Digital signature forgery
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 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.
- Choose a random signature, .
- Send the message as: .
- The recipient checks the signature: so the check will pass.
Note: The sender cannot control the message content so it will be a random message, that may help in some cases.
This forgery can be used with two messages and their signatures as follows:
- Let be the RSA signature on the message, , under the key, .
- Analogously, .
- In that case will be the valid RSA signature on the message, , under the key, .
Selective forgery is the creation (by an adversary) of a message/signature pair where has been chosen by the adversary 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 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.
- Kantarcioglu, Murat. "Digital Signatures" (PDF).
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