: `3 c 123`. To check what is the computed address, we can switch to the asm layout by running `layout asm`, and then going through instructions `ni` or `si` until we reach the line `movslq (%rdx,%rax,4),%rax` `%rax` should hold the value 3. ``` (gdb) print $rax $1 = 3 ``` ![Screenshot of GDB terminal depicting us checking the value of the instruction to be jumped to](/assets/bomb-lab/phase-3.png) We can see that this makes us jump to `` (Continue to step through the code by using `ni`) ``` 0x00005555555556f2 <+186>: mov $0x64,%eax 0x00005555555556f7 <+191>: cmpl $0x280,0x8(%rsp) 0x00005555555556ff <+199>: je 0x555555555787 0x0000555555555705 <+205>: call 0x555555555d4a ``` We see that `0x64` (Decimal 100) is being stored in `%eax`. Then, the program compares `0x280` (Decimal 640) with memory address `0x8` bytes above the stack pointer (`%rsp`). If the values are equal, then it jumps to ``, otherwise `explode_bomb` is called. ``` 0x0000555555555787 <+335>: cmp %al,0x7(%rsp) 0x000055555555578b <+339>: jne 0x555555555792 0x000055555555578d <+341>: add $0x18,%rsp 0x0000555555555791 <+345>: ret 0x0000555555555792 <+346>: call 0x555555555d4a ``` Here, the program is comparing the value of our given character to the value stored in `%al` (lower 8 bits of `EAX`), and checks if they are not equal. Knowing that the character is stored at an offset of 7 bytes to `%rsp`, we can print and check the value by running: ``` (gdb) x/1cw $rsp+7 c (gdb) print $al $1 = 100 ``` We can simply lookup the [ASCII table](https://www.cs.cmu.edu/~pattis/15-1XX/common/handouts/ascii.html), and see that 100 in decimal stands for the character `d`. Let us try this answer: ``` ... That's number 2. Keep going! 3 d 640 Breakpoint 1, 0x0000555555555638 in phase_3 () (gdb) continue Continuing. Halfway there! ``` ## Phase 4 ``` jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_4' -ex 'break explode_bomb' -ex 'run' -args ./bomb sol.txt GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1 Copyright (C) 2022 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: . Find the GDB manual and other documentation resources online at: . For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from ./bomb... Breakpoint 1 at 0x17d3 Breakpoint 2 at 0x1d4a Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb sol.txt [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Welcome to my fiendish little bomb. You have 6 phases with which to blow yourself up. Have a nice day! Phase 1 defused. How about the next one? That's number 2. Keep going! Halfway there! test string Breakpoint 1, 0x00005555555557d3 in phase_4 () (gdb) disas phase_4 Dump of assembler code for function phase_4: => 0x00005555555557d3 <+0>: endbr64 0x00005555555557d7 <+4>: sub $0x18,%rsp 0x00005555555557db <+8>: lea 0x8(%rsp),%rcx 0x00005555555557e0 <+13>: lea 0xc(%rsp),%rdx 0x00005555555557e5 <+18>: lea 0x1bba(%rip),%rsi # 0x5555555573a6 0x00005555555557ec <+25>: mov $0x0,%eax 0x00005555555557f1 <+30>: call 0x5555555552e0 <__isoc99_sscanf@plt> 0x00005555555557f6 <+35>: cmp $0x2,%eax 0x00005555555557f9 <+38>: jne 0x555555555802 0x00005555555557fb <+40>: cmpl $0xe,0xc(%rsp) 0x0000555555555800 <+45>: jbe 0x555555555807 0x0000555555555802 <+47>: call 0x555555555d4a 0x0000555555555807 <+52>: mov $0xe,%edx 0x000055555555580c <+57>: mov $0x0,%esi 0x0000555555555811 <+62>: mov 0xc(%rsp),%edi 0x0000555555555815 <+66>: call 0x555555555799 0x000055555555581a <+71>: cmp $0x2,%eax 0x000055555555581d <+74>: jne 0x555555555826 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) 0x0000555555555824 <+81>: je 0x55555555582b 0x0000555555555826 <+83>: call 0x555555555d4a 0x000055555555582b <+88>: add $0x18,%rsp 0x000055555555582f <+92>: ret End of assembler dump. (gdb) ``` Again, `gdb` has marked the string being passed to `scanf` ``` (gdb) x/1s 0x5555555573a6 0x5555555573a6: "%d %d" ``` Okay, so this time we are supposed to enter 2 numbers. ``` 0x00005555555557f6 <+35>: cmp $0x2,%eax 0x00005555555557f9 <+38>: jne 0x555555555802 ``` Checks if there were 2 values read from calling `scanf`, if not -> jump to `` which calls ``. ``` 0x00005555555557fb <+40>: cmpl $0xe,0xc(%rsp) 0x0000555555555800 <+45>: jbe 0x555555555807 ``` Compare `0xe` (14 in Decimal) and value stored at `$rsp` + `0xc` bytes (Decimal 12). If this condition is met (<= 14), jump to ``. If not, then explode bomb. ``` ... 