Lua

Diffie-Hellman Key Exchange (DH)

See more Diffie-Hellman Examples

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.

Lua

local chilkat = require("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.
local dhBob = chilkat.Dh.new()
local dhAlice = chilkat.Dh.new()

-- 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:use_known_prime(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.
local p = dhBob.p
local 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.
local success = dhAlice:set_pg(p, g)
if not success then
    print("P is not a safe prime")
    return
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).
local eBob = dhBob:create_e(256)

-- Alice does the same:
local eAlice = dhAlice:create_e(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":
local kBob = dhBob:find_k(eAlice)

-- Alice computes the shared secret from Bob's "E":
local kAlice = dhAlice:find_k(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:
local crypt = chilkat.Crypt2.new()

crypt.encoding_mode = "hex"
crypt.hash_algorithm = "md5"

local sessionKey = crypt:hash_string_enc(kBob)

print("128-bit Session Key:")
print(sessionKey)

-- Encrypt something...
crypt.crypt_algorithm = "aes"
crypt.key_length = 128
crypt.cipher_mode = "cbc"

-- Use an IV that is the MD5 hash of the session key...
local iv = crypt:hash_string_enc(sessionKey)

-- AES uses a 16-byte IV:
print("Initialization Vector:")
print(iv)

crypt:set_encoded_key(sessionKey, "hex")
crypt:set_encoded_iv(iv, "hex")

-- Encrypt some text:

crypt.encoding_mode = "base64"
local cipherText64 = crypt:encrypt_string_enc("The quick brown fox jumps over the lazy dog")
print(cipherText64)

local plainText = crypt:decrypt_string_enc(cipherText64)

print(plainText)