Protocol Encryption

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

Overview

1. Client connects to server
2. C->S 0x02 handshake
3. S->C 0xFD encryption request - server sends its public key and 4 random bytes
4. Client generates symmetric key (shared secret)
5. Client authenticates via session.minecraft.net
6. Client encrypts these 4 bytes with the servers public key.
7. C->S 0xFC encryption response - client encrypts shared secret with server's public key and sends along with encrypted 4 bytes
8. Server checks that the encrypted bytes match
9. Server decrypts shared secret with its private key
10. Server checks player authenticity via session.minecraft.net
11. S->C 0xFC encryption response - empty payload
12. Server enables AES stream encryption
13. Client enables AES stream encryption
14. C->S 0xCD - Payload of 0 (byte)
15. S->C 0x01 login
16. see Protocol FAQ to get information about what happens next.

For a Code example you can have a look at this example of a Chat Bot using a C# Protocol Library: https://minecraftprotocol.codeplex.com/SourceControl/changeset/view/68979#1204175

Key Exchange

The server generates a 1024-bit RSA keypair on startup. The key, when packed into a 0xFD 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

12w18a:

On receipt of a 0xFD from the server, the client will generate a 16-byte shared secret, to be used with the RC4 stream cipher. It then encrypts it with the server's public key (PKCS#1 padded), and sends it to the server with a 0xFC.

The server decrypts the shared secret using its private key. It then sends a 0xFC to the client with an empty payload, and enables RC4 encryption. Similarly, the client will also enable encryption upon receipt of the empty 0xFC. From this point forward, everything is encrypted.

12w19a:

On receipt of a 0xFD from the server, the client will generate a 32-byte shared secret, to be used with the HC-256 stream cipher. It then encrypts it with the server's public key (PKCS#1 padded), and sends it to the server with a 0xFC.

The server decrypts the shared secret using its private key. It then sends a 0xFC to the client with an empty payload, and enables HC-256 encryption. For the Initial Vector (IV) Both sides use the first 16 byte of the key. Similarly, the client will also enable encryption upon receipt of the empty 0xFC. From this point forward, everything is encrypted.

12w21a:

On receipt of a 0xFD 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 padded), and sends it to the server with a 0xFC.

The server decrypts the shared secret using its private key. It then sends a 0xFC to the client with an empty payload, and enables AES/CFB 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 the empty 0xFC. From this point forward, everything is encrypted.

Authentication

Both server and client need to make a request to session.minecraft.net 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 serverId string from 0xFC) 
sha1.update(shared secret) 
sha1.update(server's encoded public key from 0xFC) 
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 GET request to

http://session.minecraft.net/game/joinserver.jsp?user=username&sessionId=user_session&serverId=hash

If it returns OK then continue, otherwise stop

Server

After decrypting the shared secret in 0xFD, the sever generates the login hash as above and sends it to

http://session.minecraft.net/game/checkserver.jsp?user=username&serverId=hash

If the response is YES then the client is authenticated and allowed to join. Otherwise the client will/should be kicked (unencrypted) with "Failed to verify username!"