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8 Jan 2004

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RFC 3526 has MOD groups with large primes for example MOD Group 5 : The prime is: 2^1536  2^1472  1 + 2^64 * { [2^1406 pi] + 741804 }
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
The generator is: 2.
Does this program support this? And how would I implement it?
Thanks in advance!
Tvoice





Hi Griffer,
First thanks for the useful code posting. I have ported it to a 32 bit cortexM0 CPU. The compiler supports 64 bit ints, so it wasn't too hard. Also I converted to C from C++, also straightforward.
When I increase this defines, the algo seems to not return, and in fact stays in the while/do loop looking for a prime (even if I just double them).
#define MAX_RANDOM_INTEGER 2147483648 //Should make these numbers massive to be more secure
#define MAX_PRIME_NUMBER 2147483648 //Bigger the number the slower the algorithm
I'd like to increase the key sizes beyond 32 bits, but I believe these constants preclude that. I am happy to invest more time investigating it, but thought I'd pass a note to see what testing you might have done already.
Best regards, christop_chrisop





This is pavithra doing project on Diffiehellman.... so i need the information as well as source code of DiffieHellman Key Exchange Example" this is helpful to me doing the project..






I am unable to run the source plz help me. how to run in dev





Dear sir,
This is sudhakar doing project on Diffiehellman, so i need the information as well as source code of "DiffieHellman Key Exchange Example" this is helpful to me doing the project.
Thanking you sir






hi i am unable to setup the demo file. can u please provide steps.
thank you in advance
swati





hi swati m snehlata shaw from kolkata .m also workin project in dis topic,,same prob facing "error showing in setting demo project",,did u get n e reply?if u know hw 2 setup that plz reply .i'll be grateful to you
thank you





Hi Snehlata Shaw,
Please could you elaborate on what problems you're having exactly with the demo project  so I can provide further assitance.
Thanks
Lee Griffiths





Hi, Lee Griffiths.
I've seen this same code in UltraVNC and I want to get it into gtkvnc, a library which implements the VNC client side, and which is used in Vinagre, the GNOME VNC client.
So, gtkvnc is licensed under the LGPL and thus I cannot just copy and paste your code into the library. Could you please license your code under LGPL or compatible?
I tried to get in touch with you by email, but I think the email listed in the code is outdated.
I'd appreciate any comment from you.
Thanks in advance,
Wendell  jwendell AT gnome DOT org





Hi Wendell,
I have modified the license and this article, along with any associated source code and files, is now licensed under The Code Project Open License (CPOL).
UltraVNC have obviously borrowed the code from this article and used it in their own implementation, which is perfectly fine because that was my intention to allow anyone here to use the code for free.
HOWEVER although the source code itself may be freely used you will need to check whether the use of the Diffie Hellman algorithm is free or whether a license is required to use it. I did read somewhere that the US patent for it expired in 1997 but PLEASE do not take my word for it, and if anyone else has some definitive answers on this then please let us all know by posting a reply here.
Thanks
Lee Griffiths





i face two errors during build up, can u please help me how to solve these
Error 1: error LNK2005: "void __cdecl operator delete(void *)" (??3@YAXPAX@Z) already defined in LIBCMTD.lib(dbgdel.obj) nafxcwd.lib
Error 2 fatal error LNK1169: one or more multiply defined symbols found c:\securetalk.exe 1
Thanks
Zeeshan





Hi zeeshan51
The problem you are experiencing is to do with the use of the code generation in MT mode and the use of static MFC libraries. There is a way to keep these settings by changing the linking order of libcmt and msvcrt, but the easiest way is as follows:
In the solution configuration, change the C++/Code Generation/Runtime Library to be Multithreaded Debug DLL (for debug builds) and Multithreaded DLL (for release builds).
Then you also have to modify (in the solution configuration), General/Use of MFC to be Use MFC in a shared library.
This should resolve the problem, but be aware that you will then need to redistribute the MFC libraries for computers that do not have visual studio etc installed.
Of course this is all in respect of the demo application, and if you wish to just use the diffie hellman class by itself then there are of course no MFC requirements (it's just for the demo application).
I have modified the solution / project to fix the aforementioned problems and am awaiting the article and downloads to be updated.....
Thanks,
Lee Griffiths





