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(AutoIt) 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. $oDhBob = ObjCreate("Chilkat_9_5_0.Dh") $oDhAlice = ObjCreate("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 } $oDhBob.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). Local $sP Local $iG ; Bob will now send P and G to Alice. $sP = $oDhBob.P $iG = $oDhBob.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. Local $bSuccess = $oDhAlice.SetPG($sP,$iG) If ($bSuccess <> True) Then ConsoleWrite("P is not a safe prime" & @CRLF) Exit EndIf ; 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). Local $sEBob $sEBob = $oDhBob.CreateE(256) ; Alice does the same: Local $sEAlice $sEAlice = $oDhAlice.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. Local $sKBob Local $sKAlice ; Bob computes the shared secret from Alice's "E": $sKBob = $oDhBob.FindK($sEAlice) ; Alice computes the shared secret from Bob's "E": $sKAlice = $oDhAlice.FindK($sEBob) ; Amazingly, kBob and kAlice are identical and the expected ; length (260 characters). The strings contain the hex encoded bytes of ; our shared secret: ConsoleWrite("Bob's shared secret:" & @CRLF) ConsoleWrite($sKBob & @CRLF) ConsoleWrite("Alice's shared secret (should be equal to Bob's)" & @CRLF) ConsoleWrite($sKAlice & @CRLF) ; 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: $oCrypt = ObjCreate("Chilkat_9_5_0.Crypt2") $oCrypt.EncodingMode = "hex" $oCrypt.HashAlgorithm = "md5" Local $sessionKey $sessionKey = $oCrypt.HashStringENC($sKBob) ConsoleWrite("128-bit Session Key:" & @CRLF) ConsoleWrite($sessionKey & @CRLF) ; Encrypt something... $oCrypt.CryptAlgorithm = "aes" $oCrypt.KeyLength = 128 $oCrypt.CipherMode = "cbc" ; Use an IV that is the MD5 hash of the session key... Local $sIv $sIv = $oCrypt.HashStringENC($sessionKey) ; AES uses a 16-byte IV: ConsoleWrite("Initialization Vector:" & @CRLF) ConsoleWrite($sIv & @CRLF) $oCrypt.SetEncodedKey $sessionKey,"hex" $oCrypt.SetEncodedIV $sIv,"hex" ; Encrypt some text: Local $sCipherText64 $oCrypt.EncodingMode = "base64" $sCipherText64 = $oCrypt.EncryptStringENC("The quick brown fox jumps over the lazy dog") ConsoleWrite($sCipherText64 & @CRLF) Local $sPlainText $sPlainText = $oCrypt.DecryptStringENC($sCipherText64) ConsoleWrite($sPlainText & @CRLF) |
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