Some doubts about signature procedure

[Francesco C.]

Hi, here I come back to ask you some clarification about the signature
procedure.
The purpose of signature procedure is making sure of anybody can't modify
the file you're trying to send.

Many times they use also the check of Md5sum or SHA512Sum, but anyway my
question is:

if any spiteful person succeed in tapping a file transmission of mine, he
surely succeed in signing the modified file. So how can my addressee
realize of that?

In other words, if this spiteful person succeed in replacing a modified
file in a server he also succeed in replacing also the signature file,
doesnt' he?

In this case I can't understand the benefit of signing procedure.
I'm sorry if my exposure was not as good as an english professor :p but I
promise next times it wil be better and better.

Thank you for your patience.

--
Francesco
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