additional document for BIP32

Question

I am reading through BIP32's documentation and its implementation in Java on GitHub in the file supernode/api/src/main/java/com/bitsofproof/supernode/api/ExtendedKey.java.

Luckily, I was able to follow the code, but there are areas I am not sure about.

I have 2 questions:

1. I know it is in BIP32.json file, but why are we doing if ((sequence & 0x8000000)==0)? It is a condition for master public key to child public key or master private key to child private key... but what sequence does for? if ( (sequence & 0x80000000) == 0 )

2. My second one is why we add 4 bytes. Is there any detail document source I can find?

           extended = new byte[pub.length + 4];
           System.arraycopy (pub, 0, extended, 0, pub.length);
           extended[pub.length] = (byte) ((sequence >>> 24) & 0xff);
           extended[pub.length + 1] = (byte) ((sequence >>> 16) & 0xff);
           extended[pub.length + 2] = (byte) ((sequence >>> 8) & 0xff);
           extended[pub.length + 3] = (byte) (sequence & 0xff);
       }
       else
       {
           byte[] priv = master.getPrivate ();
           extended = new byte[priv.length + 5];
           System.arraycopy (priv, 0, extended, 1, priv.length);
           extended[priv.length + 1] = (byte) ((sequence >>> 24) & 0xff);
           extended[priv.length + 2] = (byte) ((sequence >>> 16) & 0xff);
           extended[priv.length + 3] = (byte) ((sequence >>> 8) & 0xff);
           extended[priv.length + 4] = (byte) (sequence & 0xff);
       }
   

Many thanks in advance.

Answer 1

Check https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#serialization-format

((sequence & 0x80000000)==0) means it's not a hardened key. The key is hardened (public key derivation not possible) for child keys with index higher then 0x80000000 (== 2147483648 == 2^31).

From the BIP

4 byte: version bytes (mainnet: 0x0488B21E public, 0x0488ADE4 private; testnet: 0x043587CF public, 0x04358394 private)
1 byte: depth: 0x00 for master nodes, 0x01 for level-1 derived keys, ....
4 bytes: the fingerprint of the parent's key (0x00000000 if master key)
4 bytes: child number. This is ser32(i) for i in xi = xpar/i, with xi the key being serialized. (0x00000000 if master key)
32 bytes: the chain code
33 bytes: the public key or private key data (serP(K) for public keys, 0x00 || ser256(k) for private keys)