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I am working though the paper of Goldwasser, Micali and Rivest ''A Digital Signature Scheme Secure Against Adaptive Chosen- Message Attacks''

In the hierarchy of the severity of breaking the signature scheme, what does the ''a priori'' in the definition of Selective Forgery mean?

Selective Forgery: Forge a signature for a particular message chosen priori by the enemy.

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The reference is Shafi Goldwasser, Silvio Micali, Ronald L. Rivest, A Digital Signature Scheme Secure Against Adaptive Chosen-Message Attacks, in SIAM Journal on Computing, 1988.

It defines

Selective forgery. Forge a signature for a particular message chosen a priori by the enemy.

Here, "a priori" means, informally, that the message is to some degree chosen by the adversary (or at least restricted) before the attack is carried, as opposed to being entirely a result of the attack.

The authors do not give a formal definition. One could be that the adversary must define, before being given the public key, a suitably small subset of the full message set among which s/he will later be able to exhibit a forgery (that is, an acceptable message/signature pair for which s/he has not obtained the signature by asking a signing oracle).

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