A5/0 (GSM Null Cipher)

A deprecated GSM encryption algorithm that provides no encryption (plaintext transmission). A5/0 was historically used in countries with export restrictions on strong cryptography and is a critical vulnerability. Modern networks must reject A5/0.

Categories: Radio Access NetworkSecurity ControlsThreats and Attacks

A5/0 in context

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

Security controls in telecom span signaling firewalls (SS7/Diameter/GTP screening at the network edge), transport encryption (IPsec, TLS, PRINS), identity and access on OSS/BSS, and monitoring at both the packet and log level. Controls are audited against GSMA FS.11/FS.19/FS.20/FS.36 and, in Europe, NIS2 sectoral rules.

To place A5/0 in the wider telecom-security picture, review DNS, NTP, GNSS Spoofing, ICMP, 1G and AP.

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