@zgulser When you say "certificate including the public key", do you mean "certificate that results from applying the public key", or do you mean "certificate, plus the actual value of the public key"? There wouldn't be much point in including the public key itself in the message. The point of the signature is for the recipient to verify the signature against the alleged sender's public key, to see if the public key used to sign the file is the same as the public key that the recipient has otherwise determined to be the sender's public key.

If someone wants to forge a message, and they're sending the public key with the message, all they have to do is generate a key pair, sign the file with the private key, and then send the public key with the message. All signing does is indicate that the person who sent the message is the same person who published the public key. If the public key is included in the message, then obviously the person who sent you the public key is the same person who sent the public key.

Nobody I know uses the terms "encryption" and "decryption" to refer to which key is used. The dependency is always the other way around -- if it obscures something, it's encryption; if it recovers something, it's decryption. Statements like "signing is done by decrypting the file" are outright nonsense. Signing is done by signing the file. If you were decrypting the file, the result would be that something encrypted would be decrypted, by definition.

@DavidSchwartz I never said that signing is done by decrypting a file. But signing is done by doing the exact same process that is done to encrypt a file. It calls the exact same function. So if (emphasis of if) we call that process "decryption", then signing is done by decryption. If you want to call the exact same process different names depending not on what is being done, but on the intent of the person doing it, fine, but I don't see why you object to someone pointing out the possibility for confusion.

@Acccumulation I have no idea what you're talking about and I have to wonder if you do. I don't deny that there might be some library or system which has a single function that both signs and encrypts. Sure, such a thing could exist. But WHY ARE YOU TALKING ABOUT THAT PARTICULAR LIBRARY OR SYSTEM? This question isn't about any particular library or system -- why is your answer all about this one weird quirk of this one particular system? It honestly seems to me like you're actively trying to confuse people and resisting all suggests that ou be clear and unconfusing.

@DavidSchwartz My first paragraph talked about the general principle, and the rest of my answer discussed implementation. I don't see it as clear that the OP was not asking about implementation. My answer mentioned a quirk, but it wasn't about that quirk. How am I confusing people? What suggestions am I resisting, and how?

@Acccumulation Your third paragraph goes completely off the rails as I explained in my first comment. By putting the word "generally" before describing a weird quirk, you create the confusion that the quirk is in fact the way things generally work. Your third paragraph essentially sets up definitions that nobody else in the field uses and then you continue to try to explain things with these unique definitions nobody else uses.

@DavidSchwartz "Your third paragraph goes completely off the rails as I explained in my first comment." Your first comment provided nothing resembling an "explanation". At best it was a vague allusion. You are being a bit arrogant and rude. When someone makes it clear that they don't find your comment to have presented a clear and valid objection and asks you to explain your position, simply ignoring their objections to your original comment and simply referring back to the comment they've already rebutted is not civil behavior.

"By putting the word "generally" before describing a weird quirk" You could be a lot clearer what you're talking about. "Your third paragraph essentially sets up definitions that nobody else in the field uses and then you continue to try to explain things with these unique definitions nobody else uses." I really don't see how that's true.

@Acccumulation I don't know how to be clearer. I can't provide a full cryptography education in a few sentences. I'm trying very hard not to be rude, but it's abundantly clear you're trying to teach something you don't know. For example, your second paragraph now asserts that signing has something to do with a process to recover the original signed data from the signature, which doesn't make any sense at all. I don't know how to be more polite about trying to tell someone who doesn't understand something to please not try to teach it to others. I'm honestly really trying.

@DavidSchwartz "I don't know how to be clearer." I find that difficult to believe. You haven't made ANY attempt to be clear at all. Here's how you can be clearer: present ANY explanation as to what's wrong with my answer. "For example, your second paragraph now asserts that signing has something to do with a process to recover the original signed data from the signature" No, it doesn't. How exactly do you not "know how to be more polite" than making false statements?

@Acccumulation Honestly, I've tried as hard as I can to be as clear as I can and it's not working. I don't know what else to do. I'd have to mark up your text sentence by sentence to be clearer, and there isn't room here to do that. Perhaps take a lesson from the downvotes, take a step back, and read what you wrote critically. Your fourth paragraph, for example, has nothing to do with signing.

@Acccumulation Though you never mention RSA anywhere in your answer, it seems quite clear that you learned RSA as your first (maybe only) asymmetric crypto system. And your answer painfully mixes general concepts of cryptography with specific attributes of RSA. That makes it extremely confusing and likely to propagate that same confusion you have to those who read it. Studying, for example, ECDSA would help you clarify what signing is generally versus what it is in RSA specifically.\

@DavidSchwartz "Honestly, I've tried as hard as I can to be as clear as I can and it's not working. I don't know what else to do." I've already explained how you could be clearer, and you're just ignoring me. I'm flagging your comments, as you clearly have no interest in actually contributing anything useful to theis answer.

I'm going to give this one more try on the off chance you're actually being honest and not just trolling me.

Now, here's what's wrong with it:

Either "public key encryption" refers specifically to encryption/decryption or it's a generic term that also includes things like signing and secure random number generation.

Those are the only two possibilities.

"I'm going to give this one more try on the off chance you're actually being honest and not just trolling me." I'm not reading any more of what you wrote. You call me a troll, discussion is over. Message flagged.

If that refers only to encryption specifically, then your statement, "One method of doing this is using public key encryption" is simply false. Public key encryption specifically has nothing to do with signing.

If that refers to encryption generically (encryption, signing, and so on) then your statement, " the result and applying the cryptographic process with the other key, results in the original input" is false. That doesn't happen with ECDSA signing, for example.

And this matters and is not just nitpicking because this very same confusion is a common one and it's a thread throughout your entire answer.

In fact, if you're not trolling, you likely actually are confused about this.

And you'll note that I'm not the only one who downvoted your answer, likely because it confuses things that are specific to encryption with things that are common to both encryption and signing in many places.

And I see you have no interest in understanding this or gaining the knowledge necessary to teach others about it. That's unfortunate.

All I can say is that I tried as hard as I could to educate you.