ON FEISTEL NETWORKS, S-BOXES, AND BLOCK CIPHER DESIGN
Claude Shannon defined the cryptographic principles of confusion
and diffusion. Fifty years after this paper was written, these
principles are still the cornerstone of good block cipher design.
Confusion serves to hide any relationship between the plaintext,
the ciphertext, and the key. Remember how linear and
differential cryptanalysis can exploit even a slight relationship
between these three things? Good confusion makes the
relationship statistics so complicated that even these powerful
cryptanalytic tools won't work.
Diffusion serves to spread the influence of individual plaintext
or key bits over as much of the ciphertext as possible. This
also serves to hide statistical relationships, and make
cryptanalysis more difficult.
Confusion alone is enough. A cipher that consisted of a single
lookup table of 64 bits of plaintext to 64 bits of ciphertext
based on a key would be plenty strong. The problem is that large
lookup tables require large amounts of memory to implement: 1020
bytes of memory for the above table. The whole point of block
cipher design is to create something that looks like a large
lookup table, but with much smaller memory requirements.
The trick is to repeatedly mix confusion (with much smaller
tables) and diffusion in a single cipher in different
combinations. This is called a product cipher. Sometimes a
block cipher that incorporates layers of substitution and
permutation is called a substitution-permutation network, or even
a SP network.
Look back at function f of DES. The expansion permutation and P-
box perform diffusion; the S-boxes perform confusion. The
expansion permutation and P-box are linear; the S-boxes are
nonlinear. Each operation is pretty simple on its own, but
together they work pretty well.
DES also illustrates a few more principles of block cipher
design. The first is the idea of an iterated block cipher. This
simply means taking a simple round function and iterating it
multiple times. Two-round DES isn't very strong; it takes five
rounds before all of the output bits are dependent on all of the
input bits and all of the key bits. Sixteen-round DES is strong;
32-round DES is even stronger.
Feistel Ciphers
Most block algorithms that have appeared in the literature are
Feistel Ciphers. The idea dates from the early 1970s. Take a
block of length n and divide it into two halves of length n/2: L
and R. Of course, n must be even. You can define an iterated
block cipher where the output of the ith round is determined from
the output of the previous round:
L_i = R_(i-1)
R_i = L_(i-1) XOR f(R_(i-1),K_i)
K_i is the subkey used in the ith round, and f is an arbitrary
round function.
You've seen this concept in DES, Lucifer, FEAL, Khufu, Khafre,
LOKI, and others. Why is it such a big deal? First off, the
function is guaranteed to be reversible. Because XOR is used to
combine the left half with the output of the round function, it
is necessarily true that
L_(i-1) XOR f(R_(i-1),K_i) XOR f(R_(i-1),K_i) = L_(i-1)
A cipher that uses this construction is guaranteed to be
invertible as long as the inputs to f in each round can be
reconstructed. It doesn't matter what f is; f does not have to
invertible. We can design f to be as complicated as we please,
and we don't have to implement two different algorithms--one for
encryption and another for decryption. The structure of a
Feistel network takes care of all this automatically.
Simple Relations
DES has the property that if E_K(P) = C, then E_K'(P') = C',
where P', C', and K' are the bitwise complements of P, C, and K.
This property reduces the complexity of a brute-force attack by a
factor of two. LOKI has complementation properties that reduce
the complexity of a brute-force attack by a factor of 256.
A simple relation can be defined as [KNU94]:
If E_K(P) = C, then E_f(K)(g(P,K) = h(C,K)
where f, g, and h are simple functions. By simple I mean that
they are easy to compte, much easier than an iteration of the
block cipher. In DES, f is the bitwise complement of K, g is the
bitwise complement of P, and h is the bitwise complement of C.
This is a result of XORing the key into part of the text.
In a good block cipher, there are no simple relations. Methods
for finding some of these weaknesses are in [KWA91B].
No Weak Keys
In a good block cipher, all keys are equally strong. Algorithms
with a small number of weak keys, like DES, are generally no
problem. The odds of picking one at random are very small, and
it's easy to test and discard them. However, these weak keys can
sometimes be exploited if the block cipher is used as a one-way
hash function.
