Ritter Software Engineering
2609 Choctaw Trail
Austin, Texas 78745
(512) 892-0494, ritter(a)io.com
Toward Axiomatic Fenced DES
Terry Ritter
May 26, 1994
Introduction
This article continues the development of a block cipher which I
have been calling "Fenced DES." This unique construct uses the
U.S. Data Encryption Standard (DES) as a component in a strength-
enhanced cipher. Even though DES is slow and is now becoming
vulnerable to advancing attack technology, DES is also well-known
and trusted, and industry would be grateful to continue to use it
if only it were stronger.
The time has come to replace ordinary DES. One alternative is
the complete certification of a totally new cipher at tremendous
cost in both treasure and time. Another alternative is "triple-
DES," at three times the computation of ordinary DES. But if a
strength-enhancing construction can be found which is sufficiently
clear and elegant, we may hope for a "derivative certification,"
based only assumptions about the strength of DES itself.
In this article I start the process of proving some things about
the Fenced DES cipher. In particular, I prove that the resulting
cipher is invertible and has the avalanche property, two admittedly
modest characteristics, but ones we do associate with a good block
cipher. I claim that the construct is certainly guaranteed to be
no weaker than DES. I also argue--with some theoretical support--
that the construct should be expected to be much stronger, at least
120 bits. In other words, it should be "strong enough" for the next
couple of decades.
The system of definitions, proofs and arguments which takes up the
major part of this article is by no means finished, and is known
to be casual and inconsistent in places. (Some of these problems
could be fixed by expanding the mathematical base, which I avoid
for now.) In spite of this, I believe it to be an interesting
approach, even if it is an approach to which others are probably
far better suited than myself. Therefore, let us just agree to
accept it for what it is, and see how close it gets to what we need.
The definitions apply to this particular construction. Those
generally familiar with combinatorics might start with section 7,
"Block Mixing Transforms."
Fenced DES
Here is the current 4x Fenced DES construct:
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
------------------------------mix------------------------------
--------------mix-------------- --------------mix--------------
------DES------ ------DES------ ------DES------ ------DES------
--------------mix-------------- --------------mix--------------
------------------------------mix------------------------------
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
Each "S" represents a separately-shuffled and independent 8-bit
substitution table (which also implies the presence of a keyed
cryptographic RNG to shuffle the tables). We have 32 input
substitutions and 32 output substitutions, for an overall block
size of 256 bits. This is only 32 bytes, which should be much
smaller than the typical message. Trailing 2x and 1x blocks would
reduce data expansion to only that needed by DES itself.
Each "---DES---" represents an ordinary 64-bit-block DES operation.
Each "---mix---" represents the mixing of the covered data blocks
using "block mixing transform" technology. There are two levels
of mixing on each side of the DES operations: The innermost levels
each have two mixings which combine two 64-bit blocks; the outermost
levels each have just a single mixing which combines two 128-bit
blocks, a substantial mixing operation.
This entire construct requires about 4.8 times the computation to
cipher 4 times the data. In contrast, triple-DES would of course
need 12 times the computation to cipher 4 times the data.
The Proofs
1. SETS
=======
1.1 DEFINITION: A SET is a collection of objects in which any
object either is or is not a part of the set. A set S can
be described by a list of the elements in the set, viz.
S = { a1, a2, ..., an }.
1.2 DEFINITION: The SIZE OF SET S is the number of elements in S,
and is denoted |S|.
2. CODES
========
2.1 DEFINITION: A CODE is a string of symbols in which the symbol
in each position is taken from some common set S. When S consists
of numeric values, a code can be seen as a polynomial with
coefficients in S.
2.2 DEFINITION: An N-POSITION code is a code which has n positions
for symbols, and can be denoted by S**n.
2.3 DEFINITION: A BINARY code is a code in which the common set
is the set {0,1}.
2.4 DEFINITION: An N-BIT binary code is a binary code with n
positions and can be denoted by {0,1}**n or by S**n with S = {0,1}.
2.5 THEOREM: (Size of code.) There are |S|**n distinct code
values in an n-position code.
