IDEA

Xuejia Lai and James Massey at ETH Zurich designed IDEA in 1991 as a hardened version of their earlier PES, after differential cryptanalysis went public. It mixes three algebraically incompatible operations — XOR, addition modulo 2¹⁶ and multiplication modulo 2¹⁶+1 — over eight and a half rounds, with no S-boxes at all.

Phil Zimmermann chose IDEA for PGP 2.0, so a generation of encrypted email ran on it. The patent, held by Swiss firm Ascom, kept it out of free software until it expired in 2011–2012. By then AES had taken over, and the 64-bit block left it exposed to the same weakness Sweet32 later exploited.