A New Class of Weak Keys for Blowfish

The reflection attack is a recently discovered self similarity analysis which is usually mounted on ciphers with many fixed points. In this paper, we describe two reflection attacks on r-round Blowfish which is a fast, software oriented encryption algorithm with a variable key length k. The attacks work successfully on approximately 2k+32-16r number of keys which we call reflectively weak keys. We give an almost precise characterization of these keys. One interesting result is that 234 known plaintexts are enough to determine if the unknown key is a reflectively weak key, for any key length and any number of rounds. Once a reflectively weak key is identified, a large amount of subkey information is revealed with no cost. Then, we recover the key in roughly r

• Type: Conference
• Year: 2007
• Where: FSE
• Authors: Orhun Kara, Cevat Manap
• Keywords: Cryptology, FSE 2007, Key Lengths, Unknown Key, Weak Keys