Protocol Encryption

As of 12w17a, Minecraft implements SSL-like encryption.

Overview

  C->S : Handshake State=2
  C->S : Login Start
  S->C : Encryption Key Request
  (Client Auth)
  C->S : Encryption Key Response
  (Server Auth, Both enable encryption)
  S->C : Login Success

1. see Protocol FAQ to get information about what happens next.

Server ID String

The server ID string is a randomly-generated string of characters with a maximum length of 20 code points (the client disconnects with an exception if the length is longer than 20).

The client appears to arrive at incorrect hashes if the server ID string contains certain unprintable characters, so for consistent results only characters with code points in the range U+0021-U+007E (inclusive) should be sent. This range corresponds to all of ASCII with the exception of the space character (U+0020) and all control characters (U+0000-U+001F, U+007F).

The client appears to arrive at incorrect hashes if the server ID string is too short. 15 to 20 (inclusive) length strings have been observed from the Notchian server and confirmed to work as of 1.5.2.

Key Exchange

The server generates a 1024-bit RSA keypair on startup. The key, when packed into a Encryption Request packet, is in ASN.1 format as defined by x.509. The ASN.1 structure looks as follows

SubjectPublicKeyInfo ::= SEQUENCE {
  algorithm SEQUENCE {
    algorithm         OBJECT IDENTIFIER
    parameters        ANY OPTIONAL
  }
  subjectPublicKey  BITSTRING
}

SubjectPublicKey ::= SEQUENCE {
  modulus           INTEGER
  publicExponent    INTEGER
}

If you're struggling to import this using a crypto library, you can convert it to common PEM by base64-encoding and wrapping in '-----BEGIN PUBLIC KEY-----' and '-----END PUBLIC KEY-----'.

Symmetric Encryption

On receipt of a Encryption Request from the server, the client will generate a 16-byte shared secret, to be used with the AES/CFB8 stream cipher. It then encrypts it with the server's public key (PKCS#1 v1.5 padded), and also encrypts the token received in the Encryption Request packet in the same way, then sends both to the server in a Encryption Response packet.

The server decrypts the shared secret and token using its private key, and checks if the token is the same. It then sends a Login Success, and enables AES/CFB8 encryption. For the Initial Vector (IV) and AES setup, both sides use the secret key. Similarly, the client will also enable encryption upon receipt of Login Success. From this point forward, everything is encrypted.

Authentication

Both server and client need to make a request to sessionserver.mojang.com if the server is in online-mode.

Client

After generating the shared secret, the client generates the following hash:

sha1 := Sha1()
sha1.update(ASCII encoding of the server id string from Encryption Request) 
sha1.update(shared secret) 
sha1.update(server's encoded public key from Encryption Request) 
hash := sha1.hexdigest()  # String of hex characters

Note that the Sha1.hexdigest() method used by minecraft removes leading zeros and uses the two's-complement of negative numbers prefixed with a minus sign:

sha1(Notch) :  4ed1f46bbe04bc756bcb17c0c7ce3e4632f06a48
sha1(jeb_)  : -7c9d5b0044c130109a5d7b5fb5c317c02b4e28c1
sha1(simon) :  88e16a1019277b15d58faf0541e11910eb756f6

The resulting hash is then sent via an HTTP POST request to

https://sessionserver.mojang.com/session/minecraft/join

With the following sent as post data

 {
   "accessToken": "<accessToken>",
   "selectedProfile": "<selectedProfile>",
   "serverId": "<serverHash>"
 }

Server

After decrypting the shared secret in the second Encryption Response, the server generates the login hash as above and sends it to

https://sessionserver.mojang.com/session/minecraft/hasJoined?username=username&serverId=hash

The response is a JSON object containing the user's UUID

  {"id":"<uuid>"}

Sample Code

Examples of generating Java-style hex digests:

• C#: https://gist.github.com/404223052379e82f91e6
• node.js: https://gist.github.com/4425843
• Go: https://gist.github.com/toqueteos/5372776

Additional Links

Encrypt shared secret using OpenSSL

Generate RSA-Keys and building the ASN.1v8 structure of the x.509 certificate using Crypto++

Decrypt shared secret using Crypto++

De/Encrypt data via AES using Crypto++

C# AES/CFB support with bouncy castle on Mono