Authentication in the context of Cryptographic hash function

Legal instrument is a legal term of art that is used for any formally executed written document that can be formally attributed to its author, records and formally expresses a legally enforceable act, process, or contractual duty, obligation, or right, and therefore evidences that act, process, or agreement. Examples include a certificate, deed, bond, contract, will, legislative act, notarial act, court writ or process, or any law passed by a competent legislative body in domestic or international law. Many legal instruments were written under seal by affixing a wax or paper seal to the document in evidence of its legal execution and authenticity (which often removed the need for consideration in contract law). However, today, many jurisdictions have abolished the requirement for documents to be under seal in order for them to have legal effect.

Philology (from Ancient Greek φιλολογία (philología) 'love of word') is the study of language in oral and written historical sources. It is the intersection of textual criticism, literary criticism, history, and linguistics with strong ties to etymology. Philology is also defined as the study of literary texts and oral and written records, the establishment of their authenticity and their original form, and the determination of their meaning. A person who pursues this kind of study is known as a philologist. In older usage, especially British, philology is more general, covering comparative and historical linguistics.

Classical philology studies classical languages. Classical philology principally originated from the Library of Pergamum and the Library of Alexandria around the fourth century BC, and was continued by the Ancient Greeks and then the Romans throughout the Roman and Byzantine Empire. It was eventually resumed by European scholars of the Renaissance, where it was soon joined by philologies of other European (Romance, Germanic, Celtic, Slavic, etc.), Asian (Arabic, Persian, Sanskrit, Chinese, etc.), and African (Egyptian, etc.) languages. Indo-European studies involve the comparative philology of all Indo-European languages.

In the law, a seal affixed to a contract or other legal instrument has had special legal significance at various times in the jurisdictions that recognise it. In the courts of common law jurisdictions, a contract which was sealed ("made under seal") was treated differently from other written contracts (which were "made under hand"), although this practice gradually fell out of favour in most of these jurisdictions in the 19th and early 20th century. The legal term seal arises from the wax seal used throughout history for authentication (among other purposes).

Originally, only a wax seal was accepted as a seal by the courts, but by the 19th century many jurisdictions had relaxed the definition to include an impression in the paper on which the instrument was printed, an embossed paper wafer affixed to an instrument, a scroll made with a pen, or the printed words "Seal" or "L.S." (standing for the Latin term locus sigilli meaning "place of the seal").

Cryptography, or cryptology (from Ancient Greek: κρυπτός, romanized: kryptós "hidden, secret"; and γράφειν graphein, "to write", or -λογία -logia, "study", respectively), is the practice and study of techniques for secure communication in the presence of adversarial behavior. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, information security, electrical engineering, digital signal processing, physics, and others. Core concepts related to information security (data confidentiality, data integrity, authentication and non-repudiation) are also central to cryptography. Practical applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords and military communications.

Cryptography prior to the modern age was effectively synonymous with encryption, converting readable information (plaintext) to unintelligible nonsense text (ciphertext), which can only be read by reversing the process (decryption). The sender of an encrypted (coded) message shares the decryption (decoding) technique only with the intended recipients to preclude access from adversaries. The cryptography literature often uses the names "Alice" (or "A") for the sender, "Bob" (or "B") for the intended recipient, and "Eve" (or "E") for the eavesdropping adversary. Since the development of rotor cipher machines in World War I and the advent of computers in World War II, cryptography methods have become increasingly complex and their applications more varied.

Biometrics are body measurements and calculations related to human characteristics and features. Biometric authentication (or realistic authentication) is used in computer science as a form of identification and access control. It is also used to identify individuals in groups that are under surveillance.

Biometric identifiers are the distinctive, measurable characteristics used to label and describe individuals. Biometric identifiers are often categorized as physiological characteristics which are related to the shape of the body. Examples include, but are not limited to fingerprint, palm veins, face recognition, DNA, palm print, hand geometry, iris recognition, retina, odor/scent, voice, shape of ears and gait. Behavioral characteristics are related to the pattern of behavior of a person, including but not limited to mouse movement, typing rhythm, gait, signature, voice, and behavioral profiling. Some researchers have coined the term behaviometrics (behavioral biometrics) to describe the latter class of biometrics.

Provenance (from French provenir 'to come from/forth') is the chronology of the ownership, custody or location of a historical object. The term was originally mostly used in relation to works of art, but is now used in similar senses in a wide range of fields, including archaeology, paleontology, archival science, economy, computing, and scientific enquiry in general.

The primary purpose of tracing the provenance of an object or entity is normally to provide contextual and circumstantial evidence for its original production or discovery, by establishing, as far as practicable, its later history, especially the sequences of its formal ownership, custody and places of storage. The practice has a particular value in helping authenticate objects. Comparative techniques, expert opinions and the results of scientific tests may also be used to these ends, but establishing provenance is essentially a matter of documentation. The term dates to the 1780s in English. Provenance is conceptually comparable to the legal term chain of custody.