(Proof: Each position in a code string can be any possible
symbol, there are |S| possible symbols and n positions in
each code string, so there are |S|**n possible code values
of length n.)
2.6 THEOREM: (No special positions.) Taken over all possible
code values, each string position has exactly the same number of
occurrences of each symbol.
(Proof: Each position in a code string can be any possible
symbol. For any particular combination of symbols in other
positions, in the selected position each possible symbol
occurs once. So for every possible combination of symbols
in other positions, in the selected position each possible
symbol occurs the same number of times.)
2.7 THEOREM: (Position difference counts.) The number of
n-position code values which differ in exactly m positions is
(n) m
(m) * (|S|-1) .
(n)
(Proof: There are (m) combinations of m positions out of n
possible positions, and in any particular combination of m
positions each position can take on |S|-1 other symbols
producing (|S|-1)**m other code values for each combination.)
2.8 EXAMPLE: The number of 8-bit binary codes which differ in m
bits is:
distance count
0 1
1 8
2 28
3 56
4 70
5 56
6 28
7 8
8 1
---
256 = 2**8
(Comment: There are 256 8-bit binary code values, and 255
values which differ in at least one position from any
particular code value.)
2.9 THEOREM: (Average distance and distribution.) The expected
number of elements which differ between two n-position code values
is n * (1 - 1/|S|), and the distribution is binomial.
(Proof: Assume the number of code differences is the binomial
(n) m n-m
probability of a difference B(m;n,p) = (m) p q , where
where p = 1 - 1/|S| and q = 1-p, times the total number of
n
code values (1/q) :
(n) m (n) m n-m -n
(m) * (|S|-1) = (m) p q q
(n) m
= (m) (p / (1-p))
which is correct, so the expected number of different elements
is the binomial expectation np.)
2.10 EXAMPLE: The expected number of elements which differ between
two 8-bit binary code values is:
8 * (1 - 0.5) = 4.
2.11 EXAMPLE: The probability of having two 8-bit binary code
values which differ in exactly two elements is:
(8) 2 6
(2) (0.5) (0.5) = 0.109 = 28 / 256.
2.12 EXAMPLE: The expected number of elements which differ between
two 64-bit binary code values is:
64 * (1 - 0.5) = 32.
2.13 EXAMPLE: The probability of getting a 64-bit binary code
value which differs in exactly m bits from some other value is:
difference probability
16 0.000026
28 0.061
29 0.075
30 0.088
31 0.096
32 0.099
(Comment: The 9 difference values 28..36 account for about
74 percent of all possible difference counts, even though
they are only about 14 percent of all 65 possibilities.)
3. DISCRETE FUNCTIONS
=====================
3.1 DEFINITION: A DISCRETE FUNCTION takes an input code value
to an output code value for a finite number of input code values.
3.2 DEFINITION: A RANDOM discrete function allows each output
code value to be selected independently for each possible input
condition.
3.3 THEOREM: (Number of random functions.) There are 2**2n
possible random functions with an n-bit binary input code and an
n-bit binary output code.
(Proof: An n-bit binary code can make 2**n possible
selections, each of which can be 2**n possible values,
and (2**n)*(2**n) = 2**2n.)
4. SUBSTITUTION
===============
4.1 DEFINITION: A SUBSTITUTION is a mapping from input values
or positions to output values.
(Comment: A SUBSTITUTION can be seen as an indexable vector
of substitute values. A SUBSTITUTION can also be seen as a
"codebook" with an entry for every possible input code, and
storage for each corresponding output code. A SUBSTITUTION
can also be seen as an "arbitrary" discrete function, since
any possible discrete function can be described by using a
separate output code for each possible input condition. A
SUBSTITUTION can also be seen as the relation joining
substitute values with the position of each value.)
4.2 DEFINITION: SIMPLE substitution is the operation of using a
substitution table or codebook to "encode" a string of input
values by replacing each value in the string with its associated
substitute value.
(Comment: If the substitution is invertible, we can use an
inverse substitution to "decode" the resulting encoded values
and recover the original values.)
4.3 THEOREM: (Unique substitute values.) An invertible
substitution can contain any particular output code at most once.
