Login Protocol

The login protocol begins after INIT_LOGIN. The server accepts two variable-short client packets with the same payload layout; the reconnect flag does not change how the payload is parsed.

Client packet	ID	Payload size
LOGIN	16	Variable short
RECONNECT	18	Variable short

The payload fields are consumed in this order:

Field	Encoding	Notes
Client version	4-byte integer	—
Instance ID	8-byte integer	—
Flags	1 byte	See the flag bit positions below
Settings	JStr	Applet settings string
New display name	JStr2, conditional	Present when the NEW_DISPLAY_NAME flag is set
RSA block	2-byte length and ciphertext	See the RSA/XTEA decoding guide

The flag bit positions are zero-based:

Flag	Bit position
MEMBER	1
JAGEX_DOMAIN	3
NEW_DISPLAY_NAME	4

The decrypted login payload contains four 4-byte ISAAC seed values, a 24-byte machine ID, a 2-byte affiliate ID, credentials, and a password. Credentials depend on the type selected during init:

Init credentials type	Credential encoding
1 or 2	Base-37 display name
3	JStr2 email

The password uses its packed wire representation. The server derives the nonce from the low 32 bits of seed values 2 and 3 and rejects a mismatch with the nonce returned by INIT_LOGIN.

The server removes the read timeout and starts authentication. Server reply packet types are:

Server packet	ID	Payload size	Payload
LOGIN_OK	0	Variable-any	Display name, staff level, membership days, cookie, flags, and account-related values
INVALID_CREDENTIALS	8	0	Empty payload
PICK_NEW_DISPLAY_NAME	10	Variable-any	Empty in the current helper
ERROR	2	Variable-any	Error message as JStr

The LOGIN_OK payload fields appear in this order:

Order	Field	Encoding / notes
1	Display name	Base-37 long
2	Staff level	Ordinal (1 byte)
3	Reserved	Zero byte
4	Membership days	2 bytes
5	Cookie	JStr
6	Reply flags	1 byte
7	Random data	Optional
8	Display name	JStr
9	Unread-message count	1 byte

Reply flags is a one-byte bitfield with zero-based bit positions:

Flag	Bit position	Meaning
READ_RANDOM_DATA	1	Random data follows the flags byte when this bit is set; it is omitted when clear.
CHAT_RESTRICTED	3	The account is chat-restricted.

The current success path uses the defaults: empty cookie, no random data, and no chat-restricted flag.

On successful authentication, the server writes LOGIN_OK and switches to the game-specific, ISAAC-ciphered game protocol.