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You have a valid point, given such a constrained space makes it easier to recover that stuff. For what it's worth, we used salted SHA-1 hashes, which would make it harder if somebody were to get a hold of the data, but of course, it wouldn't prevent us from doing the hash reversal ourselves if we were to turn "evil" some day.

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Can you explain how you're using salts?

Unless I'm missing something, either

1) you're using a "common salt" across all the hashes, which means I might need to generate my own rainbow tales with your common salt - but for only 10 digits worth of phone-number-space that's probably only a few bucks worth of EC2 time and S3 space to store it.

or 2) you're using a random salt for each phonenumber/hash, in which case you cant identify matching phone numbers.

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