Hawkynt

A5/1

GSM stream cipher using three irregularly clocked LFSRs with majority voting. Educational implementation demonstrating telecommunications security algorithms from cellular networks.

Properties

Property	Value
Category	Stream Ciphers
Sub-category	Stream Cipher
Security status	❌ Broken
Complexity	Intermediate
Inventor	ETSI SAGE
Year	1987
Origin	Not specified
Source	algorithms/stream/a5.js

Parameters

Parameter	Supported values
Key sizes	8 bytes (64 bits)
Block sizes	1 byte (8 bits) to 65536 bytes (524288 bits)

Security

Status: ❌ Broken

Known vulnerabilities

Issue	Description	Mitigation
Time-Memory Tradeoff Attack	Practical real-time key recovery attacks demonstrated by Biryukov-Shamir-Wagner	—
Correlation Attack	Exploits linear properties of LFSRs to recover internal state	—

Documentation

• Wikipedia: A5/1
• ETSI TS 155 226 - A5/1 Encryption Algorithm
• 3GPP TS 55.216 - A5/1 Algorithm Specification

References

• Pedagogical A5/1 and A5/2 Reference Implementation (Briceno, Goldberg, Wagner)
• libosmocore A5/1-A5/4 Reimplementation

Test vectors

2 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.

Vector 1 — A5/1 Test Vector - Key 1

Source: A5/1 reference implementation with frame=0

Field	Value
key	123456789abcdef0
input	000000000000000000000000000000
expected	b7d37880be862cdbf6d401fae7ffa2

Vector 2 — A5/1 Test Vector - Key 2

Source: A5/1 reference implementation with different key

Field	Value
key	fedcba9876543210
input	0102030405060708090a0b0c0d0e0f
expected	6a97039a899deca7df5a3f722e7bff

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