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IEEE TRANSACTIONS ON NANOBIOSCIENCE, VOL. 4, NO. 2, JUNE 2005
149
Fast Parallel Molecular Algorithms for DNA-Based
Computation: Factoring Integers
Weng-Long Chang, Minyi Guo*
, Member, IEEE
, and Michael Shan-Hui Ho
Through advances in molecular biology [1], it is now pos-
sible to produce roughly 10 DNA strands that t in a test tube.
Those 10 DNA strands can also be applied to represent 10
bits of information. In the future (perhaps after many years) if bi-
ological operations can be applied to deal with a tube with 10
DNA strands and they are run without errors, then 10 bits of
information can simultaneously be correctly processed. Hence,
in the future, it is possible that biological computing can provide
a huge amount of parallelism for dealing with many computa-
tionally intensive problems in the real world.
The fastest super computers currently available can execute
approximately 10 integer operations per second. This implies
that 128 10 bits of information can be simultaneously
processed in a second. The fastest super computers can process
128 10 bits of information in 1000 seconds. The
extract
operation is one of basic biological operations of the longest
execution time. An
extract
operation could be approximately
done in 1000 s [12]. In the future, if an
extract
operation can
be used to deal with a tube with 10 DNA strands and it is run
without errors, then 10 bits of information can simultaneously
be correctly processed in 1000 s. If it becomes true in the future,
then basic biological operations will perhaps be faster than the
fastest super computer in the future. In [12], it was pointed out
that storing information in molecules of DNA allows for an in-
formation density of approximately 1 bit/nm . Videotape is a
kind of traditional storage media and its information density is
approximately 1 bit/10 nm . This implies that an information
density in molecules of DNA is better than that of traditional
storage media.
In this paper, we rst construct solution spaces of DNA
strands for encoding every integer of bits. By using basic
biological operations, we then develop DNA-based algorithms
for a parallel subtractor, a parallel comparator, and a parallel
divider, respectively, to factor the product of two large prime
numbers of bits. We also show that cryptosystems based on
the dramatic difference between the ease of nding large prime
numbers of bits and the difculty of factoring the product of
two large prime numbers of bits can be broken. Furthermore,
this work presents clear evidence of molecular computing
ability to nish parallel mathematical operations.
The rest of this paper is organized as follows. Section II rst
introduces DNA models of computation proposed by Adleman
et al.
and compares them with other models. Section III intro-
duces the DNA program to factor the product of two large prime
numbers of bits for solution spaces of DNA strands. Discus-
sion and conclusion are drawn in Section IV and Section V,
respectively.
Abstract—
The RSA public-key cryptosystem is an algorithm
that converts input data to an unrecognizable encryption and
converts the unrecognizable data back into its original decryption
form. The security of the RSA public-key cryptosystem is based on
the difculty of factoring the product of two large prime numbers.
This paper demonstrates to factor the product of two large prime
numbers, and is a breakthrough in basic biological operations
using a molecular computer. In order to achieve this, we propose
three DNA-based algorithms for parallel subtractor, parallel
comparator, and parallel modular arithmetic that formally verify
our designed molecular solutions for factoring the product of two
large prime numbers. Furthermore, this work indicates that the
cryptosystems using public-key are perhaps insecure and also
presents clear evidence of the ability of molecular computing to
perform complicated mathematical operations.
Index Terms—
Biological parallel computing, DNA-based algo-
rithms, DNA-based computing, factoring integers, RSA public-key
cryptosystem.
I. I
NTRODUCTION
T
HE RSA public-key cryptosystem [34] is an algorithm that
converts input data to an unrecognizable encryption, and
converts the unrecognizable data back into its original decryp-
tion form. The construction of the RSA public-key cryptosystem
is based on the ease of nding large prime numbers. The security
for the cryptosystem using public-key is based on the difculty
of factoring the product of two large prime numbers. The RSA
public-key cryptosystem is the most popular cryptosystem. No
method in a reasonable amount of time can be applied to break
the RSA public-key cryptosystem.
Feynman proposed molecular computation in 1961, but his
idea was not implemented by experiment for a few decades
[37]. In 1994 Adleman [2] succeeded in solving an instance of
the Hamiltonian path problem in a test tube, just by handling
DNA strands. Lipton [3] demonstrated that the Adleman tech-
niques could be used to solve the satisability problem (the rst
NP-complete problem). Adleman
et al.