Plastic cards usually serve as identity documents, thus providing authentication. In combination with other assets that complement the data stored on the card, like PIN numbers, they also serve authorization purposes, most often as debit or credit cards for allowing their holders to do financial transactions. Early and simpler cards feature only hard-to-imitate integrated photographs, security holograms, guillochés, or a magnetic strip on which few bytes of personal data could be stored. Today, smart cards, i.e. those equipped with an electronic chip (storage, or RFID), serve as high-security active electronic documents that allow their holder to qualify for driving cars (drivers license card), receive medical treatment (health insurance cards), do banking and more.

Photo identification or photo ID is an identity document that includes a photograph of the holder, usually only their face. The most commonly accepted forms of photo ID are those issued by government authorities, such as driver's licenses, identity cards and passports, but special-purpose photo IDs may be also produced, such as internal security or access control cards.

Photo identification may be used for face-to-face authentication of identity of a party who either is personally unknown to the person in authority or because that person does not have access to a file, a directory, a registry or an information service that contains or that can render a photograph of somebody on account of that person's name and other personal information.

Internet identity (IID), also online identity, online personality, online persona or internet persona, is a social identity that an Internet user establishes in online communities and websites. It may also be an actively constructed presentation of oneself. Although some people choose to use their real names online, some Internet users prefer to be anonymous, identifying themselves by means of pseudonyms, which reveal varying amounts of personally identifiable information. An online identity may even be determined by a user's relationship to a certain social group they are a part of online. Some can be deceptive about their identity.

In some online contexts, including Internet forums, online chats, and massively multiplayer online role-playing games (MMORPGs), users can represent themselves visually by choosing an avatar, an icon-sized graphic image. Avatars are one way users express their online identity. Through interaction with other users, an established online identity acquires a reputation, which enables other users to decide whether the identity is worthy of trust. Online identities are associated with users through authentication, which typically requires registration and logging in. Some websites also use the user's IP address or tracking cookies to identify users.

SOCKS is an Internet protocol that exchanges network packets between a client and server through a proxy server. SOCKS5 optionally provides authentication, so only authorized users may access a server. Practically, a SOCKS server proxies TCP connections to an arbitrary IP address and provides a means for UDP packets to be forwarded. The SOCKS protocol operates between the application layer and the transport layer. A SOCKS server accepts incoming client connection on TCP port 1080.

Gold, alongside platinum and silver, is highly popular among precious metals as an investment. Investors generally buy gold as a way of diversifying risk, especially through the use of futures contracts and derivatives. The gold market is subject to speculation and volatility as are other markets.

An authentication protocol is a type of computer communications protocol or cryptographic protocol specifically designed for transfer of authentication data between two entities. It allows the receiving entity to authenticate the connecting entity (e.g. Client connecting to a Server) as well as authenticate itself to the connecting entity (Server to a client) by declaring the type of information needed for authentication as well as syntax. It is the most important layer of protection needed for secure communication within computer networks.

Keystroke dynamics, keystroke biometrics, typing dynamics, or typing biometrics refer to the collection of biometric information generated by key-press-related events that occur when a user types on a keyboard. Use of patterns in key operation to identify operators predates modern computing, and has been proposed as an authentication alternative to passwords and PIN numbers.

The FIDO (Fast IDentity Online) Alliance is an open industry association launched in February 2013 whose stated mission is to develop and promote authentication standards that "help reduce the world’s over-reliance on passwords". FIDO addresses the lack of interoperability among devices that use strong authentication and reduces the problems users face creating and remembering multiple usernames and passwords.

FIDO supports a full range of authentication technologies, including biometrics such as fingerprint and iris scanners, voice and facial recognition, as well as existing solutions and communications standards, such as Trusted Platform Modules (TPM), USB security tokens, embedded Secure Elements (eSE), smart cards, and near-field communication (NFC). The USB security token device may be used to authenticate using a simple password (e.g. four-digit PIN) or by pressing a button. The specifications emphasize a device-centric model. Authentication over an insecure channel happens using public-key cryptography. The user's device registers the user to a server by registering a public key. To authenticate the user, the device signs a challenge from the server using the private key that it holds. The keys on the device are unlocked by a local user gesture such as a biometric or pressing a button.

A backdoor is a typically covert method of bypassing normal authentication or encryption in a computer, product, embedded device (e.g. a home router), or its embodiment (e.g. part of a cryptosystem, algorithm, chipset, or even a "homunculus computer"—a tiny computer-within-a-computer such as that found in Intel's AMT technology). Backdoors are most often used for securing remote access to a computer, or obtaining access to plaintext in cryptosystems. From there it may be used to gain access to privileged information like passwords, corrupt or delete data on hard drives, or transfer information within autoschediastic networks.

In the United States, the 1994 Communications Assistance for Law Enforcement Act forces internet providers to provide backdoors for government authorities. In 2024, the U.S. government realized that China had been tapping communications in the U.S. using that infrastructure for months, or perhaps longer; China recorded presidential candidate campaign office phone calls—including employees of the then-vice president of the nation, and of the candidates themselves.