(Proof: Suppose not: Then two different values into a
substitution will produce the same output value. But that
output value can inverse-substitute to only one inverse
value, making the other input value unreachable, which
contradicts invertibility, so this is false.)
4.4 THEOREM: (Number of invertible substitutions.) There are
(2**n)! possible invertible substitutions for an n-bit binary
input code.
(Proof: The first substitution element can be any one of
2**n elements, the second element can be any except the first
element, or (2**n)-1 elements, the third can be any except
the first and second, for (2**n)-2 elements, and so on.)
4.5 THEOREM: (Guaranteed change propagation.) A change of even
one input bit to an invertible substitution is guaranteed to
produce a change in at least one output bit from the substitution.
(Proof: Each input bit can select between two different input
code values, which will select two different output code
values, since an invertible substitution contains no duplicate
values. Since any two different codes must be different in at
least one bit, any input bit-change will produce at least one
output bit-change.)
4.6 DEFINITION: A COMPLETE substitution contains every value
of an n-position code, for some n.
4.7 THEOREM: (Probable change propagation.) Any change whatsoever
to the input value to a complete invertible substitution is likely
to change about half the bits in the output value.
(Proof: Changing the input value selects among all remaining
output code values. If the output is considered to be binary
bits, we expect about half those bits to change (2.9).)
4.8 DEFINITION: AVALANCHE is a statistical property of a discrete
function in which any change whatsoever on the input is expected to
produce a change in about half the bits in the output value.
4.9 THEOREM: (Avalanche is automatic.) Avalanche is an inherent
property of complete invertible substitution.
(Proof: See 4.5, 4.7, and 2.9.)
4.10 THEOREM: (No special input bits.) Each input bit to an
invertible substitution has exactly the same power to produce the
same expected change in output bits.
(Proof: Consider any possible change to any possible input
value: from all possible input values any particular bit-change
will produce all possible input values. Thus, any possible
bit-change must produce the same overall expectation.)
4.11 THEOREM: (No special output bits.) Each output bit from
a complete invertible substitution has exactly the same change
expectation as any other output bit.
(Proof: See 2.6.)
4.12 THEOREM: (Not a random function.) An invertible substitution
cannot be a random function.
(Proof: Suppose a value is selected for placement somewhere
in a substitution. Since an invertible substitution cannot
allow another occurrence of that same value, other values
cannot be selected independently.)
4.13 DEFINITION: In a KEYED substitution the substitute element
values have been permuted or re-arranged as a function of some
key value or function.
4.14 THEOREM: (Reconstruction requires information linking output
values to input values.) An unknown invertible substitution cannot
be resolved without simultaneous information about both the input
value or position and the output value.
(Proof: To the extent that a particular substitution can
be said to have an identity, that identity is the relation
between substitute values and their position. This relation
is both necessary and sufficient to define the substitution.)
5. BIT MIXERS
=============
5.1 DEFINITION: A BIT-MIXER combines multiple input bits such
that each output value is defined by each and every input bit.
5.2 THEOREM: An invertible substitution is a bit-mixer.
(Proof: Each and every input bit can select between two
different input code values. Any input value change into
an invertible substitution must necessarily select a
different output value. Thus, the output value, and every bit
in the output value, inherently depends upon each and every
bit of the input value.)
6. BLOCK CIPHERS
================
6.1 DEFINITION: A CIPHER is a keyed invertible translation from
a plaintext element to a ciphertext element.
6.2 THEOREM: A CIPHER is a keyed invertible substitution.
(Proof: For "translation" read "substitution.")
6.3 DEFINITION: A BLOCK cipher is a cipher in which the size of
the code element is prohibitively large to be exhaustively explored.
6.4 THEOREM: (Not a random function.) No static block cipher can
be a random function.
(Proof: A cipher must be an invertible function, and no
invertible function can have elements which are independent.)
6.5 ASSERTION: (Just a large substitution.) There is no property
of a block cipher which is not ideally modelled by a substitution
table of appropriate size containing a key-selected permutation of
the possible output values.
