Stronger public algorithms have been added to the GSM standard: the A5/3 and A5/4 (Block ciphers), otherwise known as KASUMI or UEA1 published by ETSI. If the network does not support A5/1, or any other A5 algorithm implemented by the phone, then the base station can specify A5/0 which is the null algorithm, whereby the radio traffic is sent unencrypted. Even if mobile phones are able to use 3G or 4G (which have much stronger encryption than 2G GSM), the base station can downgrade the radio communication to 2G GSM and specify A5/0 (no encryption). This is the basis for eavesdropping attacks on mobile radio networks using a fake base station commonly called an IMSI catcher.
