[ww520]

> single long password could be used to authenticate as anyone.

Only if everyone uses the same long prefix for password.

[buzer]

No. If the value of the cache key is simply true/false then someone would first login to their own account using the long password. This would result in storing:

bcrypt(longpassword + 123456 + [e-mail]) = bcrypt(longpassword) = hash1 -> true

If they then try login as [e-mail] using same password there would be a cache lookup:

bcrypt(longpassword + 1111111 + [e-mail]) = bcrypt(longpassword) = hash1 -> true