Authentication (from Greek: αυθεντικός ; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the subject are true ("authentification" is a French language variant of this word). This might involve confirming the identity of a person, tracing the origins of an artifact, ensuring that a product is what its packaging and labeling claims to be, or assuring that a computer program is a trusted one.
There are two types of techniques for doing this.
The first is comparing the attributes of the object itself to what is known about objects of that origin. For example, an art expert might look for similarities in the style of painting, check the location and form of a signature, or compare the object to an old photograph. An archaeologist might use carbon dating to verify the age of an artifact, do a chemical analysis of the materials used, or compare the style of construction or decoration to other artifacts of similar origin. The physics of sound and light, and comparison with a known physical environment, can be used to examine the authenticity of audio recordings, photographs, or videos.
Attribute comparison may be vulnerable to forgery. In general, it relies on the fact that creating a forgery indistinguishable from a genuine artifact requires expert knowledge, that mistakes are easily made, or that the amount of effort required to do so is considerably greater than the amount of money that can be gained by selling the forgery.
In art and antiques certificates are of great importance, authenticating an object of interest and value. Certificates can, however, also be forged and the authentication of these pose a problem. For instance, the son of Han van Meegeren, the well-known art-forger, forged the work of his father and provided a certificate for its provenance as well; see the article Jacques van Meegeren.
The second type relies on documentation or other external affirmations. For example, the rules of evidence in criminal courts often require establishing the chain of custody of evidence presented. This can be accomplished through a written evidence log, or by testimony from the police detectives and forensics staff that handled it. Some antiques are accompanied by certificates attesting to their authenticity. External records have their own problems of forgery and perjury, and are also vulnerable to being separated from the artifact and lost.
Currency and other financial instruments commonly use the first type of authentication method. Bills, coins, and cheques incorporate hard-to-duplicate physical features, such as fine printing or engraving, distinctive feel, watermarks, and holographic imagery, which are easy for receivers to verify.
Consumer goods such as pharmaceuticals, perfume, fashion clothing can use either type of authentication method to prevent counterfeit goods from taking advantage of a popular brand's reputation (damaging the brand owner's sales and reputation). A trademark is a legally protected marking or other identifying feature which aids consumers in the identification of genuine brand-name goods.
Counterfeit products are often offered to consumers as being authentic. Counterfeit consumer goods such as electronics, music, apparel, and Counterfeit medications have been sold as being legitimate. Efforts to control the supply chain and educate consumers to evaluate the packaging and labeling help ensure that authentic products are sold and used.
The authentication of information can pose special problems (especially man-in-the-middle attacks), and is often wrapped up with authenticating identity.
Literary forgery can involve imitating the style of a famous author. If an original manuscript, typewritten text, or recording is available, then the medium itself (or its packaging - anything from a box to e-mail headers) can help prove or disprove the authenticity of the document.
However, text, audio, and video can be copied into new media, possibly leaving only the informational content itself to use in authentication.
Various systems have been invented to allow authors to provide a means for readers to reliably authenticate that a given message originated from or was relayed by them. These involve authentication factors like:
The opposite problem is detection of plagiarism, where information from a different author is passed of as a person's own work. A common technique for proving plagiarism is the discovery of another copy of the same or very similar text, which has different attribution. In some cases excessively high quality or a style mismatch may raise suspicion of plagiarism.
Determining the truth or factual accuracy of information in a message is generally considered a separate problem from authentication. A wide range of techniques, from detective work to fact checking in journalism, to scientific experiment might be employed.
With closed circuit television cameras in place in many public places it has become necessary to conduct video authentication to establish credibility when video CCTV recordings are used to identify crime. CCTV is a visual assessment tool. Visual Assessment means having proper identifiable or descriptive information during or after an incident. These systems should not be used independently from other security measures and their video recordings must be authenticated in order to be proven genuine when identifying an accident or crime. 
The ways in which someone may be authenticated fall into three categories, based on what are known as the factors of authentication: something you know, something you have, or something you are. Each authentication factor covers a range of elements used to authenticate or verify a person's identity prior to being granted access, approving a transaction request, signing a document or other work product, granting authority to others, and establishing a chain of authority.
