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Non-repudiation refers to a situation where a statement's author cannot successfully dispute its authorship or the validity of an associated
contract A contract is a legally enforceable agreement between two or more parties that creates, defines, and governs mutual rights and obligations between them. A contract typically involves the transfer of goods, services, money, or a promise to tran ...
. The term is often seen in a legal setting when the authenticity of a signature is being challenged. In such an instance, the authenticity is being "repudiated". For example, Mallory buys a cell phone for $100, writes a paper
cheque A cheque, or check (American English; see spelling differences) is a document that orders a bank (or credit union) to pay a specific amount of money from a person's account to the person in whose name the cheque has been issued. The pers ...
as payment, and signs the cheque with a pen. Later, she finds that she can't afford it, and claims that the cheque is a forgery. The signature guarantees that only Mallory could have signed the cheque, and so Mallory's bank must pay the cheque. This is non-repudiation; Mallory cannot repudiate the cheque. In practice, pen-and-paper signatures aren't hard to
forge A forge is a type of hearth used for heating metals, or the workplace (smithy) where such a hearth is located. The forge is used by the smith to heat a piece of metal to a temperature at which it becomes easier to shape by forging, or to th ...
, but
digital signatures A digital signature is a mathematical scheme for verifying the authenticity of digital messages or documents. A valid digital signature, where the prerequisites are satisfied, gives a recipient very high confidence that the message was created b ...
can be very hard to break.


In security

In general, ''non-repudiation'' involves associating actions or changes with a unique individual. For example, a secure area may use a key card access system where non-repudiation would be violated if key cards were shared or if lost and stolen cards were not immediately reported. Similarly, the owner of a computer account must not allow others to use it, such as by giving away their password, and a policy should be implemented to enforce this.


In digital security

In
digital security Digital security refers to various ways of protecting a computer's internet account and files from intrusion by an outside user or a hacker. Computers and the Internet Internet security involves the protection of a computer's internet account a ...
, non-repudiation means: *A service that provides proof of the
integrity Integrity is the practice of being honest and showing a consistent and uncompromising adherence to strong moral and ethical principles and values. In ethics, integrity is regarded as the honesty and truthfulness or accuracy of one's actions. Inte ...
and origin of data. *An authentication that can be said to be genuine with high confidence. *An authentication that the data is available under specific circumstances, or for a period of time: data availability. Proof of data integrity is typically the easiest of these requirements to accomplish. A data hash such as
SHA2 SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. They are built using the Merkle–Damgård construction, from a one-way compression ...
usually ensures that the data will not be changed undetectably. Even with this safeguard, it is possible to tamper with
data in transit Data in transit, also referred to as data in motion and data in flight, is data en route between source and destination, typically on a computer network. Data in transit can be separated into two categories: information that flows over the publi ...
, either through a
man-in-the-middle attack In cryptography and computer security, a man-in-the-middle, monster-in-the-middle, machine-in-the-middle, monkey-in-the-middle, meddler-in-the-middle, manipulator-in-the-middle (MITM), person-in-the-middle (PITM) or adversary-in-the-middle (AiTM) ...
or
phishing Phishing is a type of social engineering where an attacker sends a fraudulent (e.g., spoofed, fake, or otherwise deceptive) message designed to trick a person into revealing sensitive information to the attacker or to deploy malicious softwa ...
. Because of this, data integrity is best asserted when the recipient already possesses the necessary verification information, such as after being mutually authenticated. The common method to provide non-repudiation in the context of digital communications or storage is Digital Signatures, a more powerful tool that provides non-repudiation in a ''publicly verifiable'' manner. Message Authentication Codes (MAC), useful when the communicating parties have arranged to use a shared secret that they both possess, does not give non-repudiation. A misconception is that encrypting, per se, provides authentication "If the message decrypts properly then it is authentic" - Wrong! MAC can be subject to several types of attacks, like: message reordering, block substitution, block repetition, .... Thus just providing message integrity and authentication, but not non-repudiation. To achieve non-repudiation one must trust a service (a certificate generated by a trusted third party (TTP) called certificate authority (CA)) which prevents an entity from denying previous commitments or actions (e.g. sending message A to B). The difference between MAC and Digital Signatures, one uses symmetric keys and the other asymmetric keys (provided by the CA). Note that the goal is not to achieve confidentiality: in both cases (MAC or digital signature), one simply appends a tag to the otherwise plaintext, visible message. If confidentiality is also required, then an
encryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
scheme can be combined with the digital signature, or some form of
authenticated encryption Authenticated Encryption (AE) and Authenticated Encryption with Associated Data (AEAD) are forms of encryption which simultaneously assure the confidentiality and authenticity of data. Programming interface A typical programming interface for ...
could be used. Verifying the digital origin means that the certified/signed data likely came from someone who possesses the private key corresponding to the signing certificate. If the key used to digitally sign a message is not properly safeguarded by the original owner, digital forgery can occur.


