Roots of Square: Cryptanalysis of Double-Layer Square and Square+

Abstract. Square is a multivariate quadratic encryption scheme proposed in 2009. It is a specialization of Hidden Field Equations by using only odd characteristic elds and also X2 as its central map. In addition, it uses embedding to reduce the number of variables in the public key. However, the system was broken at Asiacrypt 2009 using a dierential attack. At PQCrypto 2010 Clough and Ding proposed two new variants named Double-Layer Square and Square+. We show how to break Double-Layer Square using a rened MinRank attack in 245 eld operations. A similar fate awaits Square+ as it will be broken in 232 eld operations using a mixed MinRank attack over both the extension and the ground eld. Both attacks recover the private key, given access to the public key. We also outline how possible variants such as Square or multi-Square can be attacked. Key words: Multivariate Cryptography, Algebraic Cryptanalysis, Square, Double-Layer Square, Square+, MinRank, Key Recovery

• Type: Journal
• Year: 2011
• Where: IACR
• Authors: Enrico Thomae, Christopher Wolf
• Keywords: Double Layer, Elds, Field Equations, IACR 2011