I suggest making obeying constcorrectness with the inputparameters in createRecipientInterKey.
This makes it easier to see what is input & what is output.
Also, the example concerning
DH>CreateRecipientInterKey(RInterim);
appears to be misleading. I think it's meant to say
serverkey.createRecipientInterKey (rintrim, g, n);
serverkey.createRecipientEncryptionKey(key,lintrim);
Here you can clearly see it's using the received g, n and lintrim,
and you can see where the rimtrim (which needs to be sent back) actually comes from
Regards,
Roger





Hello. First of all congratulations...
I've successfully compiled that sources on linux by making the following changes:
in DrmCrypto.h line 62:
typedef int64_t __int64;
In the DrmCrypto.cpp method __int64 CDrmCrypto::GetRTSC( void ) I've changed to:
__int64 CDrmCrypto::GetRTSC( void )
{
#ifdef _WIN32
int tmp1 = 0;
int tmp2 = 0;
__asm
{ RDTSC;
mov tmp1, eax;
mov tmp2, edx;
}
return ((__int64)tmp1 * (__int64)tmp2);
#else
__int64 result;
asm ("RDTSC" : "=A" (result));
return result;
#endif
}
That's all...





Excellent  thanks for that Filipe!





When I try to build, it displays the LNK6004 error. Can someone tell me how to solve it?





Hi Wygno,
Can you elaborate a bit further?
What operating system and what compiler are you using?
Also what is the full description of the link error/
Thanks.





Hi, I'm a beginner and I'm wondering what DRMCrypto class is? Could you be kind enough to provide a bit of explanation? Thanks





Hi wygno, the DRMCrypto class is something I wrote to demonstrate the use of the Diffie Hellman Key Exchange protocol. It isn't a Windows API.
Thanks.





Hello
My concept on diffie key exchange is not clear. If any one has diffie hellman code. Please send me simple diffie hellman code. I really need it?
Regards
Muhammad Naeem
Naeem Qazi





this might explain it a bit better (from a post further down the page) http://www.codeproject.com/cpp/DiffieHellmanExample.asp?msg=1769062#xx1769062xx[^]
as for simple ? there's easy to use, there's a good implementation or a bad implementation of an algorithm, but simple ? would you trust a 'simple' encryption algorithm .. I hope not
Modified  you might also want to google for Wei Dai's Crypto++ or CryptLib from Peter Guttman, or LibTomCrypt  all good libraries implementing algorithms in different ways  but simple ?? I think not
'g'





Hello
Thank you very much for reply. Actually by simple I mean that I dont need its GUI coding. I need the diffie hellman key exchange coding but without the use of libraries. I m mean to not use the libraries for it.
Regards
Muhammad Naeem
Naeem Qazi





Make clear about the concept: http://www.securitydocs.com/library/2978





I know the concept of Diffie Hellman Key Exchange. But i am badly in need of Diffie Hellman key exchange program written in java. If anybody finds it kindly provide me the program as early as possible. Its very urgent.
thank you one and all
Gokulakrishnan





Hi All,
I have now modified the class so that it does not use the GUID structure or the Windows API (CoCreateGuid). I have requested the download to be updated, but in the meantime if you reply to this message with your email address I can send you the latest version.
Thanks.
Lee





Please send me the code of Deffie Hellman algorithm to the email address i.e. qaziejaz@hotmail.com, please send me the code as soon as possible, I am doing my research work and in my research I need the said code.
thank's
Qazi Ejaz
31081979





I have to implement IPSEC n using omnet with .net to code it.... I find ur example very helpful... It is very easy to understand even for me when i dont know the details of the Diffie Helliman key exchange algorithm......
The compiler for ipv6suitewithinet (based on omnet n vc.net) doesnt support to resolve GUID u use in ur example....... It is giving me the Linking error to the external symbol to the function CoCreate()..... Can u plz help me... if i need to include certain file for it???? or is there any alternative i can use????
thanks a lot in advance...
Regards,
Maria





Would it be possible for you to support 1024 bit keys ?





ya..it is..but you have to create a library file to support big numbers.