[14] proposed
sticker
for
enhancing the error rate of hybridization.
Manuscript received April 7, 2004; revised November 25, 2004. This work
was supported in part by the Republic of China National Science Foundation
under Grant NSC-92-2213-E-218-026 and Japan JSPS Grant-in-Aid for Sci-
entic Research under Grant (C) 14580386.
Asterisk indicates corresponding
author.
W.-L. Chang and M. S.-H. Ho are with the Department of Information Man-
agement, Southern Taiwan University of Technology, Tainan, Taiwan, R.O.C.
(E-mail: changwl@mail.stut.edu.tw; MHoInCerritos@yahoo.com).
*M. Guo is with the School of Computer Science and Engineering, University
of Aizu, Aizu-Wakamatsu City 965-8580, Japan (E-mail: minyi@u-aizu.ac.jp).
Digital Object Identier 10.1109/TNB.2005.850474
1536-1241/$20.00 © 2005 IEEE
150
IEEE TRANSACTIONS ON NANOBIOSCIENCE, VOL. 4, NO. 2, JUNE 2005
also 3 end matches 5 end. The length of a single-stranded DNA
is the number of nucleotides composing the single strand. Thus,
if a single stranded DNA includes 20 nucleotides, then we say
that it is a 20 mer (i.e., it is a polymer containing 20 monomers).
The length of a double-stranded DNA (where each nucleotide is
base paired) is counted in the number of base pairs. Thus, if we
make a double-stranded DNA from a single stranded 20 mer,
then the length of the double stranded DNA is 20 base pairs,
also written 20 bp. Hybridization is a special technology term
for the pairing of two single DNA strands to make a double helix
and also takes advantages of the specicity of DNA base pairing
for the detection of specic DNA strands. (For more discussions
of the relevant biological background, refer to [1] and [16]).
Fig. 1.
A schematic representation of a nucleotide.
II. B
ACKGROUND
In this section we review the basic structure of the DNA mole-
cule and then discuss available techniques for dealing with DNA
that will be used to solve the problem of factoring integers. Si-
multaneously, several well-known DNA models are compared.
A. The Structure of DNA
From [1], [16], DNA (
DeoxyriboNucleic Acid
) is the
mole-
cule
that plays the main role in DNA-based computing. In the
biochemical world of large and small
molecules, polymers,
and
monomers
, DNA is a polymer, which is strung together from
monomers called
deoxyriboNucleotides
. The monomers used
for the construction of DNA are deoxyribonucleotides. Each de-
oxyribonucleotide contains three components: a
sugar
,a
phos-
phate
group, and a
nitrogenous
base. The sugar has ve carbon
atoms—for the sake of reference there is a xed numbering of
them. Because the base also has carbons, to avoid confusion the
carbons of the sugar are numbered from 1 to 5 (rather than from
one to ve). The phosphate group is attached to the 5 carbon,
and the base is attached to the 1 carbon. Within the sugar struc-
ture there is a
hydroxyl
group attached to the 3 carbon.
Distinct nucleotides are detected only with their bases, which
come in two sorts:
purines
and
pyrimidines
. Purines include
adenine
and
guanine
, abbreviated and . Pyrimidines
contain
cytosine
and
thymine
, abbreviated and . Because
nucleotides are distinguished solely from their bases, they are
simply represented as , , ,or nucleotides, depending
upon the kinds of base that they have. The structure of a
nucleotide, cited from [16], is illustrated (in a very simplied
way) in Fig. 1. In Fig. 1, B is one of the four possible bases ( ,
, ,or ), P is the phosphate group, and the rest (the “stick”)
is the sugar base (with its carbons enumerated 1 through 5 ).
Nucleotides can be linked together in two different ways [1],
[16]. The rst method is that the 5 -phosphate group of one nu-
cleotide is joined with 3 -hydroxyl group of the other forming
a
phosphodiester
bond. The resulting molecule has the 5 -phos-
phate group of one nucleotide, denoted as 5 end, and the 3 -OH
group of the other nucleotide available, denoted as 3 end, for
bonding. This gives the molecule the
directionality
, and we can
talk about the direction of 5 end to 3 end or 3 end to 5 end.
The second way is that the base of one nucleotide interacts with
the base of the other to form a
hydrogen
bond. This bonding is
the subject of the following restriction on the base pairing:
and can pair together, and and can pair together—no
other pairings are possible. This pairing principle is called the
Watson–Crick complementarity (named after J. D. Watson and
F. H. C. Crick, who deduced the famous double helix structure
of DNA in 1953 and won the Nobel Prize for the discovery).
