# AES and DES - reusing the same round keys

Both DES and AES use key expansion to generate the round keys (for Feistel network or for the AddRoundKey operation). Each round uses a different key. Why do we need that? Why we cannot use the same key in each round?

• Search "slide attack".(I lack expertise, and currently time, for a detailed answer).
– fgrieu
Nov 1, 2018 at 12:23

Before discussing the Slide attack, first, observe that the DES uses a 48-bit key per round produced by its key-schedule. If the DES uses the same key for every round then the key-space will be $$2^{48}$$ not $$2^{56}$$. Therefore it can be trivially broken by simple brute-force of two p3.16xlarge instances on AWS.

## The Slide attack

Wagner and Biryokov called your type of key-schedule homogenous. This occurs when the key-schedule produce periodic subkeys and in the simplest case, the period is 1, as in your question.

While the brute-force requires $$\mathcal{O}(2^{n})$$, the slide attack on block cipher (if works) requires $$\mathcal{O}(2^{n/2})$$ and for Feistel ciphers requires $$\mathcal{O}(2^{n/4})$$ since the $$F$$ function modifies only half of the block.

The attack is independent of round numbers, all it needs the $$F$$, the round function, is a weak permutation; i.e. given two equations $$F(x_1,k) = y_1$$ and $$F(x_2,k) = y_2$$ it is easy to extract the key $$k$$, as 3 rounds of DES which is a weak permutation.

## Idea

Let a block cipher with $$r$$ rounds, then;

$$X_j = F_1(X_{j-1})$$ where $$1 \leq j \leq r$$ and $$x_j$$ represent the internal outputs, $$X_0$$ is plaintext and $$X_r$$ is the ciphertext.

In the period 1 case, $$F_j = F_{j+1}$$ for all $$j \geq 1$$.

## Slid pair

A pair of $$(P,C)$$ and $$(P',C')$$ is called a slid pair if $$F(P)=P'$$ and $$F(C) = C'$$.

## Attack

Obtain $$2^{n/2}$$ known-plaintext $$(P_i,C_i)$$ and look for slid pairs. By birthday paradox, it is expected to find one pair with this property.

When the round function is weak, testing a slid pair condition will be easy, and some key bits of the cipher will be recovered.

Note: This slide attack requires $$2^{64}$$ known plaintexts and has a time complexity of $$2^{128}$$ encryptions for AES and this is greater than exhaustive key search!

## Efficient Slide Attacks

Bar-On et. al improved the original sliding attack with their work 1K-AES has that use the same key for every round with data and time complexity $$2^{64}$$. The result shows that 1K-AES is insecure.

## Countermeasures

For slide attacks to work, also the key schedule must exhibit a large degree of symmetry. Therefore one need methods to break the symmetry

1. Don't use periodic key scheduling algorithms
2. A simple yet efficient countermeasure is utilizing round constant, as in AES's rcon values and similarly as in PRINCE, LED, Simon and Speck,...