Suppose there's an entity which produces files. User A has one of those files and decides to send it to user B.
How can user B validate the authenticity of the file without communicating with the producer? Is it even possible?
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3$\begingroup$ Hi, take a look at digital signature and electronic signature. $\endgroup$– knok16Jul 23, 2015 at 10:25
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$\begingroup$ Do you allow for the producer to take appropriate measures so that authenticity can be validated by anyone without further communication with him, independently of the number of people who may want to check authenticity? BTW, what did you look, where did you search? What didn't you understand from available information? $\endgroup$– babouJul 23, 2015 at 11:44
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$\begingroup$ Didn't we close a very similar question not too long ago? $\endgroup$– RaphaelJul 23, 2015 at 13:41
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1$\begingroup$ What research have you done? What specifically can be assumed? Can you assume the existence of a PKI? assume that B knows A's public key in advance? Can B communicate with a trusted third party (Trent) who has previously communicated with A? (If so, then use Kerberos-like solutions, or have Trent be a certificate authority.) Please read the background material provided to you, then edit the question to provide this information. Until then, the answer will only be "it depends what your specific constraints are". $\endgroup$– D.W. ♦Jul 23, 2015 at 21:59
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It depends on what you call "without communicating". How does B know that A really exists ? Either B speaks to A, or C tells to B that A exists, and B trusts C.
So, if a trusted C exists, C may provide to B a key to authenticate A. If a trusted C doesn't exist, B can't know if A exist, and authenticate its messages makes no sense.
