Modified RSA Encryption Algorithm using Four Keys

DOI : 10.17577/IJERTCONV3IS07021

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Modified RSA Encryption Algorithm using Four Keys

Nivetha A Preethy Mary S Santosh kumar J

Dept of Information Technology Dept of Information Technology Dept of Information Technology

Anand Institute of Higher Technology,

Anand Institute of Higher Technology,

Anand Institute of Higher Technology,

Chennai, Tamil nadu Chennai, Tamil nadu Chennai, Tamil nadu

Abstract: The proposed paper enhances the RSA algorithm through the use of four prime number in combination of public and private key. Hence by using this, factoring complexity of variable is increased, this makes the analysis process with the development of equipment and tools become much easier. The use of four prime number will give the ability to the modified encryption technique to provide more security in accessing, and also increased speed. This was developed from the original RSA algorithm the additional two prime numbers are going to provide secrecy. Many experiments have been done under this proving Modified RSA encryption Algorithm using four keys to be faster and efficient than the original encryption and decryption process. This thesis presents the implementation of successive subtraction operation instead of division operation. By applying this approach we can achieve the high computational speed and reduce the complexity of the mathematical steps.

Keyword: Complexity, Prime number, Public key, four keys, algorithm.


    Encryption is one of the principal means to grant the security of sensitive Information also functioned with digital signature, authentication, secret sub-keeping, system security and etc. Therefore, the purpose of adopting encryption techniques is to ensure the information's confidentiality, integrity and certainty, prevent information from tampering, forgery and counterfeiting.

    At present, the best known and most widely used public key system is RSA, which was first proposed in paper "A method for obtaining digital signatures and public-key cryptosystems" by RL Rivest in 1978. It is an asymmetric (public key) cryptosystem based on number theory, which is a block cipher system. Its security is based on the difficulty of the large number prime factorization, which is a well-known mathematical problem that has no effective solution.


    R.L. Rivest, A. Shamir, and L. Adleman[1] proposed a method for implementing a public-key cryptosystem whose security rests in part on the difficulty of factoring large numbers. If the security of our method proves to be adequate, it permits secure communications to be established without the use of couriers to carry keys, and it also permits one to sign digitized documents. The reader is urged to and a way to break the system. Once the method has withstood all attacks for a sufficient length of time it may be used with a reasonable amount of condense.

    The encryption function is the only candidate for a trap-door one-way permutation known to the authors. The large volume of personal and sensitive information currently held in computerized data banks and transmitted over telephone lines makes encryption increasingly important. In recognition of the fact that efficient, high- quality encryption techniques are very much needed but are in short supply, the National Bureau of Standards has recently adopted a Data Encryption Standard, developed at IBM. The new standard does not have property (c), needed to implement a public-key cryptosystem.

    Xin Zhou and Xiaofei Tang[2] proposed an implementation of a complete and practical RSA encrypt/decrypt solution based on the study of RSA public key algorithm. In addition, the encrypt procedure and code implementation is provided in details. Encryption is one of the principal means to guarantee the security of sensitive information. It not only provides the mechanisms in information confidentiality, but also functioned with digital signature, authentication, secret sub-keeping, system security and etc. The encryption and decryption solution can ensure the confidentiality of the information, as well as the integrity of information and certainty, to prevent information from tampering, forgery and counterfeiting. Encryption and decryption algorithm's security depends on the internal structure of the rigor of mathematics, it also depends on the key confidentiality.

    Problem for RSA encryption on the file, it indicates the RSA mathematical algorithms in the computer industrys importance and its shortcomings. It discusses the questions of how to apply to the personal life of RSA information security issues. And also contains the use of RSA and the basic principles of data encryption and decryption. In the end, it proposed a new program to improve RSA algorithm based on RSA cryptography and the extensive application. In summary, this issue of the RSA encryption and decryption keys, RSA algorithm, the new use of the RSA and other issues to study and make some new programs, future work should be in the new RSA cryptographic algorithms and a wide range of applications continue to research.


    MREA is secure as compared to RSA as it based on the factoring problem as well as decisional composite residuosity assumptions which are the intractability

    hypothesis. This scheme also presents comparisons between RSA and MRSA cryptosystems in terms of security and performance. This algorithm uses a mod operator for computational purposes. The objective of this thesis presents the implementation of successive subtraction operation instead of using division operator. For applying this approach we have to achieve the high computational speed and reduce the complexity of the mathematical steps.

      1. Modulo Operation:

        In computing, modulo operation finds the remainder of division of one number by another. .

