I've read the open letter addressing students asking for help with their homework and I agree with it, therefore I will try to be less of a pain in the *** ;)
I need to reverse the x86-64 (AT&T) assembly back to C (fill in the blanks). I did most of the task, except a few things I didn't understand.
Here is the assembly code (the compiler did some optimizations such as making the first function inline):
0000000000401c50 <checkPasswordAux>:
401c50: 48 8d 77 11 lea 0x11(%rdi),%rsi
401c54: 31 c0 xor %eax,%eax
401c56: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
401c5d: 00 00 00
401c60: 0f b6 17 movzbl (%rdi),%edx
401c63: 84 d2 test %dl,%dl
401c65: 74 29 je 401c90 <checkPasswordAux+0x40>
401c67: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
401c6e: 00 00
401c70: 89 d1 mov %edx,%ecx
401c72: 83 e1 01 and $0x1,%ecx
401c75: 01 c8 add %ecx,%eax
401c77: d0 fa sar %dl
401c79: 75 f5 jne 401c70 <checkPasswordAux+0x20>
401c7b: 48 83 c7 01 add $0x1,%rdi
401c7f: 48 39 fe cmp %rdi,%rsi
401c82: 75 dc jne 401c60 <checkPasswordAux+0x10>
401c84: b8 64 00 00 00 mov $0x64,%eax
401c89: c3 retq
401c8a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
401c90: f3 c3 repz retq
401c92: 0f 1f 40 00 nopl 0x0(%rax)
401c96: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
401c9d: 00 00 00
0000000000401ca0 <checkPassword>:
401ca0: 4c 8d 47 11 lea 0x11(%rdi),%r8
401ca4: 48 89 fe mov %rdi,%rsi
401ca7: 31 d2 xor %edx,%edx
401ca9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
401cb0: 0f b6 06 movzbl (%rsi),%eax
401cb3: 84 c0 test %al,%al
401cb5: 74 29 je 401ce0 <checkPassword+0x40>
401cb7: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
401cbe: 00 00
401cc0: 89 c1 mov %eax,%ecx
401cc2: 83 e1 01 and $0x1,%ecx
401cc5: 01 ca add %ecx,%edx
401cc7: d0 f8 sar %al
401cc9: 75 f5 jne 401cc0 <checkPassword+0x20>
401ccb: 48 83 c6 01 add $0x1,%rsi
401ccf: 4c 39 c6 cmp %r8,%rsi
401cd2: 75 dc jne 401cb0 <checkPassword+0x10>
401cd4: 31 c0 xor %eax,%eax
401cd6: c3 retq
401cd7: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
401cde: 00 00
401ce0: 83 fa 19 cmp $0x19,%edx
401ce3: 7f ef jg 401cd4 <checkPassword+0x34>
401ce5: 0f be 07 movsbl (%rdi),%eax
401ce8: 84 c0 test %al,%al
401cea: 74 4e je 401d3a <checkPassword+0x9a>
401cec: 83 e8 61 sub $0x61,%eax
401cef: 48 98 cltq
401cf1: 48 83 f8 19 cmp $0x19,%rax
401cf5: 77 dd ja 401cd4 <checkPassword+0x34>
401cf7: 31 d2 xor %edx,%edx
401cf9: eb 1b jmp 401d16 <checkPassword+0x76>
401cfb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
401d00: 83 e8 61 sub $0x61,%eax
401d03: 48 98 cltq
401d05: 48 83 f8 19 cmp $0x19,%rax
401d09: 77 c9 ja 401cd4 <checkPassword+0x34>
401d0b: 48 89 c1 mov %rax,%rcx
401d0e: 48 f7 d1 not %rcx
401d11: 48 39 d1 cmp %rdx,%rcx
401d14: 72 be jb 401cd4 <checkPassword+0x34>
401d16: 48 8d 0c 52 lea (%rdx,%rdx,2),%rcx
401d1a: 48 83 c7 01 add $0x1,%rdi
401d1e: 48 8d 14 8a lea (%rdx,%rcx,4),%rdx
401d22: 48 8d 14 50 lea (%rax,%rdx,2),%rdx
401d26: 0f be 07 movsbl (%rdi),%eax
401d29: 84 c0 test %al,%al
401d2b: 75 d3 jne 401d00 <checkPassword+0x60>
401d2d: 31 c0 xor %eax,%eax
401d2f: 48 39 15 0a 13 20 00 cmp %rdx,0x20130a(%rip) # 603040 <hash>
401d36: 0f 94 c0 sete %al
401d39: c3 retq
401d3a: 31 d2 xor %edx,%edx
401d3c: eb ef jmp 401d2d <checkPassword+0x8d>
401d3e: 66 90 xchg %ax,%ax
And here is the C code (with the things I filled):
int checkPasswordAux(char* s) {
int sum = __0__;
while(__*s__) {
char c = *s;
if(c-'a' > 25) {
return 100;
}
while(c) {
sum += c __%2__;
c __>>=__ 1;
}
s++;
}
return sum;
}
bool checkPassword(char* s) {
char* copy = s;
if(checkPasswordAux(s) > __25__) {
return 0;
}
s = copy;
unsigned long y = 0;
while(__*s__) {
unsigned long x = *s - __97__;
if(x > __25__) {
return 0;
}
if(y > ____) {
return 0;
}
y = __x+13y__;
s++;
}
return __y==0x939f103__;
}
Basically the only thing missing is the 'if' condition in the second function. I can't really find this condition in the assembly.
Another thing is, in the previous questions I find that the value of the variable is 0x939f103.
Again, sorry for being a needy student, any help will be very much appreciated <3
