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(Mono C#) Diffie-Hellman Key Exchange (DH)Diffie-Hellman key exchange (DH) is a cryptographic protocol that allows two parties that have no prior knowledge of each other to jointly establish a shared secret key. This example demonstrates how two parties (Alice and Bob) can compute an N-bit shared secret key without the key ever being transmitted.
// This example requires the Chilkat API to have been previously unlocked. // See Global Unlock Sample for sample code. // Create two separate instances of the DH object. Chilkat.Dh dhBob = new Chilkat.Dh(); Chilkat.Dh dhAlice = new Chilkat.Dh(); // The DH algorithm begins with a large prime, P, and a generator, G. // These don't have to be secret, and they may be transmitted over an insecure channel. // The generator is a small integer and typically has the value 2 or 5. // The Chilkat DH component provides the ability to use known // "safe" primes, as well as a method to generate new safe primes. // This example will use a known safe prime. Generating // new safe primes is a time-consuming CPU intensive task // and is normally done offline. // Bob will choose to use the 2nd of our 8 pre-chosen safe primes. // It is the Prime for the 2nd Oakley Group (RFC 2409) -- // 1024-bit MODP Group. Generator is 2. // The prime is: 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 } dhBob.UseKnownPrime(2); // The computed shared secret will be equal to the size of the prime (in bits). // In this case the prime is 1024 bits, so the shared secret will be 128 bytes (128 * 8 = 1024). // However, the result is returned as an SSH1-encoded bignum in hex string format. // The SSH1-encoding prepends a 2-byte count, so the result is going to be 2 bytes // longer: 130 bytes. This results in a hex string that is 260 characters long (two chars // per byte for the hex encoding). string p; int g; // Bob will now send P and G to Alice. p = dhBob.P; g = dhBob.G; // Alice calls SetPG to set P and G. SetPG checks // the values to make sure it's a safe prime and will // return false if not. bool success = dhAlice.SetPG(p,g); if (success != true) { Debug.WriteLine("P is not a safe prime"); return; } // Each side begins by generating an "E" // value. The CreateE method has one argument: numBits. // It should be set to twice the size of the number of bits // in the session key. // Let's say we want to generate a 128-bit session key // for AES encryption. The shared secret generated by the Diffie-Hellman // algorithm will be longer, so we'll hash the result to arrive at the // desired session key length. However, the length of the session // key we'll utlimately produce determines the value that should be // passed to the CreateE method. // In this case, we'll be creating a 128-bit session key, so pass 256 to CreateE. // This setting is for security purposes only -- the value // passed to CreateE does not change the length of the shared secret // that is produced by Diffie-Hellman. // Also, there is no need to pass in a value larger // than 2 times the expected session key length. It suffices to // pass exactly 2 times the session key length. // Bob generates a random E (which has the mathematical // properties required for DH). string eBob; eBob = dhBob.CreateE(256); // Alice does the same: string eAlice; eAlice = dhAlice.CreateE(256); // The "E" values are sent over the insecure channel. // Bob sends his "E" to Alice, and Alice sends her "E" to Bob. // Each side computes the shared secret by calling FindK. // "K" is the shared-secret. string kBob; string kAlice; // Bob computes the shared secret from Alice's "E": kBob = dhBob.FindK(eAlice); // Alice computes the shared secret from Bob's "E": kAlice = dhAlice.FindK(eBob); // Amazingly, kBob and kAlice are identical and the expected // length (260 characters). The strings contain the hex encoded bytes of // our shared secret: Debug.WriteLine("Bob's shared secret:"); Debug.WriteLine(kBob); Debug.WriteLine("Alice's shared secret (should be equal to Bob's)"); Debug.WriteLine(kAlice); // To arrive at a 128-bit session key for AES encryption, Bob and Alice should // both transform the raw shared secret using a hash algorithm that produces // the size of session key desired. MD5 produces a 16-byte (128-bit) result, so // this is a good choice for 128-bit AES. // To produce the session key: Chilkat.Crypt2 crypt = new Chilkat.Crypt2(); crypt.EncodingMode = "hex"; crypt.HashAlgorithm = "md5"; string sessionKey; sessionKey = crypt.HashStringENC(kBob); Debug.WriteLine("128-bit Session Key:"); Debug.WriteLine(sessionKey); // Encrypt something... crypt.CryptAlgorithm = "aes"; crypt.KeyLength = 128; crypt.CipherMode = "cbc"; // Use an IV that is the MD5 hash of the session key... string iv; iv = crypt.HashStringENC(sessionKey); // AES uses a 16-byte IV: Debug.WriteLine("Initialization Vector:"); Debug.WriteLine(iv); crypt.SetEncodedKey(sessionKey,"hex"); crypt.SetEncodedIV(iv,"hex"); // Encrypt some text: string cipherText64; crypt.EncodingMode = "base64"; cipherText64 = crypt.EncryptStringENC("The quick brown fox jumps over the lazy dog"); Debug.WriteLine(cipherText64); string plainText; plainText = crypt.DecryptStringENC(cipherText64); Debug.WriteLine(plainText); |
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