        Figure 4: Euclidean algorithm diagram

      2. Euclidean Algorithm

    Euclid's method for finding the greatest common divisor (GCD) of two starting lengths BA and DC, both defined to be multiples of a common "unit" length. In mathematics, the Euclidean algorithm, or Euclid's algorithm, is a method for computing the greatest common divisor

    By reversing the steps in the Euclidean algorithm, the GCD can be expressed as a sum of the two original numbers each multiplied by a positive or negative integer, e.g., the GCD of 252 and 105 is 21, and 21 = [5 × 105] + [(2) × 252]. This important property is known as Bezout's identity.

    The simplest form of Euclid's algorithm starts with a pair of positive integers and forms a new pair that consists of the smaller number and the difference between the larger and smaller numbers. The process repeats until the numbers are equal; then that value is the greatest common divisor of the original pair. The division form of Euclid's algorithm starts with a pair of positive integers and forms a new pair that consists of the smaller number and the remainder obtained by dividing the larger number by the smaller number. The process repeats until one number is zero. The other number then is the greatest common divisor of the original pair.


    1. RSA System

      RSA is a commonly adopted public key cryptography algorithm. The first, and still most commonly used asymmetric algorithm RSA is named for the three

      mathematicians who developed it, Rivest, Shamir, and Adelman. RSA today is used in hundreds of software products and can be used for key exchange, digital signatures, or encryption of small blocks of data.RSA uses variable size encryption block and a variable size key.

    2. Attacks against plain RSA

      e e e

      e e e

      There are a number of attacks against plain RSA as described below. When encrypting with low encryption exponents (e.g., e = 3) and small values of the m, (i.e., m < n1/e) the result of me is strictly less than the modulus n. In this case, cipher texts can be easily decrypted by taking the eth root of the cipher text over the integers. If the same clear text message is sent to e or more recipients in an encrypted way, and the receivers share the same exponent e, but different p, q, and therefore n, then it is easy to decrypt the original clear text message via the Chinese remainder theorem. Johan Has tad noticed that this attack is possible even if the clear texts are not equal, but the attacker knows a linear relation between them. This attack was later improved by Don Coppersmith. RSA has the property that the product of two cipher texts is equal to the encryption of the product of the respective plaintexts. That is m1 m2 (m1m2) (mod n). Because of this multiplicative property a chosen-cipher text attack is possible

    3. Key generation

      RSA involves a public key and a private key. The public key can be known by everyone and is used for encrypting messages. Messages encrypted with the public key can only be decrypted in a reasonable amount of time using the private key. The keys for the RSA algorithm are generated the following way:

      1. Choose two distinct prime numbers p and q. For security purposes, the integers p and q should be chosen at random, and should be of similar bit- length. Prime integers can be efficiently found using a primality test.

      2. Compute n = pq.

        n is used as the modulus for both the public and private keys. Its length, usually expressed in bits, is the key length.

      3. Compute (n) = (p) (q) =

        (p 1)(q 1), where is Euler's totient function.

      4. Choose an integer e such that 1

        < e <(n) and gcd(e,(n)) = 1; i.e. e and (n) are co-prime is released as the public key exponent. e having a short bit-length and small Hamming weight results in more efficient encryption most commonly 216 +1=65537. However, much smaller values of e(such as 3) have been shown to be less secure in some settings.

      5. Determine d as d1 e (mod (n)), i.e., d is the multiplicative inverse of e (modulo (n)).This

        is more clearly stated as solve for d given de 1 (mod (n)).This is often computed using the extended Euclidean algorithm.d is kept as the private key exponent.

        By construction, de 1 (mod (n)). The public key consists of the modulus n and the public (or encryption) exponent e. The private key consists of the modulus n and the private (or decryption) exponent d, which must be kept secret. p, q, and (n) must also be kept

        secret because they can be used to calculate d.

        An alternative, used by PKCS#1, is to choose d matching de 1 (mod ) with

        = LCM(p1, q1), where LCM is the least common multiple. Using instead of (n) allows more choices for d. can also be defined using the Carmichael function, (n).

        The ANSI X9.31 standard prescribes, IEEE 1363 describes, and PKCS#1 allows,

        that p and q match additional requirements: being strong primes, and being different enough that Fermat factorization fails.


