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Posted 8 Jan 2004

Diffie-Hellman Key Exchange Example

, 14 Nov 2008
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An example of how an encryption key can be shared by two users using the Diffie-Hellman key exchange approach.

Introduction

It's often required that a message be encrypted between two parties for secure communication. There are plenty of algorithms out there for encryption that are very secure, but their weakness lies in transporting the encryption key. The Diffie-Hellman key exchange protocol allows people to exchange keys in a manner that does not allow an eavesdropper to calculate the key in a fast manner.

This code demonstrates the use of this type of key exchange.

How to Use the Demo Project

To demonstrate the use of the key exchange, run two copies of the demo application. Set one to be the sender and the other to be a receiver.

The sender should generate the public keys, and the sender's interim key. Paste these values into the appropriate text boxes in the receiver application. The receiver should then click to generate his interim key, and copy this key into the "receiver's interim key" textbox on the sender application. Both applications should then be able to generate the same key by clicking "Generate Key".

Using the Source Code

The DiffieHellman class is simple to use and should be integrated in the following manner:

Make an instance of the class - (i.e. CDiffieHellman *DH = new CDiffieHellman;)

The sender application then does the following:

__int64 n = 0;
__int64 g = 0;
__int64 SInterim = 0;
__int64 RInterim = 0;
__int64 key = 0; 

DH->CreateKeys(g,n);
DH->CreateSenderInterKey(SInterim);

//The sender now sends (n, g, and SInterim) to the receiving application
//This can be done unencrypted because they are public keys
//Now we wait until the reciever send us their interim key lets say RInterim

DH->CreateSenderEncryptionKey(key,RInterim);
//The shared encryption key is now the value of 'key'

The receiving application does the following:

__int64 n = 0;
__int64 g = 0;
__int64 SInterim = 0;
__int64 RInterim = 0;
__int64 key = 0;

//Wait for the values of (n,g, and SInterim) to be sent here

DH->CreateRecipientInterKey(RInterim);

//Now send the RInterim key to the sender application

DH->CreateRecipientEncryptionKey(key,SInterim);
//The shared encryption key is now the value of 'key'

Extra Functions

There are some private member functions of the CDiffieHellman class that you may find useful, and please feel free to use them.

  • The GeneratePrime() function generates a large prime number.
  • The MillerRabin and IsItPrime functions can be used in conjunction to test primality.
  • The XtoYmodN is a function to raise x to the power of y in modulus n. Even though it sounds impossible for a computer to work out, say 150 million to the power of 150 million, this can be done in modulus n by using the power chaining method.

Further Help

Should you require any additional help, please do not hesitate to contact me. I would be interested in hearing your comments, suggestions and any questions.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Griffter UK
Software Developer (Senior)
United Kingdom United Kingdom
No Biography provided

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Comments and Discussions

 
QuestionRFC 3526 Pin
Member 110532912-Sep-14 5:35
memberMember 110532912-Sep-14 5:35 
QuestionInterim Keys longer than 32 bits? Pin
christop_christop3-Jul-13 11:33
memberchristop_christop3-Jul-13 11:33 
Questiondiffie_hillman Pin
Member 990145111-Mar-13 5:50
memberMember 990145111-Mar-13 5:50 
GeneralMy vote of 5 Pin
Member 43208448-Jun-12 11:22
memberMember 43208448-Jun-12 11:22 
Questionhow to run Pin
mohdamirr12-Feb-12 6:58
membermohdamirr12-Feb-12 6:58 
GeneralInformation about Diffie hellman Pin
sudhakatr2-Feb-10 21:54
membersudhakatr2-Feb-10 21:54 
GeneralRe: Information about Diffie hellman Pin
Griffter UK2-Feb-10 22:00
memberGriffter UK2-Feb-10 22:00 
Generalerror setting up the demo Pin
swati chaudhari25-Oct-09 17:23
memberswati chaudhari25-Oct-09 17:23 
GeneralRe: error setting up the demo Pin
Snehlata Shaw17-Nov-09 9:16
memberSnehlata Shaw17-Nov-09 9:16 
GeneralRe: error setting up the demo Pin
Griffter UK17-Nov-09 22:54
memberGriffter UK17-Nov-09 22:54 
GeneralLicense of this code Pin
Jonh Wendell13-Nov-08 3:35
memberJonh Wendell13-Nov-08 3:35 
GeneralRe: License of this code Pin
Griffter UK13-Nov-08 5:32
memberGriffter UK13-Nov-08 5:32 
QuestionErrors Pin
zeeshan5114-Oct-08 22:14
memberzeeshan5114-Oct-08 22:14 
AnswerRe: Errors Pin
Griffter UK13-Nov-08 5:16
memberGriffter UK13-Nov-08 5:16 
Generalminor changes Pin
Member 31219318-Oct-08 10:37
memberMember 31219318-Oct-08 10:37 
GeneralCompile on linux Pin
Filipe Niero Felisbino2-Jun-08 8:33
memberFilipe Niero Felisbino2-Jun-08 8:33 
GeneralRe: Compile on linux Pin
MrLeeGriffiths2-Jun-08 23:21
memberMrLeeGriffiths2-Jun-08 23:21 
QuestionProblem compiling Pin
wygno29-Apr-07 17:58
memberwygno29-Apr-07 17:58 
AnswerRe: Problem compiling Pin
MrLeeGriffiths9-Apr-07 22:51
memberMrLeeGriffiths9-Apr-07 22:51 
QuestionEnquiry Pin
wygno28-Apr-07 0:46
memberwygno28-Apr-07 0:46 
AnswerRe: Enquiry Pin
MrLeeGriffiths9-Apr-07 22:49
memberMrLeeGriffiths9-Apr-07 22:49 
GeneralDiffie Hellman Key exchange Pin
Naeem Qazi21-Nov-06 9:44
memberNaeem Qazi21-Nov-06 9:44 
GeneralRe: Diffie Hellman Key exchange Pin
Garth J Lancaster21-Nov-06 10:23
memberGarth J Lancaster21-Nov-06 10:23 
GeneralRe: Diffie Hellman Key exchange Pin
Naeem Qazi21-Nov-06 23:29
memberNaeem Qazi21-Nov-06 23:29 
GeneralRe: Diffie Hellman Key exchange Pin
Hashbullet30-Mar-07 1:09
memberHashbullet30-Mar-07 1:09 

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