/*
    Copyright (C) 2011-2015 de4dot@gmail.com

    This file is part of de4dot.

    de4dot is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    de4dot is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with de4dot.  If not, see <http://www.gnu.org/licenses/>.
*/

using System.Text;
using dnlib.DotNet;
using de4dot.blocks;

namespace de4dot.code.deobfuscators.Xenocode {
	class StringDecrypter {
		const int STRING_DECRYPTER_KEY_CONST = 1789;
		ModuleDefMD module;
		TypeDef stringDecrypterType;
		MethodDef stringDecrypterMethod;

		public bool Detected {
			get { return stringDecrypterMethod != null; }
		}

		public TypeDef Type {
			get { return stringDecrypterType; }
		}

		public MethodDef Method {
			get { return stringDecrypterMethod; }
		}

		public StringDecrypter(ModuleDefMD module) {
			this.module = module;
		}

		public void Find() {
			foreach (var type in module.Types) {
				if (type.HasFields)
					continue;
				if (type.Methods.Count < 1 || type.Methods.Count > 3)
					continue;
				if (type.HasProperties || type.HasEvents)
					continue;

				MethodDef method = null;
				foreach (var m in type.Methods) {
					if (m.Name == ".ctor" || m.Name == ".cctor")
						continue;
					if (DotNetUtils.IsMethod(m, "System.String", "(System.String,System.Int32)")) {
						method = m;
						continue;
					}
					method = null;
					break;
				}
				if (method == null || method.Body == null)
					continue;

				bool foundConstant = false;
				foreach (var instr in method.Body.Instructions) {
					if (instr.IsLdcI4() && instr.GetLdcI4Value() == STRING_DECRYPTER_KEY_CONST) {
						foundConstant = true;
						break;
					}
				}
				if (!foundConstant)
					continue;

				stringDecrypterType = type;
				stringDecrypterMethod = method;
				break;
			}
		}

		public string Decrypt(string es, int magic) {
			int newLen = es.Length / 4;
			var sb = new StringBuilder(newLen);
			for (int i = 0; i < newLen * 4; i += 4) {
				char c = (char)((es[i] - 'a') +
							((es[i + 1] - 'a') << 4) +
							((es[i + 2] - 'a') << 8) +
							((es[i + 3] - 'a') << 12) - magic);
				magic += STRING_DECRYPTER_KEY_CONST;
				sb.Append(c);
			}
			return sb.ToString();
		}
	}
}