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(CkPython) 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.
import sys import chilkat # 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. dhBob = chilkat.CkDh() dhAlice = chilkat.CkDh() # 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.get_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. success = dhAlice.SetPG(p,g) if (success != True): print("P is not a safe prime") sys.exit() # 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: print("Bob's shared secret:") print(kBob) print("Alice's shared secret (should be equal to Bob's)") print(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: crypt = chilkat.CkCrypt2() crypt.put_EncodingMode("hex") crypt.put_HashAlgorithm("md5") sessionKey = crypt.hashStringENC(kBob) print("128-bit Session Key:") print(sessionKey) # Encrypt something... crypt.put_CryptAlgorithm("aes") crypt.put_KeyLength(128) crypt.put_CipherMode("cbc") # Use an IV that is the MD5 hash of the session key... iv = crypt.hashStringENC(sessionKey) # AES uses a 16-byte IV: print("Initialization Vector:") print(iv) crypt.SetEncodedKey(sessionKey,"hex") crypt.SetEncodedIV(iv,"hex") # Encrypt some text: crypt.put_EncodingMode("base64") cipherText64 = crypt.encryptStringENC("The quick brown fox jumps over the lazy dog") print(cipherText64) plainText = crypt.decryptStringENC(cipherText64) print(plainText) |
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