Strength Against Differential and Linear Cryptanalysis
The study of differential and linear cryptanalysis has shed
significant light on the theory of good block cipher design. The
inventors of IDEA introduced the concept of differentials, a
generalization of the basic idea of characteristics [LAI91B].
They argued that block ciphers can be designed in such a way to
be resistant against this attack; IDEA is the result of that work
[LAI91B]. This concept was further formalized in [NYB93],
where Kaisia Nyberg and Lars Knudsen showed how to make block
ciphers that were provably secure against differential
cryptanalysis.
Linear cryptanalysis is newer, and it is less clear what generic
design techniques will protect a cipher against linear
cryptanalysis. Knudsen has made some progress, considering some
necessary (but not necessarily sufficient) criteria for what he
calls practically secure Feistel ciphers: ciphers that are
resistant to both linear and differential cryptanalysis [KNU94].
Nyberg introduced an analogy to the concept of differentials in
differential cryptanalysis in linear cryptanalysis [NYB94].
Other work that extends the idea of linear cryptanalysis can be
found in [PRE94A,KAL94B].
Interestingly enough, there seems to be a duality between
differential and linear cryptanalysis. This duality becomes
apparent both in the design of techniques to construct good
differential characteristics and linear approximations
[BIH95,MAT95], and also in the design criteria for making
algorithms that are secure against both attacks [CHA95].
Exactly where this line of research will lead is still unknown.
S-Box Design
The strength of various Feistel ciphers--and specifically their
resistance to differential and linear cryptanalysis--is tied
directly to their S-boxes. This has prompted a spate of research
on what constitutes a good S-box.
An S-box is simply a substitution: a mapping of m-bit inputs to
n-bit outputs. Above I talked about a single lookup table of 64-
bit inputs to 64-bit outputs; that would be a single 64x64-bit S-
box. A general S-box with an m-bit input and an n-bit output is
called a mxn-bit S-box. S-boxes are generally the only non-
linear step in an algorithm; they are what give a block cipher
its security. In general, the bigger they are the better.
DES has eight different 6x4-bit S-boxes. Khufu and Khafre have a
single 8x32-bit S-box. In IDEA the modular multiplication step
is effectively the S-box; it is a 32x32-bit S-box. The larger
this S-box, the harder it is to find useful statistics about it
to attack [GOR83]. Also, while random S-boxes are usually not
optimal to protect against differential and linear attacks, it is
easier to find strong S-boxes if the S-boxes are larger. Most
random S-boxes are nonlinear, nondegenerate, and have have strong
resistance to linear cryptanalysis--and the fraction that does
not goes down rapidly as the number of input bits decreases
[OCO91,OCO94,OCO94A].
The size of m is more important than the size of n. Increasing
the size of n reduces the effectiveness of differential
cryptanalysis, but it increases the effectiveness of linear
cryptanalysis to a much greater degree. In fact, if n >= 2m - m,
then there is definitely a linear relation of the input and
output bits of the S-box. And if n >= 2m, then there is a linear
relation of only the output bits [BIH95].
Much of this work involves the study of Boolean functions. In
order to be secure, the Boolean functions used in S-boxes must
satisfy specific conditions. They should not be linear, nor
should they be close to linear [ADA90,NYB91,NYB93A]. There
should be a balance of zeros and ones, and no correlations
between different combinations of bits. The output bits should
behave independently when any single input bit is complemented.
These design criteria are also related to the study of bent
functions.
One property that seems very important is the diffusion of
information: how many output bits of an S-box change when some
subset of the input bits are changed. This is called the
avalanche effect. It's easy to impose conditions on Boolean
functions so that satisfy certain avalanche criteria, but
constructing them is a harder task. The strict avalanche
criteria (SAC) guarantees that exactly half of the output bits
change when one input bit changes [WEB86].
