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| I have been interested in encryption for a long time, however it has been hard to distribute encryption utilities because of the laws passed by various governments to restrict their export.
To work around this, I will describe here a technique for making an encryption library for MUSHclient, to be used in Lua scripting.
The trick is that you need to obtain the actual source code for the AES encryption yourself, providing you are not breaking the laws of the land (whatever that country is) to do so.
You place those files into a directory, along with the "aeslib" Lua library, whose source is given below.
AES is based on the "Rijndael" algorithm by Joan Daemen and Vincent Rijmen.
Once assembled you should have the following files:
aes.c aeslib.c lauxlib.h lua.lib sha256.c
aes.h aeslib.dll lua.h lualib.lib sha256.h
Obtain the files from the following places (or others of your choosing):
- aes.c and aes.h
http://www.cr0.net:8040/code/crypto/aes/
- sha256.c and sha256.h
http://www.cr0.net:8040/code/crypto/sha256/
- lua.h, lauxlib.h, lua.lib and lualib.lib
http://www.gammon.com.au/files/mushclient/lua.zip
You could also simply download Lua from the Lua site www.lua.org and compile it yourself to get those files.
The files lua.h and luaxlib.h are part of the Lua source, once compiled they become the Lua libraries lua.lib and lualib.lib. That was with Visual C++ 6, if you compiled them under Cygwin you would get liblua.a and liblualib.a.
- aeslib.c
This is given below, and will also be available from:
http://www.gammon.com.au/files/utils/aeslib.c
- aeslib.dll
This is the result of the compilation process below.
A simple way of compiling it to install the free Cygwin compiler for Windows. Here are some guidelines for doing that:
http://www.gammon.com.au/smaug/installingcygwin.htm
Once you have got your files assembled, compile them to produce the DLL as follows:
gcc -shared -o aeslib.dll aes.c sha256.c aeslib.c lualib.lib lua.lib
You now have a aeslib.dll file which you can place into the same directory as your other Lua DLLs.
Source for aeslib.c ...
/*
Lua library for AES encryption.
You need to obtain various files yourself to compile it. See below for details.
Author of this file: Nick Gammon
Date: 10th December 2004
For more information, see:
http://www.gammon.com.au/forum/bbshowpost.php?bbsubject_id=4988
(C) Copyright Nick Gammon 2004. Permission to copy, use, modify, sell and
distribute this software is granted provided this copyright notice appears
in all copies. This software is provided "as is" without express or implied
warranty, and with no claim as to its suitability for any purpose.
The AES implementations used here were written by Christophe Devine.
*/
#define LUA_API __declspec(dllexport)
#include "lua.h"
#include "lauxlib.h"
#include "aes.h"
#include "sha256.h"
#include <malloc.h>
#include <memory.h>
#include <stdlib.h>
#pragma comment( lib, "lua.lib" )
#pragma comment( lib, "lualib.lib" )
/* Uncomment this to provide a "test" function */
/* #define TEST */
/*
For more information about the AES encryption, see:
http://www.cr0.net:8040/code/crypto/
Source needed in addition to this file is:
aes.c
aes.h
For those two files, see: http://www.cr0.net:8040/code/crypto/aes/
sha256.c
sha256.h
For those two files, see: http://www.cr0.net:8040/code/crypto/sha256/
lua.h
lauxlib.h
For those two files, see: http://www.lua.org/download.html
To link you also need:
lua.lib
lualib.lib
To compile under Cygwin:
gcc -shared -o aeslib.dll aes.c sha256.c aeslib.c lualib.lib lua.lib
*/
/*
Usage: encrypted_text = encrypt (plaintext, key)
*/
static int encrypt (lua_State *L)
{
aes_context aes_ctx;
sha256_context sha_ctx;
unsigned char digest [32];
unsigned char IV [16]; /* for cipher block chaining */
unsigned char * buf;
size_t offset;
int i,
lastn;
/* get text to encrypt */
size_t textLength;
const unsigned char * text = luaL_checklstring (L, 1, &textLength);
/* get key */
size_t keyLength;
const unsigned char * key = luaL_checklstring (L, 2, &keyLength);
