GajuMobile-Android/app/src/main/java/swiss/qpq/gajumobile/security/AccountAirlock.java

/*
 * Copyright (c) 2026 QPQ AG <info@qpq.swiss>. All rights reserved.
 * Project: Gajumaru Mobile Wallet
 */

package swiss.qpq.gajumobile.security;

import java.nio.charset.StandardCharsets;
import java.security.SecureRandom;
import java.util.UUID;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import swiss.qpq.gajumaru.core.crypto.Ed25519;
import swiss.qpq.gajumaru.core.crypto.Vault;
import swiss.qpq.gajumaru.core.encoding.Mnemonic;
import swiss.qpq.gajumaru.core.tools.CryptoUtils;
import swiss.qpq.gajumobile.data.models.Account;

/**
 * AccountAirlock provides a secure Java-based bridge for identity operations.
 * It ensures that sensitive material only exists in local stack-allocated
 * buffers and is wiped immediately after use.
 */
public final class AccountAirlock {

    private AccountAirlock() {}

    /**
     * Returns the 4096-word Gajumaru dictionary.
     */
    public static String[] getWordList() {
        return Mnemonic.WORDS;
    }

    /**
     * Generates a new random mnemonic phrase.
     * 
     * @return The 23-word mnemonic phrase as a byte array of UTF-8 words.
     */
    public static byte[][] generateMnemonic() {
        byte[] entropy = new byte[32];
        new SecureRandom().nextBytes(entropy);
        try {
            return Mnemonic.encode(entropy);
        } finally {
            CryptoUtils.wipe(entropy);
        }
    }

    /**
     * Creates a new Account object from a mnemonic phrase.
     * 
     * @param label The user-defined label for the account.
     * @param phrase The mnemonic phrase as a list of UTF-8 word byte arrays.
     * @param masterKey The hardware-backed Master Key handle.
     * @return The new Account object.
     * @throws Exception if creation or encryption fails.
     */
    public static Account createAccount(
            String label,
            byte[][] phrase,
            SecretKey masterKey
    ) throws Exception {
        
        byte[] seed = null;
        byte[] dekBytes = null;
        
        try {
            // 1. Decode mnemonic to seed
            seed = Mnemonic.decode(phrase);

            // 2. Generate Public Key
            byte[] publicKey = Ed25519.publicKey(seed);

            // 3. Generate random Data Encryption Key (DEK)
            SecureRandom random = new SecureRandom();
            dekBytes = new byte[32];
            random.nextBytes(dekBytes);
            SecretKey dek = new SecretKeySpec(dekBytes, "AES");

            // 4. Encrypt Private Key (seed) with DEK
            // We use the provider-generated IV here for maximum hardware compatibility
            Vault.Ciphertext dataCiphertext = Vault.encrypt(dek, seed);

            // 5. Encrypt DEK with Master Key
            // Android Keystore REQUIRED to generate the IV when randomizedEncryption is true
            Vault.Ciphertext dekCiphertext = Vault.encrypt(masterKey, dekBytes);

            EncryptedEnvelope envelope = new EncryptedEnvelope(
                    dataCiphertext.getData(),
                    dekCiphertext.getData(),
                    dataCiphertext.getIv(),
                    dekCiphertext.getIv()
            );

            String id = UUID.randomUUID().toString();
            return new Account(id, label, publicKey, envelope, null);

        } finally {
            // 6. Memory Hygiene: Wipe sensitive data from the heap
            if (seed != null) {
                CryptoUtils.wipe(seed);
            }
            if (dekBytes != null) {
                CryptoUtils.wipe(dekBytes);
            }
        }
    }

    /**
     * Utility to convert a UI String into the internal airlock byte[][] format.
     */
    public static byte[][] stringToPhrase(String phrase) {
        String[] words = phrase.trim().split("\\s+");
        byte[][] result = new byte[words.length][];
        for (int i = 0; i < words.length; i++) {
            result[i] = words[i].getBytes(StandardCharsets.UTF_8);
        }
        return result;
    }

    /**
     * Utility to wipe a phrase returned by the airlock.
     */
    public static void wipePhrase(byte[][] phrase) {
        if (phrase != null) {
            for (byte[] word : phrase) {
                CryptoUtils.wipe(word);
            }
        }
    }

    /**
     * Decrypts an account's private key and signs a message.
     * 
     * @param envelope The encrypted private key envelope.
     * @param masterKey The hardware-backed Master Key handle from Android Keystore.
     * @param message The message to sign.
     * @return The Ed25519 signