In a cryptographic digital signature or MAC system, digital signature forgery is the ability to create a pair consisting of a message, , and a signature (or MAC), , that is valid for , where has not been signed in the past by the legitimate signer. There are three types of forgery: existential, selective, and universal.
Besides the following attacks, there is also a total break: when adversary can compute the signer's private key and therefore forge any possible signature on any message.
Existential forgery (existential unforgeability, EUF)
Existential forgery is the creation (by an adversary) of at least one message/signature pair, , where was not produced by the legitimate signer. The adversary need not have any control over ; need not have any particular meaning; the message content is irrelevant — as long as the pair, , is valid, the adversary has succeeded in constructing an existential forgery.
Existential forgery is essentially the weakest adversarial goal, therefore the strongest schemes are those that are existentially unforgeable. Nevertheless, many state-of-art signature algorithms allow existential forgery. For example, an RSA forgery can be done as follows:
- Let be the RSA public key.
- The legitimate signer signs a message m. This means, he calculates the signature .
- The adversarial can create the message .
- The signature of , so the adversarial has found a valid pair .
Signature of a product of two messages
This forgery can be done with two messages and their signatures since
Selective forgery (selective unforgeability, SUF)
Selective forgery is the creation (by an adversary) of a message/signature pair where has been chosen by the challenger prior to the attack. may be chosen to have interesting mathematical properties with respect to the signature algorithm; however, in selective forgery, must be fixed before the start of the attack.
The ability to successfully conduct a selective forgery attack implies the ability to successfully conduct an existential forgery attack.
Universal forgery (universal unforgeability, UUF)
Universal forgery is the creation (by an adversary) of a valid signature, , for any given message, . An adversary capable of universal forgery is able to sign messages he chose himself (as in selective forgery), messages chosen at random, or even specific messages provided by an opponent.
- Vaudenay, Serge (September 16, 2005). A Classical Introduction to Cryptography: Applications for Communications Security (1st ed.). Springer. p. 254. ISBN 978-0-387-25464-7.
- Goldwasser, Shafi; Bellare, Mihir (2008). Lecture Notes on Cryptography. Summer course on cryptography. p. 170.
- Smart, Nigel P. Cryptography Made Simple. Springer. p. 217. ISBN 978-3-319-21935-6.
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