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CryptStream.cs
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CryptStream.cs
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using System;
using System.IO;
namespace DtxCS
{
/// <summary>
/// Provides a means to decrypt an encrypted DTB file (or any other file using this crypt method).
/// </summary>
public class CryptStream : Stream
{
private long position;
private readonly int key;
private int curKey;
private long keypos;
private Stream file;
public CryptStream(Stream file, bool write = false)
{
file.Position = 0;
if (write)
{
int rKey = new Random().Next();
file.WriteInt32LE(rKey);
this.key = cryptRound(rKey);
}
else
this.key = cryptRound(file.ReadInt32LE());
this.curKey = this.key;
this.file = file;
this.Length = file.Length - 4;
}
public override bool CanRead => true;
public override bool CanSeek => true;
public override bool CanWrite => false;
public override long Length { get; }
public override long Position
{
get
{
return position;
}
set
{
Seek(value, SeekOrigin.Begin);
}
}
private void updateKey()
{
if (keypos == position)
return;
if (keypos > position) // reset key (TODO: is there a better way to "re-wind" the key?)
{
keypos = 0;
curKey = key;
}
while (keypos < position)
{
curKey = cryptRound(curKey);
keypos++;
}
}
private int cryptRound(int key)
{
int ret = (key - ((key / 0x1F31D) * 0x1F31D)) * 0x41A7 - (key / 0x1F31D) * 0xB14;
if (ret <= 0)
ret += 0x7FFFFFFF;
return ret;
}
public override int Read(byte[] buffer, int offset, int count)
{
// ensure file is at correct offset
file.Seek(this.position + 4, SeekOrigin.Begin);
if (offset + count > buffer.Length)
{
throw new IndexOutOfRangeException("Attempt to fill buffer past its end");
}
if (this.Position == this.Length || this.Position + count > this.Length)
{
count = (int)(this.Length - this.Position);
//throw new System.IO.EndOfStreamException("Cannot read past end of file.");
}
int bytesRead = file.Read(buffer, offset, count);
for (uint i = 0; i < bytesRead; i++)
{
buffer[offset + i] ^= (byte)(this.curKey);
this.position++;
updateKey();
}
return bytesRead;
}
public override void Write(byte[] buffer, int offset, int count)
{
// ensure file is at correct offset
file.Seek(this.position + 4, SeekOrigin.Begin);
if (offset + count > buffer.Length)
{
throw new IndexOutOfRangeException("Attempt to fill buffer past its end");
}
byte[] tempBuf = new byte[count];
for (uint i = 0; i < count; i++)
{
tempBuf[offset + i] = (byte)(buffer[offset + i] ^ (byte)(this.curKey));
this.position++;
updateKey();
}
file.Write(tempBuf, offset, count);
return;
}
public override long Seek(long offset, SeekOrigin origin)
{
int adjust = origin == SeekOrigin.Current ? 0 : 4;
this.position = file.Seek(offset + adjust, origin) - 4;
updateKey();
return position;
}
#region Not Used
public override void Flush()
{
throw new NotSupportedException();
}
public override void SetLength(long value)
{
throw new NotSupportedException();
}
#endregion
}
}