In a cryptosystem, weaknesses can be introduced through insecure handling of the plaintext, allowing an attacker to bypass the cryptography altogether. Plaintext is vulnerable in use and in storage, whether in electronic or paper format. Physical security deals with how media can be secured from local, physical, attacks. for instance, an attacker might enter a poorly secured building and attempt to open locked desk drawers or safes. An attacker can also engage in dumpster diving, and may be able to reconstruct shredded information. One countermeasure is to burn or thoroughly crosscut shred discarded printed plaintexts. (See Paper shredder for specifications.) If plaintext is kept in a computer file, the disk along with the entire computer and its components must be secure. Sensitive data is sometimes processed on computers whose mass storage is removable, in which case physical security of the removed disk is separately vital. In the case of securing a computer, that security must be physical (e.g., against burglary, brazen removal under cover of a repair, installation of covert monitoring devices, etc.) as well as virtual (e.g., operating system modification, illicit network access, Trojan programs, ...). The wide availability of keydrives, which can plug into most modern computers and receive hundreds of megabytes of data, poses another severe security headache. A spy (perhaps posing as a cleaning person) could easily conceal one and even swallow it, if necessary.