[relate]

You could allow for errors and use a bloom filter to avoid the space issue:

https://en.wikipedia.org/wiki/Bloom_filter

[StavrosK]

Or you could use a privacy-preserving lookup API, but that might be too much traffic. A Bloom filter could be downloaded locally and is probably a better solution.

[waterhouse]

Bloom filter as first pass and then privacy-preserving lookup API if it returns "probably a match"?

[StavrosK]

If it returns "probably a match", you can just look up with HN directly, like it does now.