/* allocate memory to work in, rounded up to next 16 byte boundary
plus 16 bytes for the IV */
buf = (unsigned char *) malloc (textLength + 15 + 16);
if (!buf)
luaL_error (L, "not enough memory for encryption");
/* generate random IV */
for (i = 0; i < 16; i++)
IV [i] = rand () & 0xFF;
/* calculate how many bytes of final block are real data */
lastn = textLength & 0x0F;
/* use last 4 bits of IV to store length of final block */
IV [15] &= 0xF0;
IV [15] |= lastn; /* number of bytes in final block */
/* hash supplied key,and IV, to give a key for encryption */
sha256_starts (&sha_ctx);
sha256_update (&sha_ctx, IV, 16);
sha256_update (&sha_ctx, (uint8 *) key, keyLength);
sha256_finish (&sha_ctx, digest);
/* use hashed supplied key (digest) for encryption */
aes_set_key( &aes_ctx, digest, 256);
/* copy initialization vector into output buffer */
memcpy (buf, IV, 16);
/* make sure all zero, in case not mod 16 bytes */
memset (&buf [16], 0, textLength + 15);
/* copy supplied text into it */
memcpy (&buf [16], text, textLength);
/* encrypt in blocks of 16 bytes (128 bits) */
for (offset = 16; offset < (textLength + 16); offset += 16)
{
/* xor in the IV for this block */
for (i = 0; i < 16; i++)
buf [i + offset] ^= IV [i];
aes_encrypt (&aes_ctx, &buf [offset], &buf [offset]);
memcpy (IV, &buf [offset], 16);
}
/* push results */
lua_pushlstring (L, buf, offset);
free (buf); /* don't need buffer any more */
return 1; /* number of result fields */
} /* end of encrypt */
/*
Usage: plaintext = decrypt (encrypted_text, key)
*/
static int decrypt (lua_State *L)
{
aes_context aes_ctx;
sha256_context sha_ctx;
unsigned char digest [32];
unsigned char IV [16]; /* for cipher block chaining */
unsigned char tmp [16];
unsigned char * buf;
size_t offset;
int i,
lastn;
const unsigned char * text ;
size_t textLength;
const unsigned char * key;
size_t keyLength;
/* get text to decrypt */
text = luaL_checklstring (L, 1, &textLength);
if (textLength < 16)
luaL_error (L, "encrypted data too short, must be at least 16 bytes");
/* encrypted block starts with 16-byte initialization vector */
memcpy (IV, text, 16);
textLength -= 16;
/* find how many bytes in final block */
lastn = IV [15] & 0x0F;
/* get key */
key = luaL_checklstring (L, 2, &keyLength);
/* hash supplied key ,and IV, to give a key for decryption */
sha256_starts (&sha_ctx);
sha256_update (&sha_ctx, IV, 16 );
sha256_update (&sha_ctx, (uint8 *) key, keyLength);
sha256_finish (&sha_ctx, digest);
/* use hashed supplied key (digest) for decryption */
aes_set_key( &aes_ctx, digest, 256);
buf = (unsigned char *) malloc (textLength + 15);
if (!buf)
luaL_error (L, "not enough memory for decryption");
/* make sure all zero, in case not mod 16 bytes */
memset (buf, 0, textLength + 15);
/* copy supplied text into it, skipping IV */
memcpy (buf, &text [16], textLength);
/* decrypt in blocks of 16 bytes (128 bits) */
for (offset = 0; offset < textLength; offset += 16)
{
memcpy (tmp, &buf [offset], 16); /* this will be the IV next time */
aes_decrypt (&aes_ctx, &buf [offset], &buf [offset]);
for ( i = 0; i < 16; i++ ) /* xor in current IV */
buf [offset + i] ^= IV [i];
memcpy (IV, tmp, 16); /* new IV */
}
/* trim final length */
if (lastn)
offset -= 16 - lastn;
/* push results */
lua_pushlstring (L, buf, offset);
free (buf); /* don't need buffer any more */
return 1; /* number of result fields */
} /* end of decrypt */
#ifdef TEST
static int test (lua_State *L)
{
aes_context aes_ctx;
sha256_context sha_ctx;
unsigned char digest[32];
unsigned char buf [16];
unsigned char result1 [16];
unsigned char result2 [16];
memset (digest, 0, sizeof (digest));
memset (buf, 0, sizeof (buf));
aes_set_key( &aes_ctx, (uint8 *) digest, 256);
aes_encrypt (&aes_ctx, buf, result1);
aes_encrypt (&aes_ctx, result1, result2);
lua_pushlstring (L, result1, sizeof (result1));
lua_pushlstring (L, result2, sizeof (result2));
/*
Results should be:
DC95C078A2408989AD48A21492842087
08C374848C228233C2B34F332BD2E9D3
See "The Design of Rijndael" by Joan Daemen and Vincent Rijmen.