A DNA strand is essentially a sequence (polymer) of four
types of nucleotides detected by one of four bases they contain.
Two strands of DNA can form (under appropriate conditions)
a double strand, if the respective bases are the Watson–Crick
complements of each other—
B. Adleman’s Experiment for Solving the Hamiltonian Path
Problem
Assume a directed graph , where and are the
set of vertices and the set of edges respectively. In general, the
Hamiltonian path problem consists of deciding whether has a
Hamiltonian path or not. with designed vertices and
is said to have a Hamiltonian path if and only if there exists a
sequence of compatible “one way” edges
(that is, a
“path”), which begins at
, ends at
, and enters every other
vertex exactly once [2].
Adleman’s
experiment
is
used
to
solve
the
Hamil-
tonian
path
problem
for
a
directed
,
where
and
[2]. The rst step of
Adleman’s experiment is to generate random paths through
the directed graph . To generate random paths, each vertex
in for was associated with a random 20-mer
sequence of DNA denoted . For each edge in ,an
oligonucleotide was created which was the 3 10 mer
of (unless , in which case it was all of ) followed
by the 5 mer of (unless , in which case it was all
of ). The 20-mer sequence Watson–Crick complementary
to was denoted . For each vertex in (except
and ) and for each edge in , large quantities
of oligonucleotides and were mixed together in a
single ligation reaction. Here the oligonucleotides served
as
splints
to bring oligonucleotides associated with compatible
edges together for ligation. Consequently, the ligation reaction
resulted in the formation of DNA molecules that can be viewed
as encoding random paths through the directed graph . From
the random paths generated, basic biological operations are
applied to remove illegal paths and select a Hamiltonian path [2].
C. The Sticker-Based Model
The sticker-based model employs two basic groups of
single-stranded DNA molecules in its representation of a bit
string [14]. Consider a
memory strand
bases in length
subdivided into nonoverlapping regions each bases long
(thus, ). Each region is identied with exactly one
bit position (or equivalently one Boolean variable) during the
course of the computation. Adleman
et al.
[14] also designed
different
sticker strands
or
simply stickers
. Each sticker is
bases long and is complementary to one and only one of
matches
and
matches
;
CHANG
et al.
: FAST PARALLEL MOLECULAR ALGORITHMS FOR DNA-BASED COMPUTATION: FACTORING INTEGERS
151
. Thus synthesis began
by assembling the two 15 base oligonucleotides with sequences
and . This process was repeated until all 6 variables had
been treated.
The probes used for separating the library strands have se-
quences complementary to the value sequences. Errors in the
separation of the library strands are errors in the computation.
Sequences must be designed to ensure that library strands have
little secondary structure that might inhibit intended probe-li-
brary hybridization. The design must also exclude sequences
that might encourage unintended probe-library hybridization.
To help achieve these goals, sequences were computer-gener-
ated to satisfy the proposed seven constraints [22]. The similar
method also is applied to solve a 20-variable of 3-SAT [46].
Fig. 2.
An example of a sticker memory.
E. DNA Manipulations
Fig. 3.
Examples of memory complexes.
In the past decade, there have been revolutionary advances in
the eld of biomedical engineering, particularly in recombinant
DNA and RNA manipulating. Due to the industrialization of
the biotechnology eld, laboratory techniques for recombinant
DNA and RNA manipulation are becoming highly standard-
ized. Basic principles about recombinant DNA can be found in
[47]–[50]. In this subsection we describe eight biological oper-
ations that are useful for solving the problem of factoring inte-
gers. The method of constructing DNA solution space for the
problem of factoring integers is based on the proposed method
in [22], [46].
A (test) tube is a set of molecules of DNA (a multiset of nite
strings over the alphabet
the memory regions. If a sticker is annealed to its matching
region on a given memory strand, then the bit corresponding
that particular region is on for that strand. If no sticker is
annealed to a region, then that region’s bit is off. Each memory
strand along with its annealed stickers (if any) represents one
bit string. Such partial duplexes are called
memory complexes
.
A large set of bit strings is represented by a large number of
identical memory strands each of which has stickers annealed
only at the required bit positions. Such a collection of memory
complexes is called as a tube.