Hye Lee
Your project is very interesting.
I have a questions. I wanted to ask you what is/are the limitation in your project.
I am waiting for your response,
Thanks
Garima Thakral





Hi Garima,
The only limitation to this current project is that the ephemeral and exchange keys are limited to the size of an __int64 (e.g. 64bit keys).
If you look under the post/reply to this article (below this one)  you should see Runtime Optimization  in my relpy you will see the best way to use DiffieHellman key exchange is by using the Microsoft Crypto API functions.
If you do want to use my code instead though, feel free to do so (like I said only limitiation is 64bit keys)
Best Regards,
Lee Griffiths





first of it's a very nice code you got there
I've noticed you have a define for maximum number, so i tried to change it to a larger number and the algorithm became extremely slow, so i haven't been able to get a 64 bit prime number, to be able to use in DES for example..
any suggestions???
H.A.





First off, the best way to use Diffie Hellman should be to use tried and tested API's
<a href="http://msdn.microsoft.com/library/default.asp?url=/library/enus/seccrypto/security/diffie_hellman_keys.asp" Using Microsoft Cryptographic Libraries for DiffieHellman[Microsoft Cryptographic Libraries for DiffieHellman]
You can use the Microsoft Cryptographic Service Providers to achieve a lot of your cryptography needs. Lookup CryptAcquireContext and go from there  everything from RSA to DES to DiffieHellman and so on. I've included an example here of how to generate a prime using the Microsoft CryptoAPI. At first glance the prime doesn't always seem to look prime because it's last byte is an even number. However please note that the BYTE array contains octects!
To use the GeneratePrime function shown below, include <wincrypt.h> and then something like:
BYTE *prime = NULL;<br />
DWORD primesize = 0;<br />
<br />
GeneratePrime(&prime, &primesize);
Here is the GeneratePrime function (using RSA).
BOOL GeneratePrime(BYTE **prime, DWORD *cbPrime) <br />
{<br />
HCRYPTPROV hCryptProv = NULL;<br />
HCRYPTHASH hHash = NULL;<br />
BOOL AcquiredContext = FALSE;<br />
BOOL AcquiredPrime = FALSE;<br />
<br />
if(CryptAcquireContext(&hCryptProv, NULL, NULL, PROV_RSA_FULL, 0)== FALSE)<br />
{<br />
if(GetLastError() == NTE_BAD_KEYSET)<br />
AcquiredContext = CryptAcquireContext(&hCryptProv, NULL, NULL, PROV_RSA_FULL, CRYPT_NEWKEYSET);<br />
}<br />
else<br />
AcquiredContext = TRUE;<br />
<br />
if (AcquiredContext==TRUE)<br />
{<br />
HCRYPTKEY hEncryptionKey = NULL;<br />
<br />
BOOL bGenEncKey = CryptGenKey(hCryptProv, CALG_RSA_KEYX, (512 << 16)  CRYPT_EXPORTABLE, &hEncryptionKey);<br />
if (bGenEncKey)<br />
{ <br />
DWORD KeyLength = 0;<br />
if (CryptExportKey(hEncryptionKey, NULL, PRIVATEKEYBLOB ,0, NULL, &KeyLength)==TRUE)<br />
{<br />
if (KeyLength>0)<br />
{<br />
BYTE *PrivateKeyBlob = (BYTE *) malloc((size_t)KeyLength);<br />
if (PrivateKeyBlob)<br />
{<br />
if (CryptExportKey(hEncryptionKey, NULL, PRIVATEKEYBLOB ,0, PrivateKeyBlob, &KeyLength)==TRUE)<br />
{<br />
PUBLICKEYSTRUC pks = {0};<br />
RSAPUBKEY rsapub = {0};<br />
<br />
memcpy(&pks, PrivateKeyBlob, sizeof(PUBLICKEYSTRUC));<br />
memcpy(&rsapub, PrivateKeyBlob + sizeof(PUBLICKEYSTRUC), sizeof(RSAPUBKEY));<br />
<br />
DWORD KeySizeBytes = rsapub.bitlen / 16;<br />
DWORD ModSizeBytes = rsapub.bitlen / 8;<br />
DWORD OffsetPrimeP = sizeof(PUBLICKEYSTRUC) + sizeof(RSAPUBKEY) + ModSizeBytes;<br />
DWORD OffsetPrimeQ = sizeof(PUBLICKEYSTRUC) + sizeof(RSAPUBKEY) + ModSizeBytes + KeySizeBytes;<br />
<br />
*prime = (BYTE *) malloc((size_t) KeySizeBytes);<br />
<br />
if (*prime)<br />
{<br />
*cbPrime = KeySizeBytes;<br />
memcpy(*prime, PrivateKeyBlob + OffsetPrimeP, KeySizeBytes);<br />
AcquiredPrime = TRUE; <br />
}<br />
}<br />
}<br />
}<br />
}<br />
<br />
CryptDestroyKey(hEncryptionKey);<br />
}<br />
<br />
CryptReleaseContext(hCryptProv, 0);<br />
}<br />
<br />
return AcquiredPrime;<br />
<br />
}
Anyway hope this helps. If you look at the CryptoAPI by Microsoft you will see that encrypting with DES is simple! It also supports many other encryption algorithms along with hashing Algorithms, PublicPrivate Key and Signature algorithms  it's a very powerful library indeed.
If you need any more help, let me know.
Best Regards,
Lee





