State Convergence in Bit-Based Stream Ciphers

Provided by: Queensland Treasury Corporation
Topic: Security
Format: PDF
Modern stream cipher applications use a secret key and a publicly known Initialisation Vector (IV) to form an initial internal state for the key-stream generator, before key-stream generation begins. Well-designed initialization and key-stream generation processes for stream ciphers should ensure that each key-IV pair generates a distinct key-stream. In this paper, the authors analyze some ciphers where this does not happen due to state convergence occurring either during initialization, key-stream generation or both. They show how state convergence occurs in each case and identify two mechanisms which can cause state convergence.

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