/// <summary>
/// Generates the xor decryption method.
/// </summary>
/// <param name="assembly">The assembly.</param>
/// <param name="body">The body.</param>
private static void GenerateXorDecryptionMethod(AssemblyDefinition assembly, MethodBody body)
{
var worker = body.GetILProcessor();
//Generate the decryption method
//Since this is XOR it is the same as the encryption method
//In reality its a bit of a joke calling this encryption as its really
//just obfusaction
/*
char[] characters = value.ToCharArray();
for (int i = 0; i < characters.Length; i++)
{
characters[i] = (char)(characters[i] ^ salt);
}
return new String(characters);
*/
//Declare a local to store the char array
body.InitLocals = true;
body.Method.AddLocal(typeof(char[]));
body.Method.AddLocal(typeof(int));
body.Method.AddLocal(typeof(string));
body.Method.AddLocal(typeof(bool));
//Start with a nop
worker.Append(worker.Create(OpCodes.Nop));
//Load the first argument into the register
Instruction ldArg0 = worker.Create(OpCodes.Ldarg_0);
worker.Append(ldArg0);
//Call ToCharArray on this -- need to find it first
var toCharArrayMethodRef = assembly.Import(typeof(string).GetMethod("ToCharArray", Type.EmptyTypes));
//Import the method first
toCharArrayMethodRef = body.ImportMethod(toCharArrayMethodRef);
Instruction toCharArray = worker.Create(OpCodes.Callvirt, toCharArrayMethodRef);
worker.Append(toCharArray);
//Store it in the first local
Instruction stLoc0 = worker.Create(OpCodes.Stloc_0);
worker.Append(stLoc0);
//Set up the loop
worker.Append(worker.Create(OpCodes.Ldc_I4_0));
Instruction stLoc1 = worker.Create(OpCodes.Stloc_1);
worker.Append(stLoc1);
//We'll insert a br.s here later....
//Insert another nop and do the rest of our loop
Instruction loopNop = worker.Create(OpCodes.Nop);
worker.Append(loopNop);
worker.Append(worker.Create(OpCodes.Ldloc_0));
worker.Append(worker.Create(OpCodes.Ldloc_1));
worker.Append(worker.Create(OpCodes.Ldloc_0));
worker.Append(worker.Create(OpCodes.Ldloc_1));
worker.Append(worker.Create(OpCodes.Ldelem_U2)); //Load the array
worker.Append(worker.Create(OpCodes.Ldarg_1));
worker.Append(worker.Create(OpCodes.Xor)); //Do the xor
worker.Append(worker.Create(OpCodes.Conv_U2));
worker.Append(worker.Create(OpCodes.Stelem_I2)); //Store back in the array
worker.Append(worker.Create(OpCodes.Nop));
worker.Append(worker.Create(OpCodes.Ldloc_1));
worker.Append(worker.Create(OpCodes.Ldc_I4_1));
worker.Append(worker.Create(OpCodes.Add));
worker.Append(worker.Create(OpCodes.Stloc_1));
Instruction ldLoc = worker.Create(OpCodes.Ldloc_1);
worker.Append(ldLoc);
//Link to this line from an earlier statement
worker.InsertAfter(stLoc1, worker.Create(OpCodes.Br_S, ldLoc));
worker.Append(worker.Create(OpCodes.Ldloc_0));
worker.Append(worker.Create(OpCodes.Ldlen));
worker.Append(worker.Create(OpCodes.Conv_I4));
worker.Append(worker.Create(OpCodes.Clt)); //i < array.Length
worker.Append(worker.Create(OpCodes.Stloc_3));
worker.Append(worker.Create(OpCodes.Ldloc_3));
worker.Append(worker.Create(OpCodes.Brtrue_S, loopNop)); //Do the loop
//Return a new string
worker.Append(worker.Create(OpCodes.Ldloc_0));
//Find the constructor we want to use
MethodReference constructor = assembly.Import(typeof(string).GetConstructor(new [] { typeof(char[])}));
constructor = body.ImportMethod(constructor);
worker.Append(worker.Create(OpCodes.Newobj, constructor));
Instruction stloc2 = worker.Create(OpCodes.Stloc_2);
worker.Append(stloc2);
Instruction ldloc2 = worker.Create(OpCodes.Ldloc_2);
worker.Append(ldloc2);
worker.InsertAfter(stloc2, worker.Create(OpCodes.Br_S, ldloc2));
worker.Append(worker.Create(OpCodes.Ret));
}
