Editing Digital signature (section)

==Additional security precautions==

===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.

===Using smart card readers with a separate keyboard===
Entering a PIN code to activate the smart card commonly requires a [[numeric keypad]]. Some card readers have their own numeric keypad. This is safer than using a card reader integrated into a PC, and then entering the PIN using that computer's keyboard. Readers with a numeric keypad are meant to circumvent the eavesdropping threat where the computer might be running a [[keystroke logging|keystroke logger]], potentially compromising the PIN code. Specialized card readers are also less vulnerable to tampering with their software or hardware and are often [[Evaluation Assurance Level|EAL3]] certified.

===Other smart card designs===
Smart card design is an active field, and there are smart card schemes which are intended to avoid these particular problems, despite having few security proofs so far.

===Using digital signatures only with trusted applications===
One of the main differences between a digital signature and a written signature is that the user does not "see" what they sign. The user application presents a hash code to be signed by the digital signing algorithm using the private key. An attacker who gains control of the user's PC can possibly replace the user application with a foreign substitute, in effect replacing the user's own communications with those of the attacker. This could allow a malicious application to trick a user into signing any document by displaying the user's original on-screen, but presenting the attacker's own documents to the signing application.

To protect against this scenario, an authentication system can be set up between the user's application (word processor, email client, etc.) and the signing application. The general idea is to provide some means for both the user application and signing application to verify each other's integrity. For example, the signing application may require all requests to come from digitally signed binaries.

===Using a network attached hardware security module===
One of the main differences between a [[Cloud computing|cloud]] based digital signature service and a locally provided one is risk.  Many risk averse companies, including governments, financial and medical institutions,  and payment processors require more secure standards, like [[FIPS 140-2]] level 3 and [[FIPS 201]] certification, to ensure the signature is validated and secure. <!--To finish: current and future applications, actual algorithms, standards, why not as adopted as widely as expected, etc.-->

===WYSIWYS===
{{Main|WYSIWYS}}
Technically speaking, a digital signature applies to a string of bits, whereas humans and applications "believe" that they sign the semantic interpretation of those bits. In order to be semantically interpreted, the bit string must be transformed into a form that is meaningful for humans and applications, and this is done through a combination of hardware and software based processes on a computer system. The problem is that the semantic interpretation of bits can change as a function of the processes used to transform the bits into semantic content. It is relatively easy to change the interpretation of a digital document by implementing changes on the computer system where the document is being processed. From a semantic perspective this creates uncertainty about what exactly has been signed. WYSIWYS (What You See Is What You Sign)<ref name="WYSIWYS_SeminalPaper">{{cite journal|last1=Landrock|first1=Peter|last2=Pedersen|first2=Torben|title=WYSIWYS? – What you see is what you sign?|journal=Information Security Technical Report|date=1998|volume=3|issue=2|pages=55–61|doi=10.1016/S0167-4048(98)80005-8}}</ref> means that the semantic interpretation of a signed message cannot be changed. In particular this also means that a message cannot contain hidden information that the signer is unaware of, and that can be revealed after the signature has been applied. WYSIWYS is a requirement for the validity of digital signatures, but this requirement is difficult to guarantee because of the increasing complexity of modern computer systems. The term WYSIWYS was coined by [[Peter Landrock]] and [[Cryptomathic|Torben Pedersen]] to describe some of the principles in delivering secure and legally binding digital signatures for Pan-European projects.<ref name="WYSIWYS_SeminalPaper" />

===Digital signatures versus ink on paper signatures===

An ink signature could be replicated from one document to another by copying the image manually or digitally, but to have credible signature copies that can resist some scrutiny is a significant manual or technical skill, and to produce ink signature copies that resist professional scrutiny is very difficult.

Digital signatures cryptographically bind an electronic identity to an electronic document and the digital signature cannot be copied to another document. Paper contracts sometimes have the ink signature block on the last page, and the previous pages may be replaced after a signature is applied.  Digital signatures can be applied to an entire document, such that the digital signature on the last page will indicate tampering if any data on any of the pages have been altered, but this can also be achieved by signing with ink and numbering all pages of the contract.