DCing algorithm

[Som1Lse]

CSHARP-Code: public class ReadFile { public static string main(String FileName) { string text = ""; StreamReader finp = new StreamReader(FileName); int counta = 0; int countTo123 = 0; byte b; byte d; while (finp.Peek() >= 0) { countTo123++; b = (byte)finp.Read(); if (countTo123 > 123) {   d = DecryptByte(counta, b); counta++; if (counta > 36) { counta = 0; } text += (char)d; } } finp.Close(); return text; }   public static byte DecryptByte(int counta, byte b) { String plainEncryptionKey = "odBearBecauseHeIsVeryGoodSiuHungIsAGo"; b -= (byte)plainEncryptionKey[counta]; return b; } }

Your code is wrong.
StreamReader.Read() returns the next character read, which is not necessarily the same as the next byte. Using a FileStream instead of StreamReader solves this as it has a ReadByte() method, which returns the next byte or -1 if the end-of-file has been reached.
I'll also point out that calling both Peek() and Read() is both redundant and slow, and the same thing goes for reading the first 123 characters only to not do anything with them. FileStream.Seek() is a better solution.
My version of the code:

CSHARP-Code: public class ReadFile { public static string main(String FileName) { string text = ""; FileStream finp = new FileStream(FileName,FileMode.Open); int counta = 0; int b; byte d; finp.Seek(123,SeekOrigin.Begin); while((b = finp.ReadByte()) != -1) { d = DecryptByte(counta, (byte)b); counta++; if (counta > 36) { counta = 0; } text += (char)d; } finp.Close(); return text; }   public static byte DecryptByte(int counta, byte b) { String plainEncryptionKey = "odBearBecauseHeIsVeryGoodSiuHungIsAGo"; b -= (byte)plainEncryptionKey[counta]; return b; } }

I would also like to add than C♯ is a dumb language, and that C++ is way better.