(Invertibility argument: A permutation of the possible
output values is just a re-arrangement of values, without
duplication. As long as there are no duplicate output values,
the substitution is invertible.)
(Avalanche argument: Avalanche is an expected property of an
invertible substitution (4.9).)
7. BLOCK MIXING TRANSFORMS
=========================
7.1 DEFINITION: A BLOCK MIXING TRANSFORM is a mapping from
multiple input code values to the same number of output code
values, in which:
1. (Invertible.) The mapping is invertible. (Every possible
input will imply a different output, and every possible
output will imply a different input.)
2. (Each Output a Function of All Inputs.) Every output code
value is a function of all input code values.
3. (Changes Propagate to All Outputs.) Any change to any one
of the input code values will change all of the output code
values.
4. (Balance and Input Independence.) Stepping any input
through all possible values (with the other inputs held
fixed) will step every output through all possible values.
7.2 ASSERTION: (We have a finite field.) Mod-2 polynomials
modulo some irreducible polynomial p generate a finite field.
(Comment: Proofs can use algebra.)
7.3 THEOREM: (Example block mixing transform.) The equations
X = 3A + 2B = A + 2(A + B)
Y = 2A + 3B = B + 2(A + B)
and the inverse
A = X + 2(X + Y)
B = Y + 2(X + Y)
mod 2 and mod p, where p is some mod 2 irreducible polynomial,
represent a block mixing transform.
(Inverse Proof: assume true, thus
A = A + 2(A + B) + 2(A + 2(A + B) + B + 2(A + B))
= A + 2(A + B) + 2(A + B) = A
and
B = B + 2(A + B) + 2(A + 2(A + B) + B + 2(A + B))
= B + 2(A + B) + 2(A + B) = B
which are both correct, so the inverse does exist for any
polynomials X and Y.)
(Function Proof: the equations for output code X includes
both input code values A and B, so X is a function of both
input codes. Y reasons similarly.)
(Change Propagation Proof: First consider one term of one
output block equation:
Suppose some change C is added to A:
X = 3A + 2B (mod 2, mod p)
X' = 3(A+C) + 2B
X' = 3A + 3C + 2B
dX = X' - X = 3C
So, for any non-zero change, X has changed. Similar reasoning
covers the other term, and the other equation.)
(Balance Proof: Suppose not. Assuming A is fixed, then
there must be two different values, B and B', which produce
the same X:
X = 3A + 2B = 3A + 2B'
so
X + 3A = 2B = 2B'
which implies that
B = B'
a contradiction. Fixing B or working on the other block
reason similarly.)
7.4 THEOREM: It is easy to manipulate both input blocks to a block
mixing transform so as to fix one of the output blocks at a constant
value.
(Proof: Just inverse-transform the desired output blocks.)
7.5 ASSERTION: A block cipher can be used as a block mixing
transform.
(Method: Just divide the input block and output block into
smaller "sub-blocks.")
(Inverse Proof: A block cipher is invertible (6.1) and (6.3).)
(Function Proof: To the extent that the block cipher can be
considered an invertible substitution, each output bit is a
function of each input bit (4.5), so each sub-block result is
certainly a function of all sub-block input values.)
(Change Propagation Argument: In a statistical sense, assuming
substantial sub-blocks, each sub-block is extremely likely to
change for any input change whatsoever (2.9).)
(Balance Argument: In a statistical sense, over all possible
inputs and all possible keys, any output value is equally
likely, so any set of input changes is likely to produce a
statistically-balanced result.)
8. 1X FENCED DES STRUCTURES
=========================
8.1 DEFINITION: A 1X INPUT-FENCED DES STRUCTURE is a 64-bit-
wide construct consisting of eight keyed invertible byte-
substitutions feeding a single DES ciphering:
S S S S S S S S
------DES------
8.2 THEOREM: Any data change whatsoever into a 1x input-fenced
DES structure will produce a different result, and is expected to
change about half of the output bits.
(Proof: Every bit in the input block enters some small
substitution which selects a keyed or arbitrary value from
its set of output codes. Any input-change into an invertible
substitution is is guaranteed to produce a change to at least
one output bit (4.5). We model the DES ciphering as a large
invertible substitution (6.5), and so expect that any change
to the input will select a different output code value, which
is likely to change about half of the output bits (4.7).)
