Crypto Dictionary by Jean-Philippe Aumasson

Author:Jean-Philippe Aumasson [Jean-Philippe Aumasson]
Language: eng
Format: epub, mobi
Publisher: No Starch Press
Published: 2021-02-11T16:00:00+00:00

L

Laconic zero-knowledge proof

An interactive proof protocol where the prover sends very few bits to the verifier.

Lai–Massey

A secure way to build a block cipher, although much less common than the Feistel substitution-permutation networks. The Lai–Massey construction is notably used by IDEA and FOX.

A LAI–MASSEY ROUND

Given a function f() whose input and output is of a length twice as short as the block size, a Lai–Massey round applies to an internal state split in two halves, L and R. Each round updates these halves as follows:

First, compute X = f(L − R). Then set L = σ(L + X) and R = R + X.

Here, σ() is an orthomorphism, that is, a permutation so that the mapping Y ↦ σ(Y) − Y is also a permutation. For example, FOX’s orthomorphism sees L as two halves L = LL||LR and returns LR||(LL ⊕ LR).

Without σ(), the difference L − R remains the same through the round operation, slightly reducing the scheme’s security (I’ll leave the proof for this as a simple exercise for you to complete).

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