Sorry for my silly question, but I have some doubts on this point. I'm trying to explain it better with an example.
Suppose you use RSA to sign a package and you have the public key. If the signature is generated by the server with its secret key you are able to decipher it because you have the public key and verify the integrity of your package. Suppose I make a "fake" package and I'm able to create a correct signature like this:
00 01 FF .. .. FF 00 ASN.1 SHA1
due to the deciphering process:
cleartext = C^e mod n
I can encrypt my signature in this way:
C mod n = root(cleartext,e)
and make a correctly padded signature without knowing the private key?
e = public exponent = 3 C mod n = cubicroot (cleartext)