HSMs may have features that provide tamper evidence such as visible signs of tampering or logging and alerting, or tamper resistance which makes tampering difficult without making the HSM inoperable, or tamper responsiveness such as deleting keys upon tamper detection.[4] Each module contains one or more secure cryptoprocessor chips to prevent tampering and bus probing, or a combination of chips in a module that is protected by the tamper evident, tamper resistant, or tamper responsive packaging. A vast majority of existing HSMs are designed mainly to manage secret keys. Many HSM systems have means to securely back up the keys they handle outside of the HSM. Keys may be backed up in wrapped form and stored on a computer disk or other media, or externally using a secure portable device like a smartcard or some other security token.[citation needed]