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I understand that one can define EUF-CMA of a signature scheme is terms of a game where the adversary is allowed to query signatures on messages of his choosing, and at the end of the game he must output a pair (m, s) such that s is a valid signature of m, and m has not been queried before. My questions are the following:

  1. How does one select selective unforgability under chosen message attack in terms of a game?

  2. Are there examples of signatures which are SUF-CMA, but not EUF-CMA?

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1) In the selective unforgeability game (often also denoted universal unforgeability), the adversary is given the public key and a target message for which it needs to produce a forgery (instead of giving the adversary only the public key and letting the adversary choose the target message).

2) No, any scheme that is EUF-CMA is also SUF-CMA (this is easy to show; build an EUF-CMA adversary from any SUF-CMA one by simply running the SUF one on an arbitrarily chosen message).

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  • $\begingroup$ Hi. Thanks for your response. Apologies, for 2) I meant the other way around (see edit) $\endgroup$ – user32609 Mar 19 '16 at 16:24
  • $\begingroup$ Ah ok. Yes, that's perfectly possible. You can show how you can modify any SUF-CMA secure scheme such that it remains SUF secure, but is trivially EUF insecure. You can think about that ;) I guess I have time tomorrow to edit my answer. $\endgroup$ – DrLecter Mar 19 '16 at 16:58

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