Editing Digital signature (section)

===Putting the private key on a smart card===
All public key / private key cryptosystems depend entirely on keeping the private key secret. A private key can be stored on a user's computer, and protected by a local password, but this has two disadvantages:
* the user can only sign documents on that particular computer
* the security of the private key depends entirely on the [[computer insecurity|security]] of the computer

A more secure alternative is to store the private key on a [[smart card]]. Many smart cards are designed to be tamper-resistant (although some designs have been broken, notably by [[Ross J. Anderson|Ross Anderson]] and his students<ref name="jKyz7">{{Cite web |url=http://sec.cs.ucl.ac.uk/users/smurdoch/papers/oakland14chipandskim.pdf |title=Chip and Skim: cloning EMV cards with the pre-play attack |access-date=2018-04-06 |archive-date=2018-05-16 |archive-url=https://web.archive.org/web/20180516203624/http://sec.cs.ucl.ac.uk/users/smurdoch/papers/oakland14chipandskim.pdf |url-status=dead }}</ref>). In a typical digital signature implementation, the hash calculated from the document is sent to the smart card, whose CPU signs the hash using the stored private key of the user, and then returns the signed hash. Typically, a user must activate their smart card by entering a [[personal identification number]] or PIN code (thus providing [[two-factor authentication]]). It can be arranged that the private key never leaves the smart card, although this is not always implemented. If the smart card is stolen, the thief will still need the PIN code to generate a digital signature. This reduces the security of the scheme to that of the PIN system, although it still requires an attacker to possess the card. A mitigating factor is that private keys, if generated and stored on smart cards, are usually regarded as difficult to copy, and are assumed to exist in exactly one copy. Thus, the loss of the smart card may be detected by the owner and the corresponding certificate can be immediately revoked. Private keys that are protected by software only may be easier to copy, and such compromises are far more difficult to detect.