Overview

To implement the encoding and decoding of HD wallet-capable public keys, we will use Swift for the client side and Java with the Bouncy Castle library for the server side. The HD wallet specification follows the BIP-32 standard, which defines hierarchical deterministic wallets.

Client Side (Swift)

We will use the HDWalletKit library to handle HD wallet operations in Swift. This library provides functionalities to create and manage HD wallets, including key derivation and encoding.

Swift Code

swift
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import HDWalletKit

func encodePublicKeyToRaw(publicKey: String) -> String? {
    guard let data = Data(base64Encoded: publicKey) else {
        return nil
    }
    let rawPublicKey = data.base58EncodedString()
    return rawPublicKey
}

// Example usage
let publicKey = "your_public_key_here"
if let rawPublicKey = encodePublicKeyToRaw(publicKey: publicKey) {
    print("Raw Public Key: \(rawPublicKey)")
} else {
    print("Failed to encode public key.")
}

Server Side (Java)

We will use the Bouncy Castle library to handle cryptographic operations in Java. The library provides functionalities to encode and decode public keys using the Base58Check encoding scheme.

Java Code

java
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import org.bitcoinj.core.Base58;
import org.bitcoinj.core.ECKey;
import org.bitcoinj.params.MainNetParams;
import org.bouncycastle.util.encoders.Hex;

public class HDPublicKeyEncoder {

    public static String encodePublicKeyToRaw(String publicKey) {
        byte[] decodedPublicKey = Hex.decode(publicKey);
        String rawPublicKey = Base58.encode(decodedPublicKey);
        return rawPublicKey;
    }

    public static String decodeRawPublicKey(String rawPublicKey) {
        byte[] decodedRawPublicKey = Base58.decode(rawPublicKey);
        ECKey key = ECKey.fromPublicOnly(decodedRawPublicKey);
        return key.getPublicKeyAsHex();
    }

    public static void main(String[] args) {
        String publicKey = "your_public_key_here";
        String rawPublicKey = encodePublicKeyToRaw(publicKey);
        System.out.println("Raw Public Key: " + rawPublicKey);

        String decodedPublicKey = decodeRawPublicKey(rawPublicKey);
        System.out.println("Decoded Public Key: " + decodedPublicKey);
    }
}

Explanation

1. Client Side (Swift):

   • The encodePublicKeyToRaw function takes a Base64-encoded public key, decodes it, and then encodes it to a Base58Check string.
   • The HDWalletKit library is used to handle the encoding process.

2. Server Side (Java):

   • The encodePublicKeyToRaw function takes a hex-encoded public key, decodes it, and then encodes it to a Base58Check string using the Bouncy Castle library.
   • The decodeRawPublicKey function takes a Base58Check-encoded raw public key, decodes it, and then converts it back to a hex-encoded public key using the Bitcoinj library.

Additional Information

• The HD wallet specification can be found here.
• The Bouncy Castle library is a widely used library for cryptographic operations in Java.
• The Bitcoinj library provides functionalities to work with Bitcoin keys and addresses in Java.

This solution ensures that the public key is securely encoded and decoded on both the client and server sides, following the BIP-32 standard for HD wallets.