In this model, a unique association of memory strands and
stickers represents each possible bit string. In the illustration
given in Fig. 2, we consider a memory strand of length ,
divided into regions, each of length . Thus, in this
case the necessary complexes are interpreted as containing four
bits of information. In particular, consider the memory com-
plexes depicted in Fig. 3. In the rst memory complex, all re-
gions are off, whereas in the last complex the last two regions
are on. The binary numbers represented by these four memory
complexes are 0000, 0100, 1001, and 0011, respectively.
). Given a tube, one can
perform the following operations.
1.
Extract
. Given a tube
and a short single strand of
DNA,
, the operation produces two tubes
and
, where
is all of the molecules of DNA
in
which contain
as a substrand and
is all of
the molecules of DNA in
which do not contain
.
2.
Merge
. Given tubes and , yield , where
. This operation is to pour two tubes
into one, without any change in the individual strands.
D. Adleman’s Experiment for Solution of a Satisfability
Problem
Adleman
et al.
[22], [46] performed experiments that were
applied to, respectively, solve a six-variable 11-clause for-
mula and a 20-variable 24-clause three-conjunctive normal
form (3-CNF) formula. A Lipton encoding [3] was used to
represent all possible variable assignments for the chosen
six-variable or 20-variable SAT problem. For each of the six
variables , two distinct 15 base value sequences
were designed. One represents
true
, , and another
represents
false
, for . Each of the truth
assignments was represented by a
library sequence
of 90 bases
consisting of the concatenation of one value sequence for each
variable. DNA molecules with library sequences are termed
library strands
and a combinatorial pool containing library
strands is termed a
library
. The six-variable library strands
were synthesized by employing a mix-and-split combinatorial
synthesis technique [24]. The library strands were assigned
library sequences with
3.
Detect
. Given a tube ,if includes at least one DNA
molecule, we have “yes,” and if
contains no DNA mol-
ecule, we have “no.”
4.
Discard
. Given a tube
, the operation will discard
.
5.
Amplify
.
Given
a
tube
,
the
operation
Amplify
,
will
produce
two
new
tubes
and
so that
and
are totally a copy of
(
and
are now identical) and
becomes an empty tube.
6.
Append
. Given a tube containing a short strand of DNA
, the operation will append
onto the end of every
strand in
.
7.
Append-head
. Given a tube
containing a short strand
of DNA,
, the operation will append
onto the head of
every strand in
.
8.
Read
. Given a tube , the operation is used to describe
a single molecule, which is contained in tube .Even
if contains many different molecules each encoding a
different set of bases, the operation can give an explicit
description of exactly one of them.
at the 5 -end and
at the 3 -end
152
IEEE TRANSACTIONS ON NANOBIOSCIENCE, VOL. 4, NO. 2, JUNE 2005
F. Comparisons of Various Famous DNA Models
eral H systems, and by Hubert and Schuler [45]. Barua
et al.
[31] proposed a recursive DNA algorithm for adding two binary
numbers, which require biosteps using only dif-
ferent type of DNA strands, where is the size of the binary
string representing the larger of the two numbers.
Adleman
et al.
[14] proposed a sticker-based model to
enhance the error rate of hybridization in the Adleman–Lipton
model. Their model can be used for determining solutions of
an instance in the set cover problem. Simultaneously, Adleman
et al.
[52] also pointed out that the data encryption standard
could be easily broken from solution space of
stickers
in the
sticker-based model. Perez-Jimenez
et al.
[15] employed the
sticker-based model [14] to resolve knapsack problems. In
our previous work, Chang
et al.
[25], [32], [36], [53] also
employed the sticker-based model and the Adleman–Lipton
model for dealing with Cook’s theorem [9], [10], the set-split-
ting problem, the subset-sum problem, and the dominating-set
problem for decreasing the error rate of hybridization.
Based on solution space of
splint
in the Adleman–Lipton
model, their methods [7], [17]–[20], [35] could be applied toward
solving the traveling salesman problem, the dominating-set
problem, the vertex cover problem, the clique problem, the inde-
pendent-set problem, the three-dimensional matching problem,
the set-packing problem, the set-cover problem, and the problem
of exact cover by three-sets. Lipton
et al.
[51] indicated that
DNA-based computing had been shown to easily be capable
of breaking the data encryption standard from solution space of
splint
. The methods used for solving problems have exponentially
increased volumes of DNA and linearly increased the time.