8.3 DEFINITION: A 1X OUTPUT-FENCED DES STRUCTURE is a 64-bit-wide
construct consisting of a single DES ciphering and eight keyed
invertible byte-substitutions on the output:
------DES------
S S S S S S S S
8.4 THEOREM: Any data change whatsoever into a 1x output-fenced
DES structure is expected to change about half of the output bits.
(Proof: We model the DES ciphering as a large invertible
substitution (6.5) and expect that any change to the input
will change about half the bits in the output value (4.7).
Since every possible DES result may occur, there are no
special bits or bit subsets (2.6). Each of the output
substitutions samples a bit subset in which about half of
the bits are expected to change. Any change into an output
substitution will select a different output code value,
thus changing about half of the output bits (4.7) in every
output substitution, and, thus, the overall output.)
(Comment: One time in 255 there is no change to an output
substitution, which is exactly what is required for an even
output distribution. )
8.5 DEFINITION: A 1X FENCED DES CIPHER is a 64-bit-wide
construct consisting of eight keyed invertible byte-substitutions
on the input, a single DES ciphering, and eight keyed invertible
byte-substitutions on the output:
S S S S S S S S
------DES------
S S S S S S S S
8.6 THEOREM: (Avalanche.) In 1x Fenced DES, any change of even
a single bit in the large input block can be expected to change
about half the bits in the large output block.
(Proof: See 8.2 and 8.4.)
8.7 THEOREM: (Invertibility.) A 1x Fenced DES cipher is
invertible.
(Proof: From the construction of 1x Fenced DES, the small
input substitutions are invertible, as are the small output
substitutions. DES is assumed to be invertible. Since
all elements in sequence from input to output are separately
invertible, the sequential combination of these elements must
also be invertible.)
9. 2X FENCED DES STRUCTURES
============================
9.1 DEFINITION: A 2X INPUT-FENCED DES STRUCTURE is a 128-bit-
wide construct consisting of 16 keyed invertible byte-substitutions
feeding a block mixing transform, which feeds two DES cipherings:
S S S S S S S S S S S S S S S S
--------------mix--------------
------DES------ ------DES------
9.2 THEOREM: Any data change whatsoever into a 2x input-fenced
DES structure will produce a different result, and is expected to
change about half of the output bits.
(Proof: Any change into an invertible substitution is
guaranteed to produce a change to at least one output bit
(4.5). Any change to either input block of a two-block
block mixing transform is guaranteed to produce a change to
both output blocks (7.1.3). We model the DES cipherings as
large invertible substitutions (6.5) and so expect that any
change to the input will select a different output code
value, which is likely to change about half of the output
bits (4.7).)
9.3 DEFINITION: A 2X OUTPUT-FENCED DES STRUCTURE is a 128-bit-
wide construct consisting of two DES cipherings which feed a two-
block block mixing transform, which feeds 16 keyed invertible byte-
substitutions.
------DES------ ------DES------
--------------mix--------------
S S S S S S S S S S S S S S S S
9.4 THEOREM: Any data change whatsoever into a 2x output-fenced
DES structure is expected to change about half of the output bits.
(Proof: We model the DES cipherings as large invertible
substitutions (6.5) and expect that any change to their inputs
will select a different output value from all possible output
values (4.5). Since any DES result is possible, any value is
possible from both block mixing transform outputs (7.1.4), so
we expect about half of the output bits to change (4.7).
Since any block mixing result value is possible, there are no
special bits (2.6), and each of the output substitutions
samples a bit subset in which about half of the bits are
expected to change. Any change into an output substitution
will select a different output code value, thus changing about
half of the output bits (4.7) in every output substitution,
and, thus, the overall output.)
