A5/3 (GSM Modern Cipher (UEA1))

A modern stream cipher used in 3G (UMTS) networks as UEA1, also called A5/3 when referenced in GSM contexts. A5/3 uses the KASUMI algorithm and provides stronger security than A5/1. However, use of A5/3 on 2G networks is not recommended for new deployments; transition to 3G or later networks is preferred. A5/3 should be treated as a legacy control with modern alternatives prioritized.

Categories: Radio Access NetworkCore NetworkInternet and Routing

A5/3 in context

The radio access network is where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations and downgrade attacks; defenses rest on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

The mobile core carries subscriber sessions, mobility and policy. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF.

To place A5/3 in the wider telecom-security picture, review Core Network, 5G, 5G SA, 5G NSA, AMF and AMPS — each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

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