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ED.java

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import java.math.*;

import java.io.*;

import java.security.*;

/*

ALGORITHM

==========

STEP 1 > pick 2 large prime numbers , say 'p' and 'q'

STEP 2 > Compute their product, say n = p * q

STEP 3 > Compute PHI(n) as PHI(n) = (p-1) * (q-1)

STEP 4 > pick a value 'e' such that GCD(e,PHI(n))=1 , here 'e' is the encrytion key

STEP 5 > calculate 'd' as d = e^(-1) mod PHI(n) , here 'd' is the decryption key

STEP 6 > now we have our PublicKey=(e,n) and PrivateKey=(d,n)

STEP 7 > Encryption : CipherText = (Message)^e mod n

STEP 8 > Decryption : PlainMsg = (CipherText)^d mod n

*/

class ED_helper

{

private int bitlen,r;

private BigInteger p,q,n,phi,e,d;

ED_helper(int bit)

{

bitlen=bit;//number of bits strong we want our encryption to be

SecureRandom r=new SecureRandom();//we use this because its a 128bit rand_generator,hence low chances of repeats.

//<================================< STEP 1 >======================================>

//SYNTAX

//BigInteger(int bitLength, int certainty, Random rnd)

//we want to generate encryption keys as a product of p and q hence each of p and q much contain half as many digits

//as bitlen.And the second value to the BigInt tells us we want to generate primes with 100% certainity using 'r'

p=new BigInteger(bitlen/2,100,r);

q=new BigInteger(bitlen/2,100,r);

System.out.println("P value :"+p+"\n\n\n "+"Q value="+q);

//<================================< STEP 2 >======================================>

n=p.multiply(q);//BigInteger Multiplication

//<================================< STEP 3 >======================================>

phi=p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE));

//<================================< STEP 4 >======================================>

e=new BigInteger(bitlen/2,100,r);//BigInteger will take care of GCD() <UNDERATED STEP>

//<================================< STEP 5 >======================================>

d=e.modInverse(phi);//e^-1 then % phi

//<================================< STEP 6 >======================================>

System.out.println("\n\n\nValue of D :"+d+"\n\n\nValue of E : "+e);//Displaying keys

}

//<================================< STEP 7 >======================================>

public BigInteger encrypt(BigInteger Msg)

{

//CipherText = (Msg)^e mod n

return(Msg.modPow(e,n));

}

//<================================< STEP 8 >======================================>

public BigInteger decrypt(BigInteger Msg)

{

//DecodedMsg = (Ciphext)^d mod n

return(Msg.modPow(d,n));

}

}

//Main Driver Class to demonstrate functionalities implemented by the above class

class ED

{

public static void main(String args[]) throws IOException

{

//Create object and pass the bit length

ED_helper rsa=new ED_helper(1200);

BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

String text1,text2;

BigInteger Pt,Ct;

//Input Message from user

System.out.println("\n\nEnter Plaintext :");

text1=br.readLine();

//pass the input message to the encrypt method to obtain cipher text

Pt=new BigInteger(text1.getBytes());

Ct=rsa.encrypt(Pt);

System.out.println("\n\nCiperText is : "+Ct);

//Pass the CipherText to the decrypt method to obtain original message

Pt=rsa.decrypt(Ct);

text2=new String(Pt.toByteArray());

System.out.println("\n\n\nData after decrypt :"+text2);

}

}