I am also no expert, but I am not sure that I agree with MattSteelblade (<a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11550845" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11550845</a>).<p>There is certainly such a thing as homomorphic encryption (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Homomorphic_encryption" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Homomorphic_encryption</a>), which allows one to perform transformations on encrypted text without being able to decrypt it.  As long as one of the transformations that can be performed is a measure of closeness (which is certainly the case for fully homomorphic encryption (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Homomorphic_encryption#Fully_homomorphic_encryption" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Homomorphic_encryption#Fully_h...</a> )), <i>and as long as you know the ciphertext of the possible numerical responses</i>, then you can read off closeness without being able to decrypt the hash.<p>The emphasised bit is a drawback, but it demonstrates the theoretical possibility; and, although I don&#x27;t know of an implementation, nor do I see anything inherently contradictory about a (non-reversible) system designed intentionally to reveal closeness information.