What is this assembly doing?

Question

I wrote a very basic crackme to learn how assembly works.

Even though I wrote it myself, I am having some trouble understanding a few pieces of the assembly:

What I know up to now is: [rbp+rax+input_buffer] is basically input_buffer[rax], xor'ing two of the same registers resets them and that's about it (apart from the very basic stuff like add, mov, inc).

I specifically don't understand what movsx .. and add ecx, 0FFF.. is doing.

The input_buffer is filled with _fgets. I'm intentionally not saying a working input to see if you can figure it out (it shouldn't be hard anyways).

A proper input would be 0123456789\n, note that I discard the newline via strcspn.

Answer 1

I guess the original code is something like:

char input_buffer[...];
// ...
int c = input_buffer[i];

Since the variable c is a int and input_buffer is a char[], your compiler has to promote the read char as a int. That's why you have the movsx instruction. It will read the current character and sign extend it, so it'll fit to a int.

About the add, it's common for a compiler to encode a sub dst, imm as add dst, -imm, if you negate 0xffffffd0 (SHIFT - in IDA), you'll obtain -0x30. This is how you convert the ASCII digit into a integer.