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 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.

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

To place A5/0 in the wider telecom-security picture, review DNS, NTP, GNSS Spoofing, ICMP, 1G and AP: each entry cross-references back to this page so you can walk the topic in either direction.

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