0x0000555555555807 <+52>: mov $0xe,%edx 0x000055555555580c <+57>: mov $0x0,%esi 0x0000555555555811 <+62>: mov 0xc(%rsp),%edi 0x0000555555555815 <+66>: call 0x555555555799 0x000055555555581a <+71>: cmp $0x2,%eax 0x000055555555581d <+74>: jne 0x555555555826 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) 0x0000555555555824 <+81>: je 0x55555555582b 0x0000555555555826 <+83>: call 0x555555555d4a ``` * ` 0x0000555555555815 <+66>: call 0x555555555799 ` calls another function called `func4` * The returned value is compared with `0x2`, if they are not equal then the program jumps to call ``. This tells us that `func4` should return 2. Let us look into `func4` ``` (gdb) disas func4 Dump of assembler code for function func4: 0x0000555555555799 <+0>: endbr64 0x000055555555579d <+4>: sub $0x8,%rsp 0x00005555555557a1 <+8>: mov %edx,%ecx 0x00005555555557a3 <+10>: sub %esi,%ecx 0x00005555555557a5 <+12>: shr %ecx 0x00005555555557a7 <+14>: add %esi,%ecx 0x00005555555557a9 <+16>: cmp %edi,%ecx 0x00005555555557ab <+18>: ja 0x5555555557b9 0x00005555555557ad <+20>: mov $0x0,%eax 0x00005555555557b2 <+25>: jb 0x5555555557c5 0x00005555555557b4 <+27>: add $0x8,%rsp 0x00005555555557b8 <+31>: ret 0x00005555555557b9 <+32>: lea -0x1(%rcx),%edx 0x00005555555557bc <+35>: call 0x555555555799 0x00005555555557c1 <+40>: add %eax,%eax 0x00005555555557c3 <+42>: jmp 0x5555555557b4 0x00005555555557c5 <+44>: lea 0x1(%rcx),%esi 0x00005555555557c8 <+47>: call 0x555555555799 0x00005555555557cd <+52>: lea 0x1(%rax,%rax,1),%eax 0x00005555555557d1 <+56>: jmp 0x5555555557b4 ``` This looks like a recursive function :( (I hate recursive functions) Let's annotate the instructions. ``` endbr64 sub $0x8,%rsp // subtract 8 bytes from the stack pointer mov %edx,%ecx // Move the value in register %edx to %ecx sub %esi,%ecx // Subtract the value in %esi from %ecx shr %ecx // Right shift the value in %ecx by one bit (dividing the value by 2) add %esi,%ecx // Add the value in %esi to %ecx cmp %edi,%ecx // Compare ja 0x5555555557b9 // If %ecx > %edi -> jump to instruction at offset +32 mov $0x0,%eax // Move 0 to %eax jb 0x5555555557c5 // If %ecx < %edi -> jump to instruction at offset +44. add $0x8,%rsp // add 8 bytes to the stack pointer ret // return lea -0x1(%rcx),%edx // LEA of $rxc - 1 into $edx call 0x555555555799 // Call itself add %eax,%eax // Double the value in %eax jmp 0x5555555557b4 // jump to the instruction at offset +27 lea 0x1(%rcx),%esi call 0x555555555799 lea 0x1(%rax,%rax,1),%eax // LEA of %rax * 2 + 1 into $eax jmp 0x5555555557b4 ``` We can either try to compute the values by hand, or write a simple script in Python to get the answer. ``` def func4(edi, esi=0, edx=20): ecx = (edx - esi) // 2 + esi if ecx > edi: return 2 * func4(edi, esi, ecx - 1) elif ecx < edi: return 2 * func4(edi, ecx + 1, edx) + 1 else: return 0 for x in range(15): # We can limit to 14 if func4(x) == 2: print(f"answer is {x}") break ``` Running this code, we get: `answer is 5` Okay, so we know that the number needed to be passed to `func4` is 5. But, what about the second digit? If we go back to the code for ``, we can see that: ``` 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) 0x0000555555555824 <+81>: je 0x55555555582b ``` The value at `$rsp+8` should be equal to 2. So, let us try passing `5 2` as our input. ``` ... Phase 1 defused. How about the next one? That's number 2. Keep going! Halfway there! 