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5

Designing your own crypto protocol (using existing primitives) is dangerous if you're not sufficiently familiar with cryptographic protocol design and the ways such protocols might be attacked. If you wish to gain such familiarity, I'd recommend taking a few introductory crypto courses that focus on protocol design and analysis.* This won't turn you ...

2

The problem with the HMAC-based solution you drew up is if the shared secret $s$ has low entropy; for example, it's actually a password that could conceivably be in a dictionary. In this case, someone could listen to the exchange $r_1, \operatorname{HMAC}(C \mathbin\| r_1, s)$, and go through his dictionary of possible values of $s$, and see if any one of ...

2

swap-or-not seems perfect for your use case.

1

If it is for completely random data you could still make a program that uses the random looking input to make different choices. For instance, you could sign two .jar files in Java, using the SHA-256 hash over the file in the META-INF folder. Then you can use the different files a property to make one choice or the other. Basically you're replacing one of ...

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