I've tested your implementation of GeneratePrime function and I found out that it doesn't generate a 512bit key, it always returns 256bit key ! KeySizeBytes was always set to 32. If I change the code to:
...
//Generate a 512bit key
BOOL bGenEncKey = CryptGenKey(hCryptProv, CALG_RSA_KEYX, (1024 << 16)  CRYPT_EXPORTABLE, &hEncryptionKey);
...
it returns a 512bit key. Any idea why?
Thanks





how can i use that without need key exchange?





I'm not sure I quite understand your question?
The whole point of the article was to describe a way to EXCHANGE ENCRYPTION KEYS between two parties.
If you're just looking for encryption/decryption algorithms I'm sure there are plenty here on CodeProject!
All the best!





I want code C++ for encryption and dencrypttion please





The DiffieHellman algo implementation was quite useful.
I'm doing some study in IKE which uses the DiffieHellman algo. I'm looking for a sample implementation of IKE. Any help? or any links?
Thanks





Hi Adam,
I found the demo very interesting, but i want the code in C.
can the code be converted to C.
Regards,
Archana
hi, all





Hello,
Could you show me a library in which I can use the generated key for the actual encryption ?
Can you import it in Microsoft's CryptoAPI ? Or even OpenSSL ?
Recently, I've successfully established an encrypted channel between a Pocket PC and Windows XP using RSA PKI in CryptoAPI. However, DiffieHelman would be nicer as the encryption itself is faster than PKIalgorithms. Moreover, your code seems portable to nonWindows clients, apart from the use of __int64.