5 2 Breakpoint 1, 0x00005555555557d3 in phase_4 () (gdb) continue Continuing. So you got that one. Try this one. ``` ## Phase 5 ``` So you got that one. Try this one. test string Breakpoint 1, 0x0000555555555830 in phase_5 () (gdb) disas phase_5 Dump of assembler code for function phase_5: => 0x0000555555555830 <+0>: endbr64 0x0000555555555834 <+4>: push %rbx 0x0000555555555835 <+5>: sub $0x10,%rsp 0x0000555555555839 <+9>: mov %rdi,%rbx 0x000055555555583c <+12>: call 0x555555555b10 0x0000555555555841 <+17>: cmp $0x6,%eax 0x0000555555555844 <+20>: jne 0x55555555588b 0x0000555555555846 <+22>: mov $0x0,%eax 0x000055555555584b <+27>: lea 0x199e(%rip),%rcx # 0x5555555571f0 0x0000555555555852 <+34>: movzbl (%rbx,%rax,1),%edx 0x0000555555555856 <+38>: and $0xf,%edx 0x0000555555555859 <+41>: movzbl (%rcx,%rdx,1),%edx 0x000055555555585d <+45>: mov %dl,0x9(%rsp,%rax,1) 0x0000555555555861 <+49>: add $0x1,%rax 0x0000555555555865 <+53>: cmp $0x6,%rax 0x0000555555555869 <+57>: jne 0x555555555852 0x000055555555586b <+59>: movb $0x0,0xf(%rsp) 0x0000555555555870 <+64>: lea 0x9(%rsp),%rdi 0x0000555555555875 <+69>: lea 0x1943(%rip),%rsi # 0x5555555571bf 0x000055555555587c <+76>: call 0x555555555b31 0x0000555555555881 <+81>: test %eax,%eax 0x0000555555555883 <+83>: jne 0x555555555892 0x0000555555555885 <+85>: add $0x10,%rsp 0x0000555555555889 <+89>: pop %rbx 0x000055555555588a <+90>: ret 0x000055555555588b <+91>: call 0x555555555d4a 0x0000555555555890 <+96>: jmp 0x555555555846 0x0000555555555892 <+98>: call 0x555555555d4a 0x0000555555555897 <+103>: jmp 0x555555555885 End of assembler dump. (gdb) ``` ``` ... 0x000055555555583c <+12>: call 0x555555555b10 0x0000555555555841 <+17>: cmp $0x6,%eax 0x0000555555555844 <+20>: jne 0x55555555588b ... 0x000055555555588b <+91>: call 0x555555555d4a ... ``` First things first, these instructions check to make sure the passed string is of length 6, otherwise `explode_bomb` is called. We can also see a similar pattern compared to Phase 2, where we had a loop: * The looping part: * `mov $0x0,%eax` - Initialise `%eax` and set it to 0 (our counter/iterator) * `movzbl (%rbx,%rax,1),%edx` - Access `%rbx + 1 * %rax` and store it in `%edx` * `and $0xf,%edx` - Take the least significant 4 bits of the byte. * `movzbl (%rcx,%rdx,1),%edx` - Use the 4 bits as an index into another array and load the corresponding byte into `%edx` * `mov %dl,0x9(%rsp,%rax,1)` - Store the transformed byte into a buffer on the stack * `add $0x1,%rax` - Increment `%rax` * `cmp $0x6,%rax` - If the index is not yet 6, loop again * `movb $0x0,0xf(%rsp)` - Null-terminate the transformed string * `lea 0x9(%rsp),%rdi` and `lea 0x1943(%rip),%rsi` * `all 0x555555555b31 ` check if the two strings loaded up just before this are equal or not. We can check the reference string we need, which `gdb` has marked as `# 0x5555555571bf`, and the lookup table marked as `# 0x5555555571f0 ` ``` (gdb) x/s 0x5555555571bf 0x5555555571bf: "bruins" (gdb) x/s 0x5555555571f0 0x5555555571f0 : "maduiersnfotvbylSo you think you can stop the bomb with ctrl-c, do you?" (gdb) ``` To summarize the transformation process: * The function takes each byte of the string * It keeps only the least significant 4 bits of each byte * It uses these 4 bits as an index into the lookup table (`array.0`) * The value from the array is then stored in a buffer Here's how the transformation process can be reversed for each character in "bruins": 1. Find the index of `b` in the lookup table (in our case, it is 13 since we index starting 0) 2. Calculate binary representation of this index (in our case 13 can be written as 1101 in binary) 3. Find ASCII character whose least significant 4 bits match (in our case, `m` has binary representation `01101101`) Repeat for all 6 characters *Hint: Using an [ASCII - Binary Table](http://sticksandstones.kstrom.com/appen.html) can save you time.* Thus, we can have the following transformation: ``` b -> m r -> f u -> c i -> d n -> h s -> g ``` Let us try out this answer: ``` ... That's number 2. Keep going! Halfway there! So you got that one. Try this one. mfcdhg Breakpoint 1, 0x0000555555555830 in phase_5 () (gdb) continue Continuing. Good work! On to the next... ``` Awesome! ## Phase 6