9.5 DEFINITION: A 2X FENCED DES STRUCTURE is a 128-bit-wide
construct consisting of 16 keyed invertible byte-substitutions which
feed a block mixing transform which feeds two DES cipherings which
feed another two-block block mixing transform, which feeds another
16 keyed invertible byte-substitutions:
S S S S S S S S S S S S S S S S
--------------mix--------------
------DES------ ------DES------
--------------mix--------------
S S S S S S S S S S S S S S S S
9.6 THEOREM: (Avalanche.) In a 2x Fenced DES cipher, any change
of even a single bit in the large input block can be expected to
change about half the bits in the large output block.
(Proof: See 9.2 and 9.4.)
9.7 THEOREM: (Invertibility.) A 2x Fenced DES cipher is
invertible.
(Proof: From the construction of 2x Fenced DES, the small
input substitutions are invertible, as are the small output
substitutions. The block mixing transform is invertible
(7.1.1). DES is assumed to be invertible. Since all elements
in sequence from input to output are separately invertible,
the sequential combination of these elements must also be
invertible.)
10. 4X FENCED DES STRUCTURES
============================
10.1 DEFINITION: A 4X FENCED DES CIPHER is a 256-bit-wide
construct consisting of 32 keyed invertible byte-substitutions
feeding a block mixing transform with two 128-bit blocks, which
then feeds two block mixing transforms each with two 64-bit blocks,
which feed four DES cipherings. The DES results feed two block
mixing transforms each with two 64-bit blocks, which feed a block
mixing transform with 128-bit blocks, which feeds 32 more keyed
invertible byte-substitutions.
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
------------------------------mix------------------------------
--------------mix-------------- --------------mix--------------
------DES------ ------DES------ ------DES------ ------DES------
--------------mix-------------- --------------mix--------------
------------------------------mix------------------------------
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
10.2 THEOREM: (Every input bit affects every DES ciphering.)
In 4x Fenced DES, every bit in the large input block will affect
at least one bit of the input to each of the DES cipherings.
(Proof: Every bit in the large block enters some small
substitution. Any input-change into a substitution is
guaranteed to produce a change to at least one output bit
(4.5). Any change into either side of the first-level block
mixing transform is guaranteed to change both sides of the
output (7.1.3), so some change is guaranteed to be present
in the input of both next-level block mixing transforms.
Again, any change anywhere on those inputs is guaranteed to
be present in both sides of both outputs, which are the
inputs to each DES ciphering.)
10.3 THEOREM: (Each output bit is affected by every DES ciphering.)
In 4x Fenced DES, any data change whatsoever into any of the four
DES cipherings is expected to change about half of the output bits.
(Proof: We model the DES cipherings as large invertible
substitutions (6.5) and expect that any change to their inputs
will select a different output value from all possible output
values (4.5). Since any DES result is possible, any value is
possible on both outputs of the first-level output block mixing
transform (7.1.4). Any possible block mixing transform result
can be produced by some BMT input, so any possible value can
occur as the input to the second-level output block mixing
transform. With any possible BMT input, every output will
occur, so there are no special bits (2.6), and each of the
output substitutions samples a bit subset in which about half
of the bits are expected to change. Any change into an output
substitution will select a different output code value, thus
changing about half of the output bits (4.7) in every
substitution, and, thus, the overall output.)
10.4 THEOREM: (Avalanche.) In 4x Fenced DES, any change of even
a single bit in the large input block can be expected to change
about half the bits in the large output block.
(Proof: See 10.2 and 10.3.)
10.5 THEOREM: (Invertibility.) 4x Fenced DES is invertible.
(Proof: From the construction of 4x Fenced DES, the small
input substitutions are invertible, as are the small output
substitutions. The block mixing transform is invertible
(7.3.1). DES is assumed to be invertible. Since all elements
in sequence from input to output are separately invertible,
the sequential combination of these elements must also be
invertible.)
11. 4X FENCED DES STRENGTH CHARACTERISTICS
==========================================
11.1 ASSERTION: (DES cipherings cannot be separated.) In 4x
Fenced DES, it is not possible to isolate and work on a single DES
ciphering unless the small input substitutions have first been
resolved.
(Argument: In order to key-search a single DES ciphering, it
is necessary to develop the input and output value for that
particular ciphering. The large input and output blocks are
known, but the values sent to the internal cipherings are
hidden by the input and output substitutions.)
