Page "Digital signature" Paragraph 33

In many scenarios, the sender and receiver of a message may have a need for confidence that the message has not been altered during transmission.

Although encryption hides the contents of a message, it may be possible to change an encrypted message without understanding it.

( Some encryption algorithms, known as nonmalleable ones, prevent this, but others do not.

) 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 ).