Why do people say that using hash twice or more is bad? yes there is key derivation that do that job, but in a low bandwidth network countries, the solution i see is to use a simple hashing system on the server to gain one second or two.
Is rainbow table effective when using more than one hash? because on internet, i only find MD5 or SHA tables, so i guess that the algorithm will work only on
hash(str) and will not work on