Simple Encryption

Here's a function to pass a secret message from one object to another. Using XOR, this is perhaps the weakest form of encryption. I have taken steps to make it harder to break using random nonce values with the password. If someone is able to decrypt one message, it will be easy for them to decrypt any other message. I've also added version support so that you can upgrade the script later and still have compatibility with older scripts using the same protocol.

Encryptor

<lsl> string ProtocolSignature = "ENC"; // your own signature float ProtocolVersion = 0.3; // can range from 0.0 to 255.255 string Password = "P@ssw0rd"; // change this to your own password integer communicationsChannel = PUBLIC_CHANNEL;

string hex(integer value) {

   string hex = "0123456789ABCDEF";
   integer digit = value & 0xF;
   string text = llGetSubString(hex, digit, digit);
   value = (value >> 4) & 0xfffFFFF;
   integer odd = TRUE;
   while(value)
   {
       digit = value & 0xF;
       text = llGetSubString(hex, digit, digit) + text;
       odd = !odd;
       value = value >> 4;
   } 
   if(odd)
       text = "0" + text;
   return text;

} string encrypt(string password, string message) {

   // get a random value
   integer nonce = (integer)llFrand(0x7FFFFFFF);

   // generate digest and prepend it to message
   message = llMD5String(message, nonce) + message;
   
   // generate one time pad
   string oneTimePad = llMD5String(password, nonce);
   
   // append pad until length matches or exceeds message
   integer count = (llStringLength(message) - 1) / 32;
   if(count)
       do
           oneTimePad += llMD5String(oneTimePad, nonce);
       while(--count);
   
   // encrypt message
   message = llXorBase64StringsCorrect(llStringToBase64(message), llStringToBase64(oneTimePad));
   
   // build header
   string header = ProtocolSignature;
   
   // append major version
   list versions = llParseString2List((string)ProtocolVersion, ["."], []);
   header += hex(llList2Integer(versions, 0));
   
   // append minor version
   string minor = llList2String(versions, 1);
   while(llGetSubString(minor, -1, -1) == "0")
       minor = llDeleteSubString(minor, -1, -1);
   header += hex((integer)minor);
   
   // append nonce to header
   header += llGetSubString("00000000" + hex(nonce), -8, -1);
   
   // return the header and encrypted message
   return header + message;

} default {

   state_entry()
   {
       llSay(communicationsChannel, encrypt(Password, "Hello, Avatar!"));
       llSay(communicationsChannel, encrypt(Password, "This is a very long text that I hope to be able to create a long one time pad to decrypt for it."));
   }

   touch_start(integer total_number)
   {
       llSay(communicationsChannel, encrypt(Password, "Touched."));
   }

} </lsl>

Decryptor

<lsl> string ProtocolSignature = "ENC"; // your own signature float ProtocolVersion = 0.3; // can range from 0.0 to 255.255 string Password = "P@ssw0rd"; // change this to your own password integer communicationsChannel = PUBLIC_CHANNEL; integer Debug = TRUE; // Set this to false for production integer listener;

init() {

   if(listener != 0)
   {
       llListenRemove(listener);
       listener = 0;
   }
   listener = llListen(communicationsChannel, "", NULL_KEY, "");

} string error(string message) {

   if(Debug) llSay(DEBUG_CHANNEL, message);
   return "";

} string decrypt(string password, string message) {

   integer signatureLength = llStringLength(ProtocolSignature);
   integer headerLength = signatureLength + 12; // version = 4, nonce = 8
   
   // verify length of encrypted message
   if(llStringLength(message) < signatureLength + 44) // digest = 32 (base64 = 44) + at least one character
       return error("Too small for secret message.");
       
   // look for protocol signature in message header
   if(llSubStringIndex(message, ProtocolSignature) != 0)
       return error("Unknown protocol.");
   
   // Parse version information from header
   integer index = signatureLength; // determine where to start parsing
   string major = "0x" + llGetSubString(message, index, ++index);
   string minor = "0x" + llGetSubString(message, ++index, ++index);
   float version = (float)((string)((integer)major) + "." + (string)((integer)minor));
   
   // verify version is supported
   if(version != ProtocolVersion)
       return error("Unknown version.");
   
   // parse nonce from header
   integer nonce = (integer)("0x" + llGetSubString(message, ++index, index + 7));
   
   // remove header from message
   message = llGetSubString(message, headerLength, -1);
   
   // create one time pad from password and nonce
   string oneTimePad = llMD5String(password, nonce);
   // append pad until length matches or exceeds message
   while(llStringLength(oneTimePad) < (llStringLength(message) * .666))
       oneTimePad += llMD5String(oneTimePad, nonce);
   
   // decrypt message
   oneTimePad = llStringToBase64(oneTimePad);
   message = llXorBase64StringsCorrect(message, oneTimePad);
   
   // decode message
   message = llBase64ToString(message);
   
   // get digest
   string digest = llGetSubString(message, 0, 31);
   
   // remove digest from message
   message = llGetSubString(message, 32, -1);
   
   // verify digest is valid
   if(llMD5String(message, nonce) != digest)
       return error("Message digest was not valid.");
   
   // return decrypted message
   return message;

} default {

   state_entry()
   {
       init();
   }
   on_rez(integer start_param)
   {
       init();
   }
   listen(integer channel, string name, key id, string message)
   {
       string message = decrypt(Password, message);
       if(message != "")
           llSay(0, message);
   }

} </lsl>

Note: The Hex and Efficient Hex examples were used to create the hex method.