In reverse engineering, what does it mean to "patch a file"?

Question

I've been solving some crackmes, and every file that I download, in the "instructions" says: "Patching is obviously not allowed". What does that mean? And how does one patch a file?

Answer 1

Following quote comes from unix.stackexchange:

To patch a file means to modify it, with the connotation that the modification is generally small.

So, patching the executable is a process of changing its content usually performed for changing its original behaviour.

Patching is obviously not allowed means that the author of a particular crackme wants you to crack it without modifying its content. For instance, consider a simple program:

ask_user_for_password();
read_input();
if (isPasswordCorrect())
    printf(“Congratulations, you have cracked it!\n”);
else
    printf(“Wrong password! Try again!\n”);

Of course one can patch it so that it always prints the "Congratulations [...]" message, but the point of this very simple crackme is to actually find that password somehow (by finding decryption key for example).

Generally, to patch a file, you want to:

1. Find the place / instruction you want to change (it may be jnz to jz as @Axel Munoz said, but it may be a jmp to the location containing the code you want to be executed as well).
2. Find out what is the machine code representing the instruction you want to put there - it can be done by first compiling the program written in assembly and then disassembling it. If you are working on x86 architecture, you can use this site as well.
3. Open the binary in a hex editor, find the offset where you want to put your code and just paste the machine code you obtained in the previous step.

Answer 2

The term "patching a [binary]" means to modify the instruction set. For example if there is a conditional jump that requires some memory to be set to a specific value, instead of modifying the memory, which might be the objective of the crackme, I could simply patch the binary to always take that jump (either by changing the condition of the jump from jnz to jz). Patching through a debugger easily allows me to change the instruction, save the new binary, and then run it.

Here is some more info.