Bach
et al.
[33] proposed a
volume,
time
molecular algorithm for the three-coloring problem and a
volume, time molecular algorithm for the independent
set problem, where and are, subsequently, the number of ver-
ticesandthenumberofedgesintheproblemsresolved.Fu[21]pre-
sented a polynomial-time algorithm with a volume for the
three-SAT problem, a polynomial-time algorithm with a
volume for the three-coloring problem, and a polynomial-time al-
gorithm with a volume for the independent set. Though the
size of those volumes [21], [33] is lower, constructing those vol-
umes is more difcult and the time complexity is higher.
Quyang
et al.
[4] showed that enzymes could be used to solve
the NP-complete clique problem. Because the maximum number
of vertices that they can process is limited to 27, the maximum
number of DNA strands for solving this problem is 2 [4]. Shin
et al.
[8] presented an encoding scheme for decreasing the error
rate of hybridization. This method [8] can be employed toward
ascertaining the traveling salesman problem for representing
integers and real values with xed-length codes. Arita
et al.
[5]
and Morimoto
et al.
[6] proposed a new molecular experimental
technique and a solid-phase method to nd a Hamiltonian path.
Amos [13] proposed a parallel ltering model for resolving the
Hamiltonian path problem, the subgraph isomorphism problem,
the three-vertex-colorability problem, the clique problem, and
the independent-set problem. The methods in [5], [6], and [13]
have lowered the error rate in real molecular experiments. In
[26], [27], and [30], the methods for DNA-based computing by
self-assembly require the use of DNA nanostructures, called tiles,
to own expressive computational power and convenient input and
output (I/O) mechanisms. That is, DNA tiles have lower error rate
in self-assembly.
One of the earliest attempts to perform arithmetic operations
(addition of two positive binary numbers) using DNA is by
Guarneiri
et al.
[38], utilizing the idea of encoding differently
bit values zero and one as single-stranded DNAs, based upon
their positions and the operands in which they appear. Gupta
et al.
[39] performed logic and arithmetic operations using the
xed bit encoding of the full corresponding truth tables. Qiu and
Lu [40] applied substitution operation to insert results (by en-
coding all possible outputs of bit by bit operation along with
second operand) in the operand strands. Ogihara and Ray [41],
as well as Amos and Dunne [42] proposed methods to realize
any Boolean circuit (with bounded fan in) using DNA strands
in a constructive fashion. Other new suggestions to perform all
basic arithmetic operations are by Atanasiu [43] using P sys-
tems and by Frisco [44] using splicing operation under gen-
III. F
ACTORING THE
P
RODUCT OF
T
WO
L
ARGE
P
RIME
N
UMBERS OF
B
ITS
A. RSA Public-Key Cryptosystem
In the RSA cryptosystem [34], a participant creates his public
and secret keys with the following steps. The rst step is to select
at random two large prime numbers and , assuming that the
length of and are both bits. The second step is to compute
by the equation . The third step is to select a small
odd integer that is relatively prime to , which is equal
to . The fourth step is to compute as the
multiplicative inverse of , module . The fth step is to
publish the pair as his RSA public key. The sixth
step is to keep secret the pair
as his secret key. A
method to factor
as
in a reasonable amount of time has
not been found.
B. Solution Space of DNA Strands for Every Unsigned Integer
of Bits
Suppose that an unsigned integer of bits is represented
as a -bit binary number, , where the value of each
bit is either one or zero for . The bits and
represent, respectively, the most signicant bit and the least
signicant bit for . The range of the value to an unsigned
integer of bits is from 0 to . From [22], [46], for every
bit ,two
distinct
15 base value sequences are designed. One
represents the value zero for and the other represents the
value one for . For convenience, we assume that denotes
the value of to be one and denes the value of to be
zero. The following algorithm is used to construct the solution
space of DNA strands for
different unsigned integer values.
Procedure InitialSolution
(T )
(1) For
j = k
down to 1
(1a) Amplify(
T
,
T
,
T
).
(1b) Append(
T
,
m
).
(1c) Append(
T
,
m
).
(1d)
T = [(T ;T )
.
EndFor
EndProcedure
CHANG
et al.
