Data Types

RuneScape uses a number of uncommon and bespoke data formats to obsfuscate communication and reduce cache file size.

Each client revision randomises the endianness and modifications used for decoding client packets. The OSRS protocol scrambling follows a decipherable order (citation needed).

Standard Java Data Types

RuneScape client uses Java’s primitive data types to store and transmit information, understanding these limitations and sizes are important regardless of client revision.

Type	Size	Minimum Value	Maximum Value
byte	1 byte	-128	127
short	2 bytes	-32,768	32,767
int	4 bytes	-2,147,483,648	2,147,483,647
long	8 bytes	-9,223,372,036,854,775,808	9,223,372,036,854,775,807
float	4 bytes
double	8 bytes
boolean	1 bit	false	true
char	2 bytes	0	65535

2,147,483,647 is the maximum stack a player can have of one item because item amounts are stored as integers.

Signed and unsigned

Data types typically are Signed, easily thought of as a single bit assigned as a binary flag denoting a positive or negative number. However when only positive values are necessary the flag can be unsigned allowing a larger number to be stored in the same amount of space.

Type	Size	Minimum Unsigned Value	Maximum Unsigned Value
byte	1 byte	0	255
short	2 bytes	0	65,535
int	4 bytes	0	‭4,294,967,296‬
long	8 bytes	0	‭18,446,744,073,709,551,616‬

Methods utilising unsigned integers commonly use ‘U’ as a prefix e.g. readUByte

Bit Access

The client also utilises reading and writing individual bits interchangeably with other data types.

TODO code example needed for both read and write which cleanly breaks down how it works

Obfuscation methods

Endianness

The read and write order of byte data is called Endianness RuneScape client occasionally uses a middle-endian (integer’s only) the order of which is determined by an inverse modification.

• Big - Most significant bit first
• Little - Least significant bit first
• Middle - Most significant bit central

Modifications

Several further modifications can be made to bytes before and reversed afterwards as obfuscation

Name	Read	Write
A	value - 128	value + 128
C	-value	-value
S	128 - value	128 - value

Implementation

Kotlin (bespoke type support needed)

Bespoke types

Medium/Tribyte

A custom data type for storing 3 byte numbers between -8,388,608 and 8,388,607

Smart

Functions that “smartly” choose the smallest data type necessary to store the value provided

e.g

if value < byte max
  write byte
else
  write short