Cryptography Reference
In-Depth Information
5.16.
This is your chance to break a cryptosystem. As we know by now, cryptogra-
phy is a tricky business. The following problem illustrates how easy it is to turn a
strong scheme into a weak one with minor modifications.
We saw in this chapter that key whitening is a good technique for strengthening
block ciphers against brute-force attacks. We now look at the following variant of
key whitening against DES, which we'll call DESA:
DESA
k
,
k
1
(
x
)=
DES
k
(
x
)
⊕
k
1
.
Even though the method looks similar to key whitening, it hardly adds to the se-
curity. Your task is to show that breaking the scheme is roughly as difficult as a
brute-force attack against single DES. Assume you have a few pairs of plaintext-
ciphertext.
