A5/1 (GSM Standard Cipher)

The standard stream cipher used in GSM networks for voice and data encryption. A5/1 was designed with 54-bit effective key length and has been cryptanalyzed successfully. A5/1 is considered broken and should be phased out, but remains operational in many networks.

Categories: Security ControlsProtocols and StandardsInternet and Routing

A5/1 in context

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.

Telecom security is written into standards from 3GPP, GSMA, ETSI, IETF and ITU-T. Understanding which body owns which specification, and how the specs interlock (for example 3GPP TS 33.501 with GSMA FS.34 for 5G SBA security), is essential for both design and audit work.

To place A5/1 in the wider telecom-security picture, review OSPF, IPv6, TR-069, ML-DSA, SLH-DSA and 3G.

Related terms

Related topic hubs

More from the TelcoSec Glossary

Browse the full TelcoSec Glossary, the Ultimate Guide to Mobile Network Security, or the P1 Arsenal of telecom-security tools.