Analysis and Improvements of the Full Spritz Stream Cipher

Subhadeep BANIK, Takanori ISOBE, Masakatu MORII

Summary :

Spritz is a stream cipher proposed by Rivest and Schuldt at the rump session of CRYPTO 2014. It is intended to be a replacement of the popular RC4 stream cipher. In this paper we propose distinguishing attacks on the full Spritz, based on a short-term bias in the first two bytes of a keystream and a long-term bias in the first two bytes of every cycle of N keystream bytes, where N is the size of the internal permutation. Our attacks are able to distinguish a keystream of the full Spritz from a random sequence with samples of first two bytes produced by 2^44.8 multiple key-IV pairs or 2^60.8 keystream bytes produced by a single key-IV pair. These biases are also useful in the event of plaintext recovery in a broadcast attack. In the second part of the paper, we look at a state recovery attack on Spritz, in a special situation when the cipher enters a class of weak states. We determine the probability of encountering such a state, and demonstrate a state recovery algorithm that betters the 2¹⁴⁰⁰ step algorithm of Ankele et al. at Latincrypt 2015. Finally we propose a simple fix that removes the bias in the first two keystream bytes. The countermeasure requires only one additional memory access and hence does not diminish software performance substantially, and in fact the loss in software speed is only around 1.5%.

• Publication: IEICE TRANSACTIONS on Fundamentals Vol.E100-A No.6 pp.1296-1305
• Publication Date: 2017/06/01
• Online ISSN: 1745-1337
• DOI: 10.1587/transfun.E100.A.1296
• Type of Manuscript: PAPER
• Category: Cryptography and Information Security

Authors

Subhadeep BANIK
  Nanyang Technological University
Takanori ISOBE
  University of Hyogo
Masakatu MORII
  Kobe University

Keyword

RC4,  Spritz,  stream cipher,  short-term bias,  long-term bias,  distinguishing attack,  plaintext recovery attack,  state recovery attack