Simplify the following text for a Grade 7 native English-speaking person.
Original: However , if a message is digitally signed , any change in the message after signature will invalidate the signature . Furthermore , there is no efficient way to modify a message and its signature to produce a new message with a valid signature , because this is still considered to be computationally infeasible by most cryptographic hash functions ( see collision resistance ) .
Simplified: The message can not be modified , and if modified a new message with a valid signature can not be produced .