Linear equation over a ring

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File:Lai Massey scheme diagram en.svg

The Lai-Massey scheme is a cryptographic structure used in the design of block ciphers. It is used in IDEA and IDEA NXT.

Construction details

Let $\mathrm{F}$ be the round function and $\mathrm{H}$ a half-round function and let $K_0,K_1,\dots ,K_n$ be the sub-keys for the rounds $0,1,\dots ,n$ respectively.

Then the basic operation is as follows:

Split the plaintext block into two equal pieces, ($L_0$, $R_0$)

For each round $i=0,1,\dots ,n$, compute

$({L_{i^{\prime }+1}}^{\prime },{R_{i^{\prime }+1}}^{\prime })=\mathrm{H}({L_i}^{\prime }+T_i,{R_i}^{\prime }+T_i)$

where $T_i=\mathrm{F}({L_i}^{\prime }-{R_i}^{\prime },K_i)$ and $({L_0}^{\prime },{R_0}^{\prime })=\mathrm{H}(L_0,R_0)$

Then the ciphertext is $(L_{n+1},R_{n+1})=({L_{n^{\prime }+1}}^{\prime },{R_{n^{\prime }+1}}^{\prime })$.

Decryption of a ciphertext $(L_{n+1},R_{n+1})$ is accomplished by computing for $i=n,n-1,\dots ,0$

$({L_i}^{\prime },{R_i}^{\prime })={\mathrm{H}}^{-1}({L_{i^{\prime }+1}}^{\prime }-T_i,{R_{i^{\prime }+1}}^{\prime }-T_i)$

where $T_i=\mathrm{F}({L_{i^{\prime }+1}}^{\prime }-{R_{i^{\prime }+1}}^{\prime },K_i)$ and $({L_{n^{\prime }+1}}^{\prime },{R_{n^{\prime }+1}}^{\prime })={\mathrm{H}}^{-1}(L_{n+1},R_{n+1})$

Then $(L_0,R_0)=({L_0}^{\prime },{R_0}^{\prime })$ is the plaintext again.

The Lai-Massey scheme offers security properties similar to those of the Feistel structure. It also shares its advantage over a substitution-permutation network that the round function $\mathrm{F}$ does not have to be invertible.

The half-round function is required to prevent a trivial distinguishing attack ($L_0-R_0=L_{n+1}-R_{n+1}$). It commonly applies an orthomorphism $\sigma$ on the left hand side, that is,

$\mathrm{H}(L,R)=(\sigma (L),R)$

where both $\sigma$ and $x\mapsto \sigma (x)-x$ are permutations (in the mathematical sense, that is, a bijection – not a permutation box). Since there are no orthomorphisms for bit blocks (groups of size $2^n$), "almost orthomorphisms" are used instead.

$\mathrm{H}$ may depend on the key. If it doesn't, the last application can be omitted, since its inverse is known anyway. The last application is commonly called "round $n.5$" for a cipher that otherwise has $n$ rounds.

Literature

• X. Lai. On the design and security of block ciphers. ETH Series in Information Processing, vol. 1, Hartung-Gorre, Konstanz, 1992
• X. Lai, J. L. Massey. A proposal for a new block encryption standard. Advances in Cryptology EUROCRYPT'90, Aarhus, Denemark, LNCS 473, p. 389-404, Springer, 1991
• Serge Vaudenay: A Classical Introduction to Cryptography, p. 33