**The block cipher NSABC (public domain)**

*Alice Nguyenova-Stepanikova and Tran Ngoc Duong*

**Abstract: **We introduce NSABC/w – Nice-Structured Algebraic Block Cipher using w -bit word arithmetics, a 4w -bit analogous of Skipjack [NSA98] with 5w -bit key. The Skipjack's internal 4-round Feistel structure is replaced with a w -bit, 2-round cascade of a binary operation (x,z)\mapsto(x\boxdot z)\lll(w/2) that permutes a text word x under control of a key word z . The operation \boxdot , similarly to the multiplication in IDEA [LM91, LMM91], bases on an algebraic group over w -bit words, so it is also capable of decrypting by means of the inverse element of z in the group. The cipher utilizes a secret 4w -bit tweak – an easily changeable parameter with unique value for each block encrypted under the same key [LRW02] – that is derived from the block index and an additional 4w -bit key. A software implementation for w=64 takes circa 9 clock cycles per byte on x86-64 processors.

**Category / Keywords: **secret-key cryptography / block ciphers, tweakable, algebraic, modular multiplication, IDEA, Skipjack

**Date: **received 12 May 2011, last revised 17 May 2011

**Contact author: **tranngocduong at gmail com

**Available format(s): **PDF | BibTeX Citation

**Note: **Terminology correction.

**Version: **20110518:021153 (All versions of this report)

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