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.

Downloads for Windows/Linux and Install Instructions

require 'rubygems'
require 'chilkat'

#  Create two separate instances of the DH object.
dhBob = Chilkat::CkDh.new()
dhAlice = Chilkat::CkDh.new()

#  Unlock the component once at program startup...
success = dhBob.UnlockComponent("Anything for 30-day trial")
if (success != true)
    print dhBob.lastErrorText() + "\n"
    exit
end

#  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" + "\n"
    exit
end

#  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:" + "\n";
print kBob + "\n";
print "Alice's shared secret (should be equal to Bob's)" + "\n";
print kAlice + "\n";

#  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.

#  Here's how you would use Chilkat Crypt (a separate Chilkat component) to
#  produce the session key:
crypt = Chilkat::CkCrypt2.new()
success = crypt.UnlockComponent("Anything for 30-day trial.")
if (success != true)
    print crypt.lastErrorText() + "\n"
    exit
end

crypt.put_EncodingMode("hex")
crypt.put_HashAlgorithm("md5")

sessionKey = crypt.hashStringENC(kBob)

print "128-bit Session Key:" + "\n";
print sessionKey + "\n";

#  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:" + "\n";
print iv + "\n";

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 + "\n";

plainText = crypt.decryptStringENC(cipherText64)

print plainText + "\n";