Tradeoff Attacks on Symmetric Ciphers

Tradeoff attacks on symmetric ciphers can be considered as the generalization of the exhaustive search. Their main objective is reducing the time complexity by exploiting the memory after preparing very large tables at a cost of exhaustively searching all the space during the precomputation phase. It is possible to utilize data (plaintext/ciphertext pairs) in some cases like the internal state recovery attacks for stream ciphers to speed up further both online and offline phases. However, how to take advantage of data in a tradeoff attack against block ciphers for single key recovery cases is still unknown. We briefly assess the state of art of tradeoff attacks on symmetric ciphers, introduce some open problems and discuss the security criterion on state sizes. We discuss the strict lower bound for the internal state size of keystream generators and propose more practical and fair bound along with our reasoning. The adoption of our new criterion can break a fresh ground in boosting the security analysis of small keystream generators and in designing ultra-lightweight stream ciphers with short internal states for their usage in specially low source devices such as IoT devices, wireless sensors or RFID tags

Finding Slid Pairs for the Plantlet Stream Cipher

This paper presents the results of an investigation into relationships between (key, IV) pairs that give rise to phase shifted keystreams for the stream cipher Plantlet. We demonstrate that such slid pairs exist for Plantlet, and that very small phase shifts are possible, including shifts of length 1. We also show that the keyspace can be partitioned into sets of keys which give rise to initial states that lie on the same state cycle during keystream generation. For a given key used to generate Plantlet keystreams, we provide a method for generating all other keys that give rise to phase shifted keystreams as they move through internal states on the same state cycle.</p