Security research has determined that for a positive identification, elements from at least two, and preferably all three, factors be verified. The three factors (classes) and some of elements of each factor are:
When elements representing two factors are required for identification, the term two-factor authentication is applied. . e.g. a bankcard (something the user has) and a PIN (something the user knows). Business networks may require users to provide a password (knowledge factor) and a random number from a security token (ownership factor). Access to a very high security system might require a mantrap screening of height, weight, facial, and fingerprint checks (several inherence factor elements) plus a PIN and a day code (knowledge factor elements), but this is still a two-factor authentication.
Historically, fingerprints have been used as the most authoritative method of authentication, but recent court cases in the US and elsewhere have raised fundamental doubts about fingerprint reliability. Outside of the legal system as well, fingerprints have been shown to be easily spoofable, with British Telecom's top computer-security official noting that "few" fingerprint readers have not already been tricked by one spoof or another. Hybrid or two-tiered authentication methods offer a compelling solution, such as private keys encrypted by fingerprint inside of a USB device.
In a computer data context, cryptographic methods have been developed (see digital signature and challenge-response authentication) which are currently not spoofable if and only if the originator's key has not been compromised. That the originator (or anyone other than an attacker) knows (or doesn't know) about a compromise is irrelevant. It is not known whether these cryptographically based authentication methods are provably secure since unanticipated mathematical developments may make them vulnerable to attack in future. If that were to occur, it may call into question much of the authentication in the past. In particular, a digitally signed contract may be questioned when a new attack on the cryptography underlying the signature is discovered.
In litigation electronic authentication of computers, digital audio recordings, video recordings analog and digital must be authenticated before being accepted into evidence as tampering has become a problem. All electronic evidence should be proven genuine before used in any legal proceeding.
To distinguish "authentication" from the closely related term "authorization," the short-hand notations A1 (authentication) and A2 (authorization) are occasionally used. The terms AuthN / AuthZ (AuthR) or Au / Az are also used to make this distinction in some communities.
The problem of authorization is often thought to be identical to that of authentication; many widely adopted standard security protocols, obligatory regulations, and even statutes are based on this assumption. However, more precise usage describes authentication as the process of verifying a claim made by a subject that it should be treated as acting on behalf of a given principal (person, computer, smart card etc.), while authorization is the process of verifying that an authenticated subject has the authority to perform a certain operation. Authentication, therefore, must precede authorization. For example, when you show proper identification to a bank teller, you could be authenticated by the teller as acting on behalf of a particular account holder, and you would be authorized to access information about the accounts of that account holder. You would not be authorized to access the accounts of other account holders. A different perspective: a merchant in the credit card network does not require a personal identification number for authentication of the claimed identity of the buyer; but, in contrast, the merchant usually does require a signature from the authenticated person to use as proof of authorization of the transaction by the authenticated person.
Since authorization cannot occur without authentication, the former term is sometimes used to mean the combination of authentication and authorization.
One familiar use of authentication and authorization is access control. A computer system supposed to be used only by those authorized must attempt to detect and exclude the unauthorized. Access to it is therefore usually controlled by insisting on an authentication procedure to establish with some established degree of confidence the identity of the user, thence granting those privileges as may be authorized to that identity. Common examples of access control involving authentication include:
In some cases, ease of access is balanced against the strictness of access checks. For example, the credit card network does not require a personal identification number for authentication of the claimed identity; and a small transaction usually do not even require a signature of the authenticated person for proof of authorization of the transaction. The security of the system is maintained by limiting distribution of credit card numbers, and by the threat of punishment for fraud.
Security experts argue that it is impossible to prove the identity of a computer user with absolute certainty. It is only possible to apply one or more tests which, if passed, have been previously declared to be sufficient to proceed. The problem is to determine which tests are sufficient, and many such are inadequate. Any given test can be spoofed one way or another, with varying degrees of difficulty.
Authentication (pronounced /ɔˈθɛntɪˌkeɪt/ "aw-then-ti-keyt") (from Greek αυθεντικός; real or genuine, from authentes; author) is the actions taken to establish or verify something (or someone) as authentic. It is the method of verifying claims made by someone about the thing. This might involve confirming the identity of a person, the origins of an object (or something), or to make sure that a computer program (or user) is a trusted one.