I would suggest not using Microsofts CryptoAPI (you never can trust them, their probably sending your generated keys to the NSA or something). I would suggest using blowfish, an encryption algorithm developed by Bruce Schneier. It's apparently very strong, and equally as good  its open source freeware (no royalties to pay for using it)!
You can probably find it here:
http://www.schneier.com/blowfishdownload.html
If you want to make it portable to a pocket pc you can change all instances of '__int64' to an 'unsigned long' if you wish, but you'll have to change the #defines for both of the maximum values to 2147483648.
Let me know how you get on.
Kind Regards,
Lee Griffiths





If I replace '__int64' with 'unsigned long', doesn't that reduce the key size, and therefore also the security level ?





Yes it sure will. It all depends on how secure you want your communication. It's probably best to get your hands on the GMP libraries (free on sourceforge.net). And you can then assign your integers to a massive number of digits.





Hello again,
I wrote a small sample app to use it with Bruce Schneier's Blowfish implementation.
It seems the resulting encryption key is not the same for the sender and the recipient.
(see the commented exit(1) statement below)
Could you tell me what I do wrong ?
#include "DrmCrypto.h"
#include "BLOWFISH.H"
struct KeyData
{
__int64 Generator;
__int64 Modulus;
__int64 InterKey;
__int64 EncryptionKey;
};
int _tmain(int argc, _TCHAR* argv[])
{
KeyData SenderKeyData, RecipientKeyData;
CDrmCrypto Sender, Recipient;
Sender.CreateKeys(SenderKeyData.Generator, SenderKeyData.Modulus);
Recipient.CreateKeys(RecipientKeyData.Generator, RecipientKeyData.Modulus);
Sender.CreateSenderInterKey(SenderKeyData.InterKey);
Recipient.CreateRecipientInterKey( RecipientKeyData.InterKey, RecipientKeyData.Generator,
RecipientKeyData.Modulus);
Sender.CreateSenderEncryptionKey( SenderKeyData.EncryptionKey, RecipientKeyData.InterKey);
Recipient.CreateRecipientEncryptionKey( RecipientKeyData.EncryptionKey, SenderKeyData.InterKey);
if (SenderKeyData.EncryptionKey != RecipientKeyData.EncryptionKey)
{
printf("Sender & Recipient encryption keys do not match.\n");
// exit(1);
}
if (InitializeBlowfish((char*)&SenderKeyData.EncryptionKey, (short)sizeof(SenderKeyData.EncryptionKey) ))
{
printf("InitializeBlowfish returned an error.\n");
exit(1);
}
unsigned long xl1 = 123456789;
unsigned long xr1 = 987654321;
unsigned long xl2 = xl1;
unsigned long xr2 = xr1;
Blowfish_encipher(&xl1,&xr1);
Blowfish_decipher(&xl1,&xr1);
if (xl1 != xl2  xr1 != xr2)
{
printf("Encryption failed.\n");
exit(1);
}
printf("Done.\n");
return 0;
}





You are using the class incorrectly. The sender is the only one that calls createkeys. The sender then creates the sender interim key. The reciever then uses the sender's interim, and public keys (ones generated from createkeys), to make his interim key. The reciever can now call CreateRecipientEncryptionKey, and the sender can call CreateSenderEncryptionKey (and pass the reciever's interim key). Both parties should now have the same encryption key.
Please read the example usage notes (i think they should make it clear what needs to be done).
Have fun.
Best Regards,
Lee Griffiths





Ever wanted to use encryption, but got stuck trying to get the encrption key to the recipient? A real chicken and egg problem. The solution is that you can use DiffieHellmanMerkle key exchange.
DiffieHellmanMerkle key exchange uses y = g^x (mod p) to create a onceoff shared secret encryption key of arbitrary length. This secret key can then be used to exchange data encrypted using the key. What makes DH key exchange fascinating is that is is not necessary to exchange secret information in order to 'share' the encryption key. Anyone evesdropping on the line cannot derive the key from the information exchanged during the negotiation process.
Security is based on the fact that it is mathematically infeasible to discover the key IF THE VALUES ARE LARGE ENOUGH. Adequate security typically utilizes keys in of 1000 bits. Security is enhanced due to the fact that a new, unique encryption key is generated each time.
The primary limitation is in performing largeinteger arithmetic. Fortunately there are public/free implementations available. Or you can read Knuth and write one yourself. Check out the BigDigits library at www.dimgt.com.au. Be aware that using this code it is possible to create arbitrarily long keys, which may have legal implications in your area. The key buffer is hardlimited to 51 4byte integers (51*4=204bytes=1632bits), and will crash if you use a larger key buffer, so be warned.
The Way It Works
DH key exchange uses the following pattern (Bob (recipient) and Alice (sender) exchange messages, with Eve evesdropping).
First, the key format. This consists of three components g, x, and p.
g is the GENERATOR. Use a small prime, say 2, 3, 5, 7 etc.
x is the EXPONENT. This is the key, a random number of, say, 1024 bits.
p is the MODULUS. This is a large prime, typically of the same order as x (1024 bits).
Use CryptoApi, .NET crypto or any other service provider to create secure values and primes of this magnitude. Do not use Rnd!
1. Bob chooses a private key. (g, x1, and p) (generates these values)
The magnitude of x1 is equivalent to the resulting encryption key size.
x1 must be kept secret.
2. Bob calculates a new x2 value using x2 = g ^ x1 (mod p)
Publishes this key (in a directory, web page, or email).
g, x2, p
[ EVE INTERCEPTS ]
3. Alice wants to send Bob secure data. She requires an encryption key.
She can establish a secret key with Bob, over a public medium, this way:
Alice chooses a new random x3 value of the same order of magnitude as x2.
She uses Bob's g and p values, leaving her with g, x3, p.
x3 must be kept secret.
4. Alice calculates a public key x4 = g ^ x3 (mod p).
She sends this to Bob over a public medium (g, x4, p). Bob aleady knows the values of g and p, since they are of his choosing. This can be a validation step  the values must match.
[ EVE INTERCEPTS ]
5. Alice generates the encryption key x5 = x2 ^ x3 (mod p)
Bob generates the encryption key x5 = x4 ^ x1 (mod p).
Alice and bob are now in possession of the same secret key. Eve intercepts all exchanges, but cannot (unless she is NSA!) derive the key value.
How does it work? To use an analogy from Simon Singh's The Code Book:
Say that keys are paint, in 3pint tins. Alice and Bob agree on a public key  one pint of yellow paint. They each pick a secret color. Bob mixes one pint of yellow with 1 pint of his secret color, and sends this to Alice.
Alice mixes one pint of yellow with her secret color, and sends this to Bob.
Then Alice adds a pint of her secret color to Bob's tin, and Bob adds a pint of his secret color to Alice's tin.
Both tins are now the same color.
Eve can intercept the tins as they pass to and fro, she can even know that the public key is a pint of yellow. But she cannot determine the secret color because mixing paint is a oneway function.
Test Case (ALL NUMBERS IN HEX)
Follow this test case to see it working  note that in the real world, keys of 1000 bits or more would be used.
The symbol => refers to the result of calculating g ^ x (mod p). g, p are carried over.
g=3 x1=9A2E p=10001 (Bob's private key, reused)<br />
=> g=3 x2=C366 p=10001 (Bob's public key, published, reused)<br />
<br />
g=3 x3=4C20 p=10001 (Alice's onceoff random key, using Bob's g, p)<br />
=> g=3 x4=6246 p=10001 (Alice publishes this to Bob)<br />
<br />
g=C366 x1=4C20 p=10001 (Alice calculates x2 ^ x3 (mod p))<br />
=> x5=DED4<br />
<br />
g=6246 x4=9A2E p=10001 (Bob calculates x4 ^ x1 (mod p))<br />
=> x5=DED4
Alice and Bob have both arrived at the same value: DED4. This is the key! Alice encrypts the data using this secret key, and sends the encrypted data to Bob, along with the public value x4, which Bob uses to calculate the key. Eve cannot take advantage of this information due to the oneway function g^x(mod p).
Web Services Secure Exchange Example
The following schematic shows how DHM key exchange could be used in a distributed environment, say between Web Services.
Note: x is the private key on each side. This is not exchanged. p is the public key which is exchanged.
ALICE (CLIENT) BOB (SERVER)
 
b = acquire_dh_key(bob) > get_dh_public( return p; )
x = make_dh_private(b)
p = make_dh_public(b,x)
c = make_dh_crypt(b,x)
encrypt_data(c)
send_data_and_key(bob,p,data) > c = make_dh_crypt(p,x)
decrypt_data(c)
In this way, one request is made to determine the server's public key, another is made to transfer the encrypted data along with the public info required for the server to calculate the encryption key. If the server's public key is known/cached, then the data can be sent using a single methodcall each time. It is up to Alice to select a random message key (x) each time data is sent.
Since encryption keys are not reused, security is enhanced.
Implementations
Be aware that you will likely write flaws into your encryption code if you implement it all yourself. If possible, use a service from an established CSP (Crypto Service Provider). The CryptoAPI on Win32 supports creating and calculating DH keys, so take a look around and see what use you can make of existing code, even if it's just the secure random number generators. .NET supports various key exchange mechanisms, but they rely on asymetric algorithms such as RSA to keep the actual key safe whilst in transit.
DH key exchange never transmits the actual encryption key. This is what makes it useful, and well suited to a distributed environment. Use a proper encryption algorithm like Rijndael (AES), but derive and transmit the key using DH mechanisms.
One possible implementation is to use a simple XML framework to transmit the data/key package, possibly similar to the following:
<br />
<?xml version="1.0" ?><br />
<encryptedpackage><br />
<key>NK89FNOF8LNKASDFW0934LNF09VNOECR3M089DFCMFHE7823JRS==</key><br />
<data>FGKMSDFVCFN879EC89SDFASDFUHNOGS7834RMIUSHDF78T3BOLKJ34289=</data><br />
</encryptedpackage><br />
Key and Data values are base64 encoded for embedding in an XML document.
Happy coding.
Caractacus
www.caradoc.co.za
Ah, yes, I thought so.





Thanks for the post. CP could do with a bit more in the way of crypto articles.
I've not looked at your source code, but I'm curious what if anything you do to mitigate small subgroup attacks? In particular, do you generate safe primes (prime p such that (p1)/2 is also prime) or small subgroups (p1) = kq for some integer k and prime q of length 256 bits? Absent such protections, DH key exchange can be attacked fairly easily by sending a g^x which is a member of a small subgroup.
See Practical Cryptography by Schneier and Ferguson for details.
Adam





Thanks for the reply. You are correct in noting that the diffiehellman key exchange can suffer from certain types of attacks if the numbers or chosen badly. In the code I have posted it only generates 64bit primes, and keys. Obviously this is not of a sufficient size for good security, but it serves as a demonstration of how it could be employed, and besides for general applications; for instance if someone wrote their own messenger this level of security would be adequate. I have not as yet generated "safe" primes, but it would be very simple to test this, and generate ones that are safe  I will modify my code shortly to encompass that (cheers ). There is further trouble with diffiehellman key exchange in that it suffers the maninthemiddle attack which can only be resolved by use of digital signatures for authenticity checking.
I am looking into generating larger primes, and keys (i.e. 256bit, and so on), but since the largest type you can have in c in __int64, I'd have to write my own number representation and calculation functions, do you have any suggestions of how this can be done?
When I find a way of representing and calculating in larger values, I will repost the code, and also add the "safe" prime checking, that way it should be of a good security level. I've read that 128bit keys can be broken fairly trivially  do you know what of 256bit or 512bit (ignoring export controls for now!)?
Anyway, thanks for the post (it's nice to see some people on CP that take an interest in Cryptography).
Best Regards,
Lee Griffiths







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