unforgeable

Unforgeable is a security term used to describe cryptographic and anti-counterfeiting mechanisms that resist the creation of valid, unauthorized outputs by an attacker. In practice, unforgeability means that it is computationally infeasible for a malicious party to produce a legitimate item that would be accepted as valid by the intended system, given the proper keys and within a reasonable amount of time. It is a property, not an absolute guarantee, and depends on formal security assumptions and proper implementation.

In cryptography, unforgeability is a central goal of digital signatures. A digital signature created with a private key should not be forgeable by anyone who does not possess the corresponding key. After observing legitimate signatures on other messages, a forger should not be able to produce a valid signature on a new message. The standard formal notion is existential unforgeability under adaptive chosen-message attack (EUF-CMA) for signatures, which captures resilience against an attacker who can request signatures on chosen messages.

Message authentication codes (MACs) also aim for unforgeability. With access to a MAC oracle for chosen messages, an attacker should not be able to forge a valid tag for any new message. The corresponding security definitions ensure that, under reasonable computational assumptions, successful forgery remains negligible.

Applications of unforgeability include secure communications (TLS), code signing, digital certificates, software distribution, and anti-counterfeiting in physical goods. Real-world guarantees rely on proper key management, randomness, and protection against side-channel or implementation flaws.