A few years ago cryptographers proposed choosing S-boxes so that
the different distribution table for each S-box is uniform. This
would provide immunity against differential cryptanalysis by
smoothing out the differentials in any particular round
[ADA92,DAW91A,DAW91,NYB91]. LOKI is an example of this
design. However, this approach can sometimes aid in differential
cryptanalysis [BIH92B]. Actually, a better approach is making
sure that the maximum differential is as small as possible.
Kwangjo Kim proposed five criteria for the construction of S-
boxes [KIM93A], similar to the design criteria for the DES S-
boxes.
Choosing good S-boxes is not an easy task, and there are many
competing ideas on how to do it. Four general approaches can be
identified:
Choose randomly: It is clear that small random S-boxes are
insecure, but large random S-boxes may be good enough.
Random S-boxes with 8 or more inputs are quite strong. And
even more strength is added if the S-boxes are both random
and key-dependent. IDEA uses both large and key-dependent
S-boxes.
Choose and test: Some ciphers generate random S-boxes and
then test them for the requisite properties. See [ADA90]
for an example of this approach.
Man-made: This technique uses little mathematics; S-boxes
are generated using more intuitive techniques. Bart Preneel
stated that "...theoretically interesting criteria are not
sufficient [for choosing Boolean functions for S-boxes]..."
and that "...ad hoc design criteria are required" [PRE93].
Math-made: Generating S-boxes according to mathematical
principles so that they have proven security against
differential and linear cryptanalysis, and good diffusive
properties. See [NYB94A] for an excellent example of this
approach.
There has been some call for a combination of he "math-made" or
"man-made" approaches [ROB94], but the real debate seems to be
between randomly-chosen S-boxes and S-boxes--whether created or
culled--that have certain properties. Certainly the latter
approach has the advantage of being optimal against known
attacks--linear and differential cryptanalysis--but it offers
unknown protection against unknown attacks. The designers of DES
knew about differential cryptanalysis, and the DES S-boxes were
optimized against it. They did not seem to know about linear
cryptanalysis, and the DES S-boxes are very weak against it
[MAT95]. Random S-boxes in DES would be weaker against
differential cryptanalysis and stronger against linear
cryptanalysis.
On the other hand, random S-boxes may not be optimal against
known attacks but they can be made sufficiently large and
therefor sufficiently resistant. And they are more likely to be
sufficiently resistant against unknown attacks. The debate is
still going on, but my personal feeling is that S-boxes should be
as large as possible, random, and key-dependent.
[ADA92] C.M. Adams, "On Immunity Against Biham and Shamir's
'Differential Cryptanalysis,'" Information Processing
Letters, v. 41, n. 2, 1992, pp. 77-80.
[ADA90] C.M. Adams and S.E. Tavares, "The Structured Design of
Cryptographically Good S-Boxes," Journal of Cryptology, v.
3, n. 1, 1990, pp. 27-41.
[BIH95] E. Biham "On Matsui's Linear Cryptanalysis," Advances in
Cryptology--EUROCRYPT '94 Proceedings, Springer-Verlag,
1995, to appear.
[BIH92B] E. Biham and A. Shamir, Differential Cryptanalysis of
the Data Encryption Standard, Springer-Verlag, 1993.
[CHA95] F. Chabaud and S. Vaudenay, "Links Between Differential
and Linear Cryptanalysis," Advances in Cryptology--EUROCRYPT
'94 Proceedings, Springer-Verlag, 1995, to appear.
[DAW91A] M.H. Dawson and S.E. Tavares, "An Expanded Set of Design
Criteria for Substitution Boxes and their Use in
Strengthening DES-like Cryptosystems," IEEE Pacific Rim
Conference on Communications, Computers, and Signal
Processing, IEEE, Victoria, BC, Canada, 9-10 Mary 1991, pp.
191-195.
[DAW91] M.H. Dawson and S.E. Tavares, "An Expanded Set of S-box
Design Criteria Based on Information Theory and its Relation
to Differential-like Attacks," Advances in Cryptology--
EUROCRYPT '91 Proceedings, Springer-Verlag, 1991, pp. 352-
367.
[GOR83] J.A. Gordon and R. Retkin, "Are Big S-boxes Best?"