Trusted third parties (TTPs)

To mitigate the risk of people repudiating their own signatures, the standard approach is to involve a
trusted third party In cryptography, a trusted third party (TTP) is an entity which facilitates interactions between two parties who both trust the third party; the Third Party reviews all critical transaction communications between the parties, based on the ease of c ...
. The two most common TTPs are
forensic analyst Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and criminal p ...
s and
notaries A notary is a person authorised to perform acts in legal affairs, in particular witnessing signatures on documents. The form that the notarial profession takes varies with local legal systems. A notary, while a legal professional, is disti ...
. A forensic analyst specializing in handwriting can compare some signature to a known valid signature and assess its legitimacy. A notary is a witness who verifies an individual's identity by checking other credentials and affixing their certification that the person signing is who they claim to be. A notary provides the extra benefit of maintaining independent logs of their transactions, complete with the types of credentials checked, and another signature that can be verified by the forensic analyst. This double security makes notaries the preferred form of verification. For digital information, the most commonly employed TTP is a
certificate authority In cryptography, a certificate authority or certification authority (CA) is an entity that stores, signs, and issues digital certificates. A digital certificate certifies the ownership of a public key by the named subject of the certificate. Thi ...
, which issues public key certificates. A public key certificate can be used by anyone to verify digital signatures without a shared secret between the signer and the verifier. The role of the certificate authority is to authoritatively state to whom the certificate belongs, meaning that this person or entity possesses the corresponding private key. However, a digital signature is forensically identical in both legitimate and forged uses. Someone who possesses the private key can create a valid digital signature. Protecting the private key is the idea behind some
smart card A smart card, chip card, or integrated circuit card (ICC or IC card) is a physical electronic authentication device, used to control access to a resource. It is typically a plastic credit card-sized card with an embedded integrated circuit (IC) c ...
s such as the
United States Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national secu ...
's
Common Access Card The Common Access Card, also commonly referred to as the CAC is a smart card about the size of a credit card. It is the standard identification for Active Duty United States Defense personnel, to include the Selected Reserve and National Guard, ...
(CAC), which never lets the key leave the card. That means that to use the card for encryption and digital signatures, a person needs the
personal identification number A personal identification number (PIN), or sometimes redundantly a PIN number or PIN code, is a numeric (sometimes alpha-numeric) passcode used in the process of authenticating a user accessing a system. The PIN has been the key to facilitat ...
(PIN) code necessary to unlock it.


See also

* Plausible deniability *
Shaggy defense A Shaggy defense is a legal defense strategy in which a person denies an accusation with the simple defense of "it wasn't me", despite overwhelming evidence to the contrary. The strategy's name is derived from reggae musician Shaggy's 2000 sing ...
*
Designated verifier signature A designated verifier signature is a signature scheme in which signatures can only be verified by a single, designated verifier, designated as part of the signature creation. Designated verifier signatures were first proposed in 1996 by Jakobsson M ...
*
Information security Information security, sometimes shortened to InfoSec, is the practice of protecting information by mitigating information risks. It is part of information risk management. It typically involves preventing or reducing the probability of unauthorize ...
*
Undeniable signature An undeniable signature is a digital signature scheme which allows the signer to be selective to whom they allow to verify signatures. The scheme adds explicit signature repudiation, preventing a signer later refusing to verify a signature by omissi ...


References

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External links


"Non-repudiation in Electronic Commerce" (Jianying Zhou), Artech House, 2001

'Non-repudiation' taken from Stephen Mason, Electronic Signatures in Law (3rd edn, Cambridge University Press, 2012)

'Non-repudiation' in the legal context in Stephen Mason, Electronic Signatures in Law (4th edn, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2016) now open source
Public-key cryptography Contract law Notary