Lua

Tips on Matching Encryption with another System

See more Encryption Examples

This example provides tips on matching encryption results produced by another system. Lua

local chilkat = require("chilkat")

-- This example assumes the Chilkat API to have been previously unlocked.
-- See Global Unlock Sample for sample code.

local crypt = chilkat.Crypt2.new()

-- Let's examine 256-bit AES encryption in CBC mode.
-- CBC mode is Cipher Block Chaining, and it uses an IV (initialization vector)
crypt.crypt_algorithm = "aes"
crypt.cipher_mode = "cbc"
crypt.key_length = 256
crypt.padding_scheme = 0
local ivHex1 = "000102030405060708090A0B0C0D0E0F"
local ivHex2 = "FF0102030405060708090A0B0C0D0E0F"
crypt:set_encoded_iv(ivHex1, "hex")
local keyHex = "000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F"
crypt:set_encoded_key(keyHex, "hex")

-- Matching encryption requires all of the above settings to be matched exactly.
-- Let's get our output in hex format so we can easily see the values of the encrypted bytes.
crypt.encoding_mode = "hex"

-- Encrypt something small:
print(crypt:encrypt_string_enc("Hello"))
-- The result is 5B827AB3B4F9F2292C2B74C8A6C99A3D
-- This 16 bytes -- exactly one AES encryption block.

-- Let's change only the padding scheme.
crypt.padding_scheme = 3

-- Encrypt again:
print(crypt:encrypt_string_enc("Hello"))
-- The result is entirely different: 469C28CC576069F807891FEE2DE76D68

-- The padding scheme only affects the very last block of output.  Therefore,
-- if all settings match except for the padding scheme, we're unable to
-- know if we encrypt a very small amount of data. However, if we encrypt
-- a larger amount of data, the single difference becomes apparent:
print("-- Only the padding scheme differs --")
crypt.padding_scheme = 0
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))
crypt.padding_scheme = 3
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))

-- Now examine the outputs:
-- F6A201F8E0B6595FA20E4A212A2AD9A5046DAF29E8B35AD15CEE56A1A69F2A3A7B347A7C15E26E7A6760533C7A8E0D44
-- F6A201F8E0B6595FA20E4A212A2AD9A5046DAF29E8B35AD15CEE56A1A69F2A3A292CA61D03A85E1AC39B50D4DA71691E
-- We can see the output matches except for the last block, which is affected by the padding scheme.

-- If we are able to easily use ECB mode w/ the other system
-- we are trying to match, then eliminate the IV from the picture.
-- If the encryption matches in ECB mode, but not in CBC mode,
-- then we know all correct except for the IV.
-- For example, you can see how the IV changes everything with CBC mode,
-- but it's not used in ECB mode:
crypt.padding_scheme = 0
crypt.cipher_mode = "cbc"
print("-- Only the IV differs, CBC mode produces different output. --")
crypt:set_encoded_iv(ivHex1, "hex")
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))
crypt:set_encoded_iv(ivHex2, "hex")
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))

crypt.cipher_mode = "ecb"
print("-- Only the IV differs, ECB does not use the IV.  The outputs are the same. --")
crypt:set_encoded_iv(ivHex1, "hex")
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))
crypt:set_encoded_iv(ivHex2, "hex")
print(crypt:encrypt_string_enc("HelloHelloHelloHelloHelloHelloHello"))

-- If we can eliminate the padding scheme and IV from the degrees of freedom,
-- then the only remaining likely differences are (1) the secret key,
-- and (2) the input data itself.

-- The secret key is composed of binary bytes of exactly KeyLength bits.
-- For 256-bit AES encrytion, the key length is 256, and therefore the
-- secret key is exactly 32 bytes.  (32 * 8 bits/byte = 256 bits)
-- If the secret key is derived from an arbitrary password string, then one must
-- exactly duplicate the derivation scheme (such as PBKDF2, for example)
-- The input bytes to the derivation scheme must also match.  For example,
-- is it the utf-8 byte representation of the password string that is used
-- as the starting point for the derivation, or perhaps utf-16, or ANSI (1 byte per char)?

-- Likewise, if the data being encrypted is a string, what byte representation of
-- the string is being encrypted?  If the bytes presented to the encryptor are different,
-- then the output is different.