Cryptography, Proceedings of the Workshop on Cryptography,
Burg Feuerstein, Germany, March 29-April 2, 1982, Springer-
Verlag, 1983, pp. 257-262.
[KAL94B] B.S. Kaliski and M.J.B. Robshaw, "Linear Cryptanalysis
Using Multiple Approximations," Advances in Cryptology--
CRYPTO '94 Proceedings, Springer-Verlag, 1994.
[KIM93A] K. Kim, "Construction of DES-like S-boxes Based on
Boolean Functions Satisfying the SAC," Advances in
Cryptology--ASIACRYPT '91 Proceedings, Springer-Verlag,
1993, pp. 59-72.
[KNU94] L.R. Knudsen, "Practically Secure Feistel Ciphers," Fast
Software Encryption, Cambridge Security Workshop
Proceedings, Springer-Verlag, 1994, pp. 211-221.
[KWA91B] M. Kwan and J. Pieprzyk, "A General Purpose Technique
for Locating Key Scheduling Weakness in DES-like
Cryptosystems," Advances in Cryptology--ASIACRYPT '91
Proceedings, Springer-Verlag, 1991, pp. 237-246.
[LAI91B] X. Lai, J. Massey, and S. Murphy, "Markov Ciphers and
Differential Cryptanalysis," Advances in Cryptology--
EUROCRYPT '91 Proceedings, Springer-Verlag, 1991, pp. 17-38.
[MAT95] M. Matsui, "On Correlation Between the Order of the S-
boxes and the Strength of DES," Advances in Cryptology--
EUROCRYPT '94 Proceedings, Springer-Verlag, 1995, to appear.
[NYB91] K. Nyberg, "Perfect Nonlinear S-boxes," Advances in
Cryptology--EUROCRYPT '91 Proceedings, Springer-Verlag,
1991, pp. 378-386.
[NYB93A] K. Nyberg, "On the Construction of Highly Nonlinear
Permutations," Advances in Cryptology--EUROCRYPT '92
Proceedings, Springer-Verlag, 1991, pp. 92-98.
[NYB94] K. Nyberg, "Provable Security Against Differential
Cryptanalysis," presented at the rump session of Eurocrypt
'94, May 1994.
[NYB94A] K. Nyberg, "Differentially Uniform Mappings for
Cryptography," Advances in Cryptology--EURORYPT '93
Proceedings, Springer-Verlag, 1994, pp. 55-64.
[NYB93] K. Nyberg and L.R. Knudsen, "Provable Security Against
Differential Cryptanalysis," Advances in Cryptology--CRYPTO
'92 Proceedings, Springer-Verlag, 1993, pp. 566-574.
[OCO91] L. O'Connor, "Enumerating Nondegenerate Permutations,"
Advances in Cryptology--EUROCRYPT '93 Proceedings, Springer-
Verlag, 1994, pp. 368-377.
[OCO94] L. O'Connor, "On the Distribution of Characteristics in
Bijective Mappings," Advances in Cryptology--EUROCRYPT '93
Proceedings, Springer-Verlag, 1994, pp. 360-370.
[OCO94A] L. O'Connor, "On the Distributino of Chracteristics in
Composite Permutations," Advances in Cryptology--CRYPTO '93
Proceedings, Springer-Verlag, 1994, pp. 403-412.
[PRE93] B. Preneel, "Analysis and Design of Cryptographic Hash
Functions," Ph.D. diss., Katholieke Universiteit Leuven, Jan
1993.
[PRE94A] B. Preneel and V. Rijmen, "On Using Maximum Liklihood to
Optimize Recent Cryptanalytic Techniques, " presented at the
rump session of EUROCRYPT '94, May 1994.
[ROB94] M.J.B. Robshaw, "Block Ciphers," Technical Report TR-601,
RSA Laboratories, Jul 1994.
[WEB86] A.F. Webster and S.E. Tavares, "On the Design of S-
Boxes," Advances in Cryptology--CRYPTO '85 Proceedings,
Springer-Verlag, 1986, pp. 523-534.
