Editing File verification (section)

== Authenticity verification ==
It is often desirable to verify that a file hasn't been modified in transmission or storage by untrusted parties, for example, to include malicious code such as [[virus]]es or [[Backdoor (computing)|backdoor]]s. To verify the authenticity, a classical hash function is not enough as they are not designed to be [[collision resistance|collision resistant]]; it is computationally trivial for an attacker to cause deliberate hash collisions, meaning that a malicious change in the file is not detected by a hash comparison.  In cryptography, this attack is called a [[preimage attack]].

For this purpose, [[cryptographic hash function]]s are employed often. As long as the hash sums cannot be tampered with &mdash; for example, if they are communicated over a secure channel &mdash; the files can be presumed to be intact. Alternatively, [[digital signature]]s can be employed to assure [[tamper resistance]].