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I've had an assignment to sign a file using hash and salt, so I read about salt and I got why it's important when it comes to password, but not to sign messages.

  1. How do I sign something using salt? The only way I could think of is that I add the salt to the message then hash both and then send the hash along with the salt (encrypted of course)… but it makes no sense to me.

  2. Even if what I said earlier is how it's done, it's not useful at all. How and why would I use salt to sign a message ?

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marked as duplicate by e-sushi, DrLecter, rath, archie, Stephen Touset Apr 30 at 21:01

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It is not really clear which kind of signature scheme you are assigned to use. As far as I know, the best known signature scheme based on hash function is the Merke signature scheme, but I doubt you mean that in your question. –  tylo Mar 24 at 16:01
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1 Answer 1

There are a couple of reasons why a salted signature would be helpful. It really depends on the particular implementation of a system.

For example if your message is encrypted, then signing with a salt could give you a different signature for the same message each time. This can be very important depending on the situation.

If the salt was actually provided by the client, then it could be used to prevent man-in-the-middle replay attacks. But there may be better ways to achieve this.

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it also might make two different messages that are supposed to have different signatures have the same signature. –  Mohe TheDreamy Mar 21 at 13:44
    
There is an arbitrarily small chance that two different messages will have the same signature. The presence or absence of a salt does not affect these chances. –  James Watkins Mar 21 at 21:41
    
If the signature scheme provides existential unforgeability against chosen message attacks (EUF-CMA), then using a salt does not change anything. –  tylo Mar 24 at 16:09
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