Paper 2026/1519

Classic Full Plaintext Recovery Attacks on Low Round Generalized Feistel Networks

Abstract

The Generalized Feistel Network (GFN) underpins widely standardized block ciphers, yet its resistance to full plaintext recovery remains largely unexplored. This paper extends the full-plaintext attack framework for standard Feistel ciphers to multi-branch scenarios, mainly makes the following $3$ research contributions: (1) Developed classic full plaintext recovery attacks on $d$-round Type-I GFN ($d\ge 3$) under CPA with $d+1$ encryption queries and $2d$-round Type-I GFN under CCA with $d$ decryption queries. Query complexity depends on d, increasing branches can enhance anti-interference capability. (2) Developed classic full plaintext recovery attack on 2-round Type-II GFN ($d\ge 4$) under CPA with $3$ encryption queries and 3-round Type-II GFN under CCA with $1$ encryption plus $2$ decryption queries. Query complexity is independent of $d$, increasing branches does not improve resistance. (3) Finded that all attacks treat round functions as black boxes, confirming that the weakness resides in the GFN topology rather than specific round-function designs. Strengthening S-boxes or diffusion matrices cannot mitigate it, only increasing rounds beyond the security threshold provides effective defense.