Cryptography Reference
In-Depth Information
keys the same, is immune to this attack, as the differential would carry into the first and last steps of both en-
cryption and decryption.
Finally, the authors of Reference [8] give some guidance on how to avoid related-key attacks.
First, avoid using key-exchange algorithms that do not have key integrity; always check the key before using
it blindly to encrypt something and broadcasting it.
Second, making efforts to not derive round keys in such linear manners will go a long way. The authors make
a very good suggestion of running the key first through a cryptographic hash function, such as SHA-1, and then
using it to derive a key schedule. Any sufficiently strong hash function will obliterate any structures that may
have been artificially induced, preventing them from meaningfully modifying the ciphertext.
7.17 Summary
In this chapter, I demonstrated many modern differential attacks against modern ciphers. The standard differen-
tial attack has been extended and studied extensively over the past two decades and continues to be in confer-
ences and papers every year.
There is always more to learn in the field of cryptanalysis. This topic only covered some of the more popular
and influential topics in the field. To the reader wanting to learn even more about the field, you can look through
the archives and current issues of
The Journal of Cryptology, Cryptologia,
the proceedings of conferences such
as CRYPTO, EUROCRYPT, ASIACRYPT, Fast Software Encryption, and many others.
Again, we must reiterate, though: The best way to become a better cryptanalyst is to practice, practice, prac-
tice!
Exercises
Exercise 1
ChangetheS-boxandP-boxoftheEASY1ciphertorandomlygeneratedvalues.Usetheanalysisinthischapter
to create a differential attack on this for three rounds.
Exercise 2
Use the basic one-round version of the cipher from the previous exercise as a round function to a simple Feistel
cipher. Use a simple key schedule, such as using the same 36-bit key every round.
Use differential cryptanalysis to break this Feistel cipher for eight rounds. Give analysis for how many plain-
text-ciphertext pairs this will require.
Exercise 3
Perform a differential analysis of the S-boxes of DES. Discuss your results.
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