[alexwright]

A form to enter the sha1 of your email address so they can check against a list of addresses they've already sha1ed? And a form that takes the email for people that don't give a shit or don't know wtf a sha is.

Maybe just a .txt of the hashes too, but then no ones coming to your web service I guess.

[FireBeyond]

That makes no sense - they have the plaintext of the email addresses they SHA1, so they can correlate it just fine when you enter it in, too.

[alexwright]

But if they don't have your address, ie it wasn't leaked or they're full of shit and never had the list, when you enter the SHA1 you don't give them the address.