: FAST PARALLEL MOLECULAR ALGORITHMS FOR DNA-BASED COMPUTATION: FACTORING INTEGERS
153
TABLE I
R
ESULT FOR
T
UBE
T
I
S
G
ENERATED BY THE
A
LGORITHM
I
NITIAL
S
OLUTION
(T )
TABLE II
R
ESULT FOR
T
UBE
T
I
S
G
ENERATED BY THE
A
LGORITHM
I
NITIAL
P
RODUCT
(T )
Consider that the number of bits for is 3 bits. Eight values
for are, respectively, 000, 001, 010, 011,100, 101 110, and
111. Tube is an empty tube and is regarded as an input tube
for the algorithm InitialSolution . Because the value for
is three, Steps (1a) through (1d) will be run three times. After
the rst execution of Step (1a) is nished, tube , tube
, and tube . Next, after the rst execution for
Step (1b) and Step (1c) is performed, tube and tube
. After the rst execution of Step (1d) is run, tube
, tube , and tube . Then, after
the second execution of Step (1a) is nished, tube , tube
, and tube . After the rest of
operations are performed, tube , tube , and the
result for tube is shown in Table I. Lemma 1 is applied to
demonstrate correction of the algorithm InitialSolution
unsigned integer of bits denoted in Section III-B, is one
of two large prime numbers if the remainder is equal to zero.
Assume that in a divider the length of a dividend is bits
and the length of a divisor is bits, where .Itis
very obvious that the division instruction is nished through
successive compare, shift, and subtract operations of at most
times. Therefore, suppose that is represented as a
-bit binary number, , where the value of
each bit is either one or zero for and
. The bits and , respectively, rep-
resent the most signicant bit and the least signicant bit for .
One binary number and another binary number
are, respectively, applied to represent the
minuend and the difference for the successive compare, shift,
and subtract operations of the th time. This is to say that the
binary number is the minuend for the suc-
cessive compare, shift, and subtract operations of the
.
Lemma 1:
The algorithm InitialSolution
is used to con-
struct the solution space of DNA strands for
different un-
signed integer values.
Proof:
The algorithm InitialSolution is implemented
by means of the
amplify, append
, and
merge
operations. Each
execution of Step (1a) is used to amplify tube and to generate
two new tubes, and , which are copies of . Tube
then becomes empty. Then, Step (1b) is applied to append a
DNA sequence, representing the value one for , onto the end
of every strand in tube . This is to say that those integers
containing the value one to the th bit appear in tube . Step
(1c) is also employed to append a DNA sequence, representing
the value zero for , onto the end of every strand in tube .
That implies that these integers containing the value zero to the
th bit appear in tube . Next, Step (1d) is used to pour tubes
and into tube . This indicates that DNA strands in tube
include DNA sequences of and . At the end
of Step (1), tube consists of DNA sequences representing
different unsigned integer values.
From InitialSolution , it takes
amplify
operations,
append
operations,
merge
operations, and three test tubes to
construct the solution space of DNA strands. A value sequence
for every bit contains 15 bases. Therefore, the length of a DNA
strand, encoding an unsigned integer value of bits, is
bases consisting of the concatenation of one value sequence for
each bit.
th
time.
For every bit ,two
distinct
15 base value sequences were
designed. One represents the value zero for and the other
represents the value one for . For convenience, we assume
that denotes the value of to be one and denes
the value of to be zero. The following algorithm is used to
construct a DNA strand for the value of
.
Procedure InitialProduct
(T )
(1) For
q =1
to
2 k
(1a) Append-head
(T ;n ;q)
.
EndFor
EndProcedure
Consider that the number of bits for is 6 bits and the value
for is 001 111. Tube with the result shown in Table I is
regarded as an input tube for the algorithm, InitialProduct .
Because the value for is six, Step (1a) will be executed six
times. After each operation for Step (1a) is performed, the result
is shown in Table II. Lemma 2 is used to prove correction of the
algorithm InitialProduct
.
C. The Construction to the Product of Two Large Prime
Numbers of Bits
Assume that the length for , the product of two large prime
numbers of bits, denoted in Section III-A, is bits. Also
suppose that the product is used to represent the minuend
(dividend) and the difference for successive compare, shift, and
subtract operations in a divider. When
Lemma 2:
A DNA strand for the value of
can be con-
structed from InitialProduct
.
Proof:
Refer to Lemma 1.
From InitialProduct , it takes
append-head
oper-
ations and one test tube to construct a DNA strand. The length
of the DNA strand, encoding the value of ,is bases con-
sisting of the concatenation of one value sequence for each bit.
is divided by
,an
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