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Primality Test


A primality test is a
test to decide if a given number is prime, instead of really deteriorating the
number into its constituent prime elements.

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Primality tests come in
two assortments: deterministic and probabilistic. Deterministic tests decide
with outright assurance whether a number is prime. Cases of deterministic tests
incorporate the Lucas-Lehmer test and elliptic bend primality demonstrating.
Probabilistic tests can conceivably (in spite of the fact that with little
likelihood) dishonestly recognize a composite number as prime (despite the fact
that not the other way around). Nonetheless, they are as a rule considerably
quicker than deterministic tests. Numbers that have finished a probabilistic
prime test are in this manner legitimately alluded to as plausible primes until
the point that their primality can be shown deterministically.

A number that breezes
through a probabilistic test yet is in truth composite is known as a
pseudoprime. There are numerous particular kinds of pseudoprimes, the most
well-known being the Fermat pseudoprimes, which are composites that in any case
fulfill Fermat’s little hypothesis.


The Rabin-Miller solid
pseudoprime test is an especially proficient test. The Wolfram Language
executes the various Rabin-Miller test in bases 2 and 3 joined with a Lucas
pseudoprime test as the primality test utilized by the capacity PrimeQn. In
the same way as other such calculations, it is a probabilistic test utilizing
pseudoprimes. With a specific end goal to ensure primality, a much

slower deterministic calculation
must be utilized. In any case, no numbers are really realized that finish
progressed probabilistic tests, (for example, Rabin-Miller) yet are really


The Miller-Rabin:

The Miller– Rabin
primality test or Rabin– Miller primality test is a primality test: a
calculation which decides if a given number is prime, like the Fermat primality
test and the Solovay– Strassen primality test. Its unique rendition is because
of Russian mathematician M. M. Artjuhov.1 Gary L. Mill operator rediscovered
it; Miller’s variant of the test is deterministic, yet the accuracy depends on
the problematic expanded Riemann hypothesis.2 Michael O. Rabin altered it to
acquire an unrestricted probabilistic calculation.


Much the same as the
Fermat and Solovay– Strassen tests, the Miller– Rabin test depends on a balance
or set of equities that remain constant for prime esteems, at that point checks
regardless of whether they hold for a number that we need to test for

We now look at the
Miller-Rabin primality test in view of the utilization of WITNESS. Once more,
we expect that n is an odd numbear more prominent than 2



operator RABIN(n, s)

       for j D 1 to s


       a = RANDOM(1, n – 1)


       if WITNESS.(a, n)


       return COMPOSITE               /certainly


5        return PRIME                         //most likely



Greatest Common

A positive integer
d is called a common divisor of the integers a and b, if d divides a and b. The
greatest possible such d is called the greatest common divisor of a and b,
denoted gcd(a,b).If = 1 gcd(a,b) then a,b are called relatively prime.

The Euclidean
Algorithm For Finding GCD:


If b==0

Return a

Else return EUCLID(b,a
mod b)

As an example of
the running of EUCLID, consider the computation of  gcd(30,21)


= EUCLID(9,3)

= EUCLID(3,0)


This computation calls EUCLID recursively three times.

The algorithm
returns a in line 2, if b = 0, so that equation (31.9) implies that gcd(a,b) =
gcd.(a,0) = a. The algorithm cannot recurse inde?nitely, since the second
argument strictly decreases in each recursive call and is always non negative.
Therefore, EUCLID always terminates with the correct answer.


Running Time Analysis of Euclid’s

analyze the worst-case running time of EUCLID as a function of the size of a
and b. We assume with no loss of generality that a>b>= 0. To justify this
assumption, observe that if b>a>= 0, then EUCLID(a,b) immediately makes
the recursive call EUCLID(b,a). That is, if the ?rst argument is less than the
second argument, EUCLID spends one recursive call swapping its arguments and
then proceeds. Similarly, if        b =
a>0, the

terminates after one recursive call, since a mod b =0.

The overall
running time of EUCLID is proportional to the number of recursive calls it


The Extended
Euclidean Algorithm For Finding GCD:

We extend the algorithm to compute the integer coef?cients x
and y such that

d =gcd(a,b)= ax


If b==0


(d’,x’,y’)=EXTENDED-EUCLID(b,a mod b)



EXTENDED-EUCLID procedure is a variation of the EUCLID procedure. Line 1 is
equivalent to the condition in SIMPLE EUCLIDEAN b == 0 in line 1 of EUCLID. If
b = 0, then EXTENDED-EUCLID returns not only d=a in line 2 but also the
coefficients x=1 and y=0 so that a=ax+by.If b not equal to zero,
EXTENDED-EUCLID first computes(d’,x’,y’) such that d’=gcd(b,a mod b) and
d’=bx’+(b,a mod b).

the number of recursive calls made in EUCLID is equal to the number of
recursive calls made in EXTENDED-EUCLID, the running times of EUCLID and EXTENDED-EUCLID
are the same, to within a constant factor. That is, for a>b>0, the number
of recursive calls is O.lgb


The RSA Public-Key Cryptosystem:

RSA (Rivest– Shamir– Adleman) is one of the main open key
cryptosystems and is broadly utilized for secure information transmission. In
such a cryptosystem, the encryption key is public and it is unique in relation
to the decoding key which is kept private. In RSA, this asymmetry depends on
the practically trouble of the factorization of the result of two extensive
prime numbers, the “factoring issue”.




Private Key cryptography, or asymmetric
cryptography, is an encryption conspire that utilizations two numerically
related, however not indistinguishable, keys – an open key and a private key.
Not at all like symmetric key calculations that depend on one key to both encode
and decode, each key plays out a remarkable capacity. General society key is
utilized to scramble and the private key is utilized to decode.

It is computationally infeasible to register
the private key depend on public (general) key. With these lines, open keys can
be free of cost shared, permitting clients a simple and advantageous strategy
for encryption content and checking advanced marks, and private keys can be
kept private, guaranteeing just the proprietors of the private keys can
decryption content and make computerized marks.

Since open keys should be shared however
are too huge to be effectively recalled that, they are put away on computerized
declarations for secure transport and sharing. Since private keys are not
shared, they are essentially put away in the product or working framework you
utilize, or on equipment (e.g., USB token, equipment security module)
containing drivers that enable it to be utilized with your product or working


Public key algorithm

i- Key1 (public key use for encryption).

ii- Key2 (private key use for decryption).

Encrypting and decrypting use modular exponentiation


Choose two large prime no. P and Q such
that P != Q

Calculate N=P*Q

Choose E (Public Key) such that E
is not a factor of (P-1)*(Q-1).

Choose  D (Private Key) such that (D*E)mod

Cipher Text (C.T) = (P.T) E mod

Plain Text (P.T) = (C.T) D
mod N.



                      A (Sender)                                                                              B(Receiver)

            (Sender A
want to send 5)

P=7, Q=13


            (P-1)(Q-1)=>6*10=60                                    (D*E) mod

            E=13 (Public
Key)                                         D=37
(Private Key)

mod 77                                                                   C.T=26

            C.T=26                                                                                   P.T=(26)37
mod 77

receive the value 5 by sender)


Modular linear equation:

As u realizes
that this sort of condition is utilized as a part of Cryptography, so it is
issue that how to unravel this condition to discovering key, utilized. For
instance in the event that somebody got an information having some esteem which
is scrambled with some component, now recipient need to decode it however don’t
have a clue about the unscrambling key so it might be conceivable that key
esteem is found by taking modulus with number n (it is rely upon encryption
strategy key if utilizing same procedure for encryption or it is predefined).

So for this Problem we have answer for fathom this sort of
condition utilizing following advances: assume that a, b, and n are given

Find gcd(a,n)=d i.e d=ax+by

On the off chance that d|b at that point there is arrangement
(at that point there is further strides to settle)

Else no any arrangement                                                              

Algorithm for
Modular equation

For measured Equation there is calculation (Algorithm) known

(d, x’, y’) = EXTENDED-EUCLID (a, n)

If d|b

3.           Xo= x’ (b/d) mod n

4.           For i = 0 to d -1

Print Xi= (Xo+ i(n/d)) mod n

Else print “no solutions”

Presently I will
clarify how these lines functions when we give some estimation of a, b, n. For
instance of the operation of this strategy, consider the condition 15x ? 40(mod
50) (here, a =14, b =30, and n =100). Calling EXTENDED-E UCLID in line 1, we
register (d, x’, y’) = (5,- 3, and 1). Since 5 | 50 at that point, lines 3– 5
will execute. Line 3 figures Xo = (- 3) (8) mod 50 = 26. At that point the
circle on lines 4– 5 prints the five arrangements 26, 36, 46, 6 and 16 by
executing line 5


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