I want to send short messages to cell phones.Commonly a short message is no longer than 70 bytes. I need 20 bytes or more holding information and the rest bytes are for signature provided by asymmetric schemes from server. It is because short messages can be easily faked with faked radio towers.
My app will get public key through https channel from the server.After getting public key, the phone may always be offline.When the server wants to contact a phone, it sends a short message to it and sign by private key.So the phone can trust the message and do the orders on the message.The public key will be refreshed every month and has a expired time of several month.
I've read some papers introducing RSA,DSA and BLS.The shortest RSA has a 128 bytes sign, while a DSA sign with readable charset like BASE64 is 64 bytes.Is there any shorter signature scheme? I saw BLS but it's not a international standard now and no jvm implementation found.
We assume the raw text is visible since attackers can read client code and know what happens to it.So is it still secure using a short signature with 30 bytes(others being redundancy)?
sign length is around 30 bytes
expired time is several months
asymmetric scheme so messages cannot be mock
raw text is some how visible