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Diffie Hellman key exchange

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Created: vineet.jain (15 Feb 2010)
Last edited: hamishwillee (20 Jul 2012)

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The following code snippet demonstrates the Diffie-Hellman key exchange algorithm which is a cryptography algorithm that allows two parties to jointly establish a shared secret key over an insecure communication channel. Diffie Hellman method is used to jointly arrive at the same Shared key and you can read more here.

Header Required

#include <cryptoasymmetric.h>


Library Required

LIBRARY cryptography.lib

Following are the steps involved in this algorithm:

1) Create two objects of RInteger class(a TInteger derived class allowing the construction of variable length big integers) each for a prime Number 'P'& a generator':

RInteger PrimeNum = RInteger::NewPrimeL(1024)//pass here the number of bits of prime number you wish to generate e.g. passing 1024 will generate a 1024 bit prime number.
RInteger Generator=RInteger::NewL(5); // generator is generated by passing a constant which is usually 2 or 5


2) Create an object of CDHKeyPair class using the RInteger objects created in Step 1 :

TRAPD(err,iDHKeyPair = CDHKeyPair::NewL(PrimeNum ,Generator));


3) Next get the 'G'(generator) & 'N'(Prime Number) parameters of Diffie Hellman using the CDHkeyPair object created in Step 2 :

const TInteger& G =iDHKeyPair->PublicKey().G();
const TInteger& N =iDHKeyPair->PublicKey().N();
//Get the Prime Number in a buffer as below :
HBufC8 *PrimeBuffer=N.BufferLC();
TPtr8 PtrPrime=PrimeBuffer->Des();
//Also get the DH value 'x'(a random large integer) as:
const TInteger& xparam =iDHKeyPair->PrivateKey().x();


4) Next generate the DH public value/parameter(PublicVal) which is to be exchanged with the party other side as below:

const TInteger& PublicVal = (G.TimesL(xparam )).ModuloL(N);
HBufC8 *Buffervalue = PublicVal .BufferLC();
TPtr8 NewPublicVal = Buffervalue ->Des();


5) Now send the Prime Number which is generated in Step 3 to the other party by the way you wish to send.


6) The receiving party(which will receive the Prime Number) will now repeat the Steps 1 to 4, with Step 1 as below:

RInteger PrimeNumReceiver = RInteger::NewL(const TDesC8&//Prime Number Received from Sender side)
RInteger GeneratorReceiver=RInteger::NewL(5);
 
//Rest of the steps will be same on receiver side but will require the use of
//received prime number where it is required.


7) Now there will be two public values/parameters generated, one at the Sender's end & one at receiver's end, both generated using the same prime Number.So now exchange the two public values/parameters i.e. the sender will send its value to receiver & vice-versa.


8) Now with public values got exchanged the two parties will now generate a common/shared secret at their ends:

RInteger PrivateValue = RInteger::NewL(const TDesC8& privateParam//private parameter 'x' generated at each side in Step 3);
RInteger PrimeNum = RInteger::NewL(const TDesC8& Prime//the common Prime Number);
RInteger ReceivedPublic = RInteger::NewL(const TDesC8& publicParam //the exchanged public value);
 
//The common/Shared secret key will be generated at each end as below:
 
const TInteger& SharedKey = (ReceivedPublic.TimesL(Private)).ModuloL(PrimeNum );


  • The above generated shared secret key will be same as generated at both the ends(sender & receiver) & now they can use it as they want like using it as an encryption/decryption key in any of the Symmetrical Cryptographic algorithms.


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This page was last modified on 20 July 2012, at 07:14.
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