Existential forgery

In a cryptographic digital signature or MAC system, digital signature forgery is the ability to create a pair consisting of a message, ${\displaystyle m}$, and a signature (or MAC), ${\displaystyle \sigma }$, that is valid for ${\displaystyle m}$, where ${\displaystyle m}$ has not been signed in the past by the legitimate signer. There are three types of forgery: existential, selective, and universal.

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