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Say I have a string that I want to keep hidden. You can then enter a known string which is added to the end of the hidden string and you are given the sha256 hash of the combined string. If you were to do this many times with whatever strings you wanted and the hidden string remained fixed, would there be a practical way to determine the hidden string?

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If this is motivated by a homework question asking about the security of H(K|M) as a MAC (for hash function H, key K, and message M), you should consider the possibility that an attacker may not need to learn K in order to produce a forgery. Think about how SHA256 works internally. –  Seth May 22 at 17:22
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How much entropy does the hidden string have? What have you tried? Are you familiar with the random oracle model? Have you tried figuring out what answer you'd get in the random oracle model? For that matter, what have you tried? Please edit the question to show your work. –  D.W. May 22 at 21:52
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