11.2 ASSERTION: (Input substitutions cannot be separated.) In
4x Fenced DES, it is not possible to isolate and work on any one
small input substitution unless all four of the DES keys and at
least one element in each of the 32 small output substitutions
have first been resolved.
(Argument: Even though their input values are known, resolving
the content of the small input substitutions requires some
information about their output values. Since these values
flow through the internal DES cipherings, if DES is effective,
these values cannot be known without the DES keys. Further,
each of the DES keys is required, since all of the DES
cipherings combined produce the known output.
There can be no statistical effects which identify particular
values from the input substitutions, because any change of
any number of bits whatsoever affects the large output block
similarly.
There can be no statistical effects which isolate individual
input substitutions because each input substitution has the
same effect on the large output block. Any change whatsoever
from any input substitution changes about half the bits in
the output block, making statistical issues about the content
of the substitutions completely irrelevant.)
11.3 ASSERTION: (Output substitutions cannot be separated.) In
4x Fenced DES, it is not possible to isolate and work on any one
small output substitution unless all four of the DES keys and at
least one element in each of the 32 small input substitutions have
first been resolved.
(Argument: Even though their output values are known, resolving
the content of the small output substitutions requires some
information about their input values. Since the input values
flow from the internal DES cipherings, if DES is effective,
these values cannot be known without the DES keys. Further,
each of the DES keys is required, since each DES ciphering
affects all of the output substitutions.
There can be no statistical effects which identify particular
input values to the output substitutions, because any change
of any number of bits whatsoever affects the output from the
substitution similarly.
There can be no statistical effects which isolate individual
output substitutions, because each of their input values come
from the the output of the DES cipherings, and these values
are "random like." So there can be no statistic to use for
attack.)
12. FENCED DES EXPECTED STRENGTH
================================
12.1 THEOREM: (Absolute minimum strength of 1x Fenced DES.)
Assuming a known-plaintext attack, further assuming that all the
input and output substitutions are known, if DES has a strength
of 56 bits, the 1x Fenced DES construct has a keyspace of 56 bits.
(Proof: All data flows through each layer; if the input and
output substitutions are known, they do not confuse the data,
but they also do not undo whatever confusion DES provides.)
12.2 ASSERTION: (Expected strength of the substitution layers in
1x Fenced DES.) Assuming a known-plaintext attack, and further
assuming that the DES key is known, the 1x Fenced DES construct
has a keyspace exceeding 64 bits.
(Argument: The overall input is known, so the small input
substitution _positions_ are all known; the uncertainty lies
wholly in the _values_ at those positions. There are 256
possible values at the known position for each of eight input
substitutions, for 256**8, or 2**64 possibilities. (A 63-bit
expectation.)
The uncertainty in the output substitution positions is
the same, but the input and output substitutions are not
independent: Since the DES key is known, defining the input
substitutions implies what the output substitutions must be
(or vise versa), so only one substitution level contributes
to the keyspace.
When working on the small input substitutions, the individual
substitutions are independent: If even one of the input
substitute values is wrong, we expect that half of the DES
result bits will be wrong, which will imply wrong positions
for most output substitutions. The process is similar if we
choose to work on the output substitutions instead.
A 64-bit keysearch is guaranteed to identify one element in
each of the eight small input substitutions (for example).
Then, assuming infinite known-plaintext, we just look for
data blocks which are the same as the solved block in seven
of the eight bytes. For each possible value of the eighth
byte we can easily try each of the 254, 253,..., 2 remaining
values (which will implicitly define many of the output
substitutions) at almost no cost beyond holding and finding
appropriate messages.
With only a limited amount of known-plaintext there will be
fewer if any messages which differ in just one byte, few if
any quick byte searches, and many more-substantial searches
until the input substitutions are filled in.)
(Comment: DES with a known key is an example of a block
mixing transform with absolutely no strength at all by itself,
which nevertheless adds strength through bit mixing.)
12.3 ASSERTION: (Expected strength of 1x Fenced DES.) Assuming
a known-plaintext attack, the 1x Fenced DES construct has a
keyspace exceeding 120 bits.
