High Quality Content by WIKIPEDIA articles! In cryptography, a preimage attack on a cryptographic hash is an attempt to find a message that has a specific hash value. Some significant preimaging attacks have already been discovered, but they are not yet practical. If a practical preimaging attack is discovered, it would drastically affect many Internet protocols. In this case, "practical" means that it could be executed by an attacker in a meaningful amount of time for a meaningful amount of money. A preimaging attack that costs trillions of dollars and takes decades to preimage one desired hash value or one message is not practical; one that costs a few thousand dollars and takes a few weeks might be very practical.