/// <summary>
/// Processes the instructions replacing all strings being loaded with an encrypted version.
/// </summary>
/// <param name="assemblyDef">The assembly definition.</param>
/// <param name="body">The body.</param>
/// <param name="decryptMethod">The decrypt method.</param>
private void ProcessInstructions(AssemblyDefinition assemblyDef, MethodBody body, MethodReference decryptMethod)
{
var instructions = body.Instructions;
var il = body.GetILProcessor();
List<Instruction> instructionsToExpand = new List<Instruction>();
List<int> offsets = new List<int>();
foreach (Instruction instruction in instructions)
{
//Find the call statement
switch (instruction.OpCode.Name)
{
case "ldstr":
//We've found a string load message - we need to replace this instruction
if (instruction.Operand is string) //Only do the direct strings for now
instructionsToExpand.Add(instruction);
break;
}
}
//Fix each ldstr instruction found
foreach (Instruction instruction in instructionsToExpand)
{
//What we do is replace the ldstr "bla" with:
//ldstr bytearray encrypted_array
//ldc.i4 random_integer
//call string class Decrypt(string, int32)
//First get the original value
string originalValue = instruction.Operand.ToString();
offsets.Add(instruction.Offset);
//Secondly generate a random integer as a salt
int salt = random.Next(5000, 10000);
//Now we need to work out what the encrypted value is and set the operand
OutputHelper.WriteLine("Encrypting string \"{0}\"", originalValue);
string byteArray = EncryptString(originalValue, salt);
Instruction loadString = il.Create(OpCodes.Ldstr, byteArray);
il.Replace(instruction, loadString);
//Now load the salt
Instruction loadSalt = il.Create(OpCodes.Ldc_I4, salt);
il.InsertAfter(loadString, loadSalt);
//Process the decryption
Instruction call = il.Create(OpCodes.Call, decryptMethod);
il.InsertAfter(loadSalt, call);
}
//Unfortunately one thing Mono.Cecil doesn't do is adjust instruction offsets for branch statements
//and exception handling start points. We need to fix these manually
if (offsets.Count == 0)
return;
//Do the adjustments
il.AdjustOffsets(body, offsets, 6 + assemblyDef.GetAddressSize());
}
