Why editing hexadecimal "code" affect programs and certification so much?

[RikTelner]

Recently I opened game with "hex editor". You know, it's kind of program for newbies that makes it able to read program in hexadecimal code and make it able to turn, or rather read code and try to reverse engineer it by turning hexa's into Assembler commands(?).

And then I met two face-hitting why's. I am not close to comprehending even how it is possible.

Why is certification so good?
First of all, whenever you modify a single byte, you just edit ONE REGULAR LETTER, for example, you change A into E. Certification drops, it doesn't say "Author: Daniweb" it says "Author: Unknown", just ONE LETTER is other, and the certificate is already considered false, how? I know you can hash program and then look into each time you double-click on it. But how would this be possible? Do I have EVERY SINGLE hash on my computer?

Modifying sentence, crashes application
I tried to modify sentence from for example "D.a.n.i. .W.e.b" to "D.o.n.k.e.y." (the replaced strings were same length), after I started application, it said at once, that it's missing a file, but it is surely there and fully accessable since normal, unmodified application (I had working/real copy) works perfectly and when I modify innocent text, it at once throws error at me. Why? It is just normal string, isn't it? I didn't replace like link to file, it was a text that it prompts you when it says "You're disconnected", I replaced something like this.

[RikTelner]

"First of all, whenever you modify a single byte, you just edit ONE REGULAR LETTER, for example, you change A into E."

I actually meant to say

"First of all, whenever you modify single letter, you just edit TWO REGULAR BYTES, for example 41 (A) into 45 (E)."

[Taywin]

Hmm... If the part you modified is the data part, it could break certification because the checksum may be involved in the process when issue the certification. If the part you modified is the execution part, then it could easily break the assembly code. Would that give you some idea?

[gusano79]

it could break certification because the checksum may be involved in the process when issue the certification

This is one of the major reasons to have a certificate; to protect against modifications. Though the signature algorithm is usually much more secure than a simple checksum.

after I started application, it said at once, that it's missing a file

The actual error could be something completely different, and probably is. "Missing file" errors are notoriously uninformative, but it's all you can get from some software. It probably means some sort of I/O problem, but I've also seen similar messages when the real failure is security-related.

[Maritimo]

I don't think this web is to teach how to crack a programm. Anyway, if you want to crack a code, you need to know a lot that seems you don't. So I recomend don't try to solve this here, you must study a lot. Your first step shuld be to learn assembler.

[RikTelner]

@Taymin Yes, I also mentioned checksums idea in my main post. I think you should read it.
@gusano79 It's not like the program didn't work before. I just modified ONE sentence which didn't look like part of program at all and then it became unworking code at once.
@Maritimo (read comment in downvote)

[gusano79]

which didn't look like part of program at all

So the lesson here is software is often more complicated than it seems, especially compiled and optimized software.