(Argument: When the DES key is known, the strength is 64
bits; the unknown DES key adds 56 bits more, for a total
of 120 bits. (This is 2**64 times the complexity of DES.)
It is not possible to separate the substitution layers from
the cipher layer and so work on either independently, because
the data flows through both.
In addition, each DES operation is a function of every input
bit (8.2) and each output bit is a function of every DES
output (8.4), so individual DES operations cannot be isolated
by particular input or output bits.
A 120-bit keysearch will identify the DES key and one element
in each of the eight small substitutions, and then we need
to fill out the rest of each substitution as above.)
12.4 THEOREM: (Absolute minimum strength of 4x Fenced DES.)
Assuming a known-plaintext attack, further assuming that all the
input and output substitutions are known, if DES has a strength
of 56 bits, the 4x Fenced DES construct has a keyspace exceeding
56 bits.
(Proof: All data flows through each layer. The information
content of the data is 256 bits; to recover that data, all
four DES operations must be solved. Even if we assume that
some aspect of the construction allows the DES operations to
be solved separately, the resulting strength is still somewhat
more than a single DES cipher.)
12.5 ASSERTION: (Expected strength of separated 4x Fenced DES.)
Assuming a known-plaintext attack, and assuming that the internal
ciphers _can_ be isolated and worked on separately, the 4x Fenced
DES construct has an overall keyspace of not less than 120 bits.
(Argument: The substitution and ciphering occur in series,
consequently, at least one eight-byte substitution (input or
output) and one DES ciphering must be solved simultaneously,
even if the block mixing transform fails.)
12.6 ASSERTION: (Expected strength of 4x Fenced DES.) Assuming
a known-plaintext attack, and assuming that the internal ciphers
_cannot_ be isolated and worked on separately, the 4x Fenced DES
construct has an overall keyspace exceeding 480 bits.
(Argument: The small substitutions (input or output) jointly
contribute 256 bits, and the four DES keys contribute 224 bits
for a total of 480 bits. That is, searching a 480-bit keyspace
will solve the system for a particular input (or output) block.
This identifies the DES keys, but only solves 1/256th of each
of 32 substitutions.
Once the system is solved for a particular block, the 255
other entries in each of 32 substitutions must be filled in
to completely solve the cipher.)
Results
It appears that Fenced DES can reasonably be proven to be an
invertible block cipher which has the avalanche property (provided,
of course, that DES has that property) with a strength at least
that of DES itself.
Reasonable-sounding arguments suggest that the internal ciphers
cannot be separated and worked on independently, and that the
resulting cipher has substantial strength. It would be nice to
tighten this up; any and all suggestions are welcome.
Appendix
Some Fenced DES constructions:
1x Fenced DES
S S S S S S S S
------DES------
S S S S S S S S
2x Fenced DES
S S S S S S S S S S S S S S S S
--------------mix--------------
------DES------ ------DES------
--------------mix--------------
S S S S S S S S S S S S S S S S
4x Construct with 1x Strength
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
------DES------ ------DES------ ------DES------ ------DES------
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
Original 4x Fenced DES
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
--------------mix-------------- --------------mix--------------
------------------------------mix------------------------------
------DES------ ------DES------ ------DES------ ------DES------
------------------------------mix------------------------------
--------------mix-------------- --------------mix--------------
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
Current 4x Fenced DES
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
------------------------------mix------------------------------
--------------mix-------------- --------------mix--------------
------DES------ ------DES------ ------DES------ ------DES------
--------------mix-------------- --------------mix--------------
------------------------------mix------------------------------
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
4x Fenced DES with Less Storage and Strength
(A..H and S..Z represent 16 keyed byte-substitutions, each
used four times.)
A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H
------------------------------mix------------------------------
--------------mix-------------- --------------mix--------------
------DES------ ------DES------ ------DES------ ------DES------
--------------mix-------------- --------------mix--------------
------------------------------mix------------------------------
S T U V W X Y Z S T U V W X Y Z S T U V W X Y Z S T U V W X Y Z
---
Terry Ritter ritter(a)io.com