Test vector results for block length 128, key length 256.
*/
return 2; /* number of result fields */
} /* end of test */
#endif
/* table of operations */
static const struct luaL_reg aeslib [] =
{
{"encrypt", encrypt},
{"decrypt", decrypt},
#ifdef TEST
{"test", test},
#endif
{NULL, NULL}
};
/* register library */
LUALIB_API int luaopen_aes(lua_State *L)
{
luaL_openlib(L, "aes", aeslib, 0);
return 1;
}
We are now ready to start encrypting.
In your script (or command window with scripting prefix) we need to load the DLL (once per session) like this:
assert (loadlib ("aeslib.dll", "luaopen_aes")) ()
The loadlib function returns a function (if possible) from the DLL (the luaopen_aes function) and the final parentheses on that line run that function. This installs two functions:
aes.encrypt
aes.decrypt
In Lua 5.1 (MUSHclient 3.80 onwards) you need to use package.loadlib, instead of loadlib on its own. Also, you need to make sure that the loadlib function has not been removed in the Lua sandbox.
The encrypt and decrypt functions need to use 256-bit keys, so they take the supplied key (the second argument) and use the sha256 function (Secure Hash Algorithm, 256 bit version) to turn whatever string you supply into a 256-bit key.
They then take the text you supply to be encrypted or decrypted, and make a temporary buffer, rounded up in size to the next 16-byte boundary, as the encryption works in blocks of 128 bits (16 times 8).
The functions then iterate over the text, encrypting each 16-byte block, and then returning the result.
An example of use:
x = aes.encrypt ("Nick Gammon", "swordfish")
y = aes.decrypt (x, "swordfish")
print (y) --> Nick Gammon
Note that since the encryption works in 16-byte blocks, the result will always be a multiple of 16, even if the text isn't. In addition, the encrypted text will be 16 bytes larger, because the Initialization Vector (IV) is stored at the start of it.
The last 4 bits of the initialization vector are used to remember how many bytes in the last block are "real", so that decryption will return the correct length string.
Encryption mode
For added security the encryption is done in cipher-block chaining mode. What this means is that a random 16-byte string is generated at the commencement of encryption. This string is stored at the start of the encrypted block, so it can be used for decryption.
The important part about the random string (the so-called Initialization Vector or IV) is that it is XOR'ed with each block prior to encrypting it. This guarantees that each block will encrypt differently, even if you happen to have a sequence of letters (like "aaaaaaaaaa") in your plain text. Without the cipher-block chaining, it is also possible for someone to reassemble a message by cutting and pasting (in 16-byte blocks) parts from different messages encrypted with the same key.
Disclaimer
Security and encryption are big fields. The methods described above are intended to get you started, if you want to use encryption with MUSHclient (or Lua). You should make enquiries about other aspects of security before relying upon a single encryption algorithm to protect your data or privacy.
Also, the source file given above has not been subject to peer review, there may be errors in the implementation that are not obvious. If you find any please let me know.
|
- Nick Gammon
www.gammon.com.au, www.mushclient.com | | Top |
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