Legacy Mojang Authentication

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Revision as of 16:26, 15 July 2013 by Sadimusi (talk | contribs) (Moved here from Session)
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The minecraft client and server communicate with Mojang to validate player sessions. This page describes some of the operations.


Minecraft 1.6 introduced a new authentication scheme called Yggdrasil which completely replaces the previous authentication system.

Request format

All requests to Yggdrasil are made to the following server:


Further, they are expected to fulfill the following rules:

  • Are POST requests
  • Have the Content-Type header set to application/json
  • Contain a JSON-encoded dictionary as payload

If a request was successful the server will respond with:

  • Status code 200
  • A JSON-encoded dictionary according to the specifications below

If however a request fails, the server will respond with:

  "error": "Short description of the error",
  "errorMessage": "Longer description which can be shown to the user",
  "cause": "Cause of the error"        // optional

Session ID

Whenever a Mojang service requires a session ID, you can simply combine a valid accessToken with a profile identifier as follows:

 token:<accessToken>:<profile ID>


This method is used whenever a user tries to log in using his password.




  "agent": {                             // optional
    "name": "Minecraft",                 // So far this is the only encountered value
    "version": 1                         // This number might be increased
                                         // by the vanilla client in the future
  "username": "mojang account name",     // Can be an email address or player name for
                                         // unmigrated accounts
  "password": "mojang account password",
  "clientToken": "client identifier"     // optional

The clientToken should be a randomly generated identifier and must be identical for each request. In case it is omitted the server will generate a random token based on Java's UUID.toString() which should then be stored by the client. This will however also invalidate all previously acquired accessTokens for this user across all clients.


  "accessToken": "random access token",  // hexadecimal
  "clientToken": "client identifier",    // identical to the one received
  "availableProfiles": [                 // only present if the agent field was received
      "id": "profile identifier",        // hexadecimal
      "name": "player name"
  "selectedProfile": {                   // only present if the agent field was received
    "id": "profile identifier",
    "name": "player name"

Note: If a user wishes to stay logged in on his computer you are strongly advised to store the received accessToken instead of the password itself.

Currently each account will only have one single profile, multiple profiles per account are however planned in the future.


This method is used to refresh an expired but valid accessToken. It can be uses to keep a user logged in between gaming sessions and is preferred over storing the user's password in a file (see lastlogin).




  "accessToken": "valid accessToken",
  "clientToken": "client identifier"     // This needs to be identical to the one used
                                         // to obtain the accessToken in the first place

Note: The provided accessToken gets invalidated.


  "accessToken": "random access token",  // hexadecimal
  "clientToken": "client identifier",    // identical to the one received
  "selectedProfile": {
    "id": "profile identifier",          // hexadecimal
    "name": "player name"

Joining a Server

See Protocol Encryption for details on encrypting connections.

Client operation

  1. Client connects to server
  2. Client sends a 0x02 handshake containing the current player name
  3. Client receives an 0xFD encryption request with the server's public key and four verification bytes.
  4. Client generates a symmetric key.
  5. Client sends a HTTP request to
    http://session.minecraft.net/game/joinserver.jsp?user=<player name>&sessionId=<session id>&serverId=<server hash>
    Note: See the Session ID section on how the <session id> is generated.
    If the response is OK then continue, otherwise stop
  6. Client sends 0xFC encryption response - client encrypts shared secret and verification token with the server's public key
  7. Server checks validity and sends 0xFC encryption response with two empty arrays
  8. Both sides enable AES/CFB2 encryption using the shared secret generated by the client
  9. Client sends 0xCD client status with a payload of 0 (ready to spawn)
  10. Server sends 0x01 login
  11. ... receive map chunks, etc...

Server operation

  1. Server generates a 1024-bit RSA keypair.
  2. ...
  3. Server answers TCP connection request and receives a 0x02 handshake from the client.
  4. Server sends 0xFD encryption request with its public key and a verification token.
  5. Server receives 0xFC encryption response from client and decrypts the shared secret.
  6. Server verifies the validity of the verification token. If this isn't verified, it kicks the client.
  7. Server sends a HTTP request to
    http://session.minecraft.net/game/checkserver.jsp?user=<username>&serverId=<server hash>
    If it returns YES then the client is authenticated and allowed to join. Otherwise the client will/should be kicked with “Failed to verify username!”
  8. Server sends a 0xFC encryption response packet with two empty arrays.
  9. Both sides enable AES/CFB2 encryption.
  10. Server receives 0xCD client status with a payload of , indicating "ready to spawn"
  11. Server sends 0x01 login
  12. ... send map chunks, etc...