ReadUTF from DataInput (RotMG/Java)

Hello. I'm currently in the final working stages of my personal Java RotMG MITM Proxy. I have a question about reading UTF strings from a DataInput object in Java. I'm not sure if anyone here can help me with this but its worth a shot.

The Issue

After constructing a DataInputStream from the ciphered bytes read in each packet some packet structures require UTF strings to be read from their bytes. The DataInput object has a native method for .readUTF() However despite the bytes being ciphered correctly and fully read, the .readUTF() method always throws a End Of File exception meaning it reached the end of the bytes before forming a complete readable string. I am able to exchange packets of Unknown type between the server and the client but whenever i try to build a packet requiring a UTF string to be read this issue occurs. If any Java/Networking gurus out there could help me solve this i will be right on my way.

Thanks for your time

yeah my dude try stackoverflow.com
java is boring and outdated huehue

Originally Posted by Riigged

yeah my dude try stackoverflow.com
java is boring and outdated huehue

Bruhhhh you say that as if my first choice for troubleshooting is MPGH. This is a last resort.

Originally Posted by ruusey

Bruhhhh you say that as if my first choice for troubleshooting is MPGH. This is a last resort.

damn mpgh being the last resort is not a good thing, its the other way around for me, would come to mpgh and if nobody could help would post on S/O (well, used to be, dont even do anything anymore so what would i need help with now hue hue)

Reading/writing a UTF string is: ushort prefix which tells you the length of the string, then a byte array for the actual string.

My relay had to do this:

Code:

		public static void WriteUTF(string str, EndianBinaryWriter ebw)
		{
			if (str == null)
				ebw.Write((short)0);
			else
			{
				byte[] bytes = Encoding.UTF8.GetBytes(str);
				ebw.Write((short)bytes.Length);
				ebw.Write(bytes);
			}
		}

Originally Posted by 059

Reading/writing a UTF string is: ushort prefix which tells you the length of the string, then a byte array for the actual string.

My relay had to do this:

Code:

public static void WriteUTF(string str, EndianBinaryWriter ebw) { if (str == null) ebw.Write((short)0); else { byte[] bytes = Encoding.UTF8.GetBytes(str); ebw.Write((short)bytes.Length); ebw.Write(bytes); } }

Here is a description of DataInput readUTF method

Code:

Open Declaration String java****.DataInput.readUTF() throws IOException Reads in a string that has been encoded using a modified UTF-8 format. The general contract of readUTF is that it reads a representation of a Unicode character string encoded in modified UTF-8 format; this string of characters is then returned as a String. First, two bytes are read and used to construct an unsigned 16-bit integer in exactly the manner of the readUnsignedShort method . This integer value is called the UTF length and specifies the number of additional bytes to be read. These bytes are then converted to characters by considering them in groups. The length of each group is computed from the value of the first byte of the group. The byte following a group, if any, is the first byte of the next group. If the first byte of a group matches the bit pattern 0xxxxxxx (where x means "may be 0 or 1"), then the group consists of just that byte. The byte is zero-extended to form a character. If the first byte of a group matches the bit pattern 110xxxxx, then the group consists of that byte a and a second byte b. If there is no byte b (because byte a was the last of the bytes to be read), or if byte b does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character: (char)(((a & 0x1F) << 6) | (b & 0x3F)) If the first byte of a group matches the bit pattern 1110xxxx, then the group consists of that byte a and two more bytes b and c. If there is no byte c (because byte a was one of the last two of the bytes to be read), or either byte b or byte c does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character: (char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F)) If the first byte of a group matches the pattern 1111xxxx or the pattern 10xxxxxx, then a UTFDataFormatException is thrown. If end of file is encountered at any time during this entire process, then an EOFException is thrown. After every group has been converted to a character by this process, the characters are gathered, in the same order in which their corresponding groups were read from the input stream, to form a String, which is returned. The writeUTF method of interface DataOutput may be used to write data that is suitable for reading by this method.

So like it says it does read a short first to determine the length and then parses the string based on the length of the short. Going to keep working on this.

edit: i checked out AMPBedu's Realm Relay code on GitHub and he uses the exact same DataInput.readUTF() method im trying to use which i assume was successful.

Update: All methods for reading data from the packet byte array returns negative values readShort(), readInt() etc... Not sure if this is to do with the nature of java's bytes being unsigned? If that was the case i dont know how Realm Relay would have accomplished what it did.

Solved. Please close @minions

@Raple
@Ahlwong

OP has solved this

ReadUTF from DataInput (RotMG/Java)

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