Key generation


  2. Calculate n=p*q*r*s.

3. Calculate f(n)=(p-1)*(q-1) *(r-1)*(s-1)

  1. Select integers e; gcd(f(n),e)=1;1<e<f(n).

  2. Calculate d; d=e-1 mod f(n)

  3. Public key KU ={e,n}

  4. Private Key KR ={d,N}.


Plain text: M<n

Cipher text : C= Memod n. Decryption:

Cipher text to Plain text: M=Cd mod n.

Here encryption and decryption using division operation instead of using that use of successive subtraction which is reduce the mathematical steps. And also to achieve the high computational speed.

    1. Proposed Algorithm Step1: Start the process Step 2: initialize i =0

      Step 3: Calculate power of ei

      Step 4: If ei value is less than phi value then go to step5 Step 5: if ei value is greater than phi value then goto step 6 Step 6: ei value store in b go to step 7

      Step 7: exit in for loop go to step 5

      Step 8: subtract phi value in b and store to phi

      Step 9: if phi value is negative the last value of phi vale is mod value

      Step 10: Stop the process.

    2. Previous work:

      Let us take example of 125 mod 2. For that we take mod operator performing, the following steps,125- 2=123-2=121-2=1193-2=1. So that, we want compute more (21 steps) steps want to evaluate this expression. Finally got the output as 1.

    3. Proposed work:

Now introduce the concept, of power subtraction. That is 125-64=61-32=29-16=13-8=5-4=1. In this concept, we use only the 4 steps of evaluation. 125 mod 2, the powers of 2 can be used. That is, nearest value of power is 64=2^6, so use of successful powers of sub tractor is used to get the answer as 1.So that speed can be increased whereas the previous system. And also reduced the complexity of the computation.


This paper presents the purpose about modification in modified RSA encryption and decryption. Here artificially small parameters are used to clarify the concept. However, the method is applicable in general to all suitably selected parameters. Here four prime numbers will be used to get the public key and private key.

Select four prime numbers.

1. Calculate n=p*q*r*s

P=2, q=3, r=5,s=17


2. Calculate f(n)=(p-1) (q-1) (r-1) (s-1)

f(510) = (2-1) (3-1) (5-1)(17-1) =128 f (n)=128

  1. Select any number 1<e<128

    F (n) must not be divisible by e Let e=3

  2. Select d, multiplicative of e(mod f(n)) d= 43

    The Public Key is (n = 510, e = 3) Private Key is (n = 510, d = 43) Given message m= 11.

  3. Encryption:

    C= 113mod 510 = 311; C = 311

  4. Decryption:

    M = 31143 mod 510 = 11

    B got the original message (11) which is sent by A. In proposed algorithm use of mod function want to perform a division operation, instead of using that use of successive subtraction which is reduce the mathematical steps. The above encryption and decryption as follows:

    Encryption: 113 mod 510 = 1331 mod 510=821-510=311

    Decryption: Same as Encryption


    The MREA cryptosystem is based on additive homomorphism properties and RSA cryptosystem, additive homomorphism scheme required four prime numbers, it will be more difficult and take long time to factor modulus, If RSA which is based on single modulus, and additive homomorphism based on dual modulus, then time required for MREA algorithm is higher than the proposed algorithm.


    In this thesis calculate the encryption and decryption time for 4 prime number, In modified RSA algorithm, the time taken for encryption and decryption is high using division of arithmetic operation in the following table1 and table2.

    Calculate Total (Encryption and Decryption) Time for MREA MODULUS OPERATION


    Calculate Total time for RSA:

    TABLE 2:

    Comparison of MREA and RSA algorithm:









    MREA with

    Mod function










    MREA with

    Mod function


    16 16 32 128

    16 16 32 128

    Figure 5: Analysis of RSA and MREA

    The above table and figure shows that the process of encryption and decryption in the proposed method are faster than he original method by the apparent results. In modulus function instead of using that division operation, use of successive subtraction which is reduce the mathematical steps and also to achieve the high computational speed. And also this proposed algorithm is more secure for mathematical attacks


Encryption algorithm plays an important role in communication Security. The proposed cryptography procedure is the enhanced proposal of our previous research work where the concept of speed enhancement of modulus functions. Using this idea to reduce the mathematical steps to solve that expression. So we conclude that easily computation can be performed and complexity was reduced. Complexity time also decreased

by using successive subtraction technique. So that the speed can be increased.

In this thesis an algorithm is proposed for Modified RSA modulus factorization. The new algorithm aims to obtain the prime factors of modulus in Modified RSA algorithm. This algorithm is relatively simple and scalable. This method can be used for factorization of subtraction, very helpful to generate results in high speed.


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