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(VBScript) 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.
Dim fso, outFile Set fso = CreateObject("Scripting.FileSystemObject") Set outFile = fso.CreateTextFile("output.txt", True) ' 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. set dhBob = CreateObject("Chilkat_9_5_0.Dh") set dhAlice = CreateObject("Chilkat_9_5_0.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). ' 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 0 if not. success = dhAlice.SetPG(p,g) If (success <> 1) Then outFile.WriteLine("P is not a safe prime") WScript.Quit End If ' 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). eBob = dhBob.CreateE(256) ' Alice does the same: 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. ' 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: outFile.WriteLine("Bob's shared secret:") outFile.WriteLine(kBob) outFile.WriteLine("Alice's shared secret (should be equal to Bob's)") outFile.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: set crypt = CreateObject("Chilkat_9_5_0.Crypt2") crypt.EncodingMode = "hex" crypt.HashAlgorithm = "md5" sessionKey = crypt.HashStringENC(kBob) outFile.WriteLine("128-bit Session Key:") outFile.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... iv = crypt.HashStringENC(sessionKey) ' AES uses a 16-byte IV: outFile.WriteLine("Initialization Vector:") outFile.WriteLine(iv) crypt.SetEncodedKey sessionKey,"hex" crypt.SetEncodedIV iv,"hex" ' Encrypt some text: crypt.EncodingMode = "base64" cipherText64 = crypt.EncryptStringENC("The quick brown fox jumps over the lazy dog") outFile.WriteLine(cipherText64) plainText = crypt.DecryptStringENC(cipherText64) outFile.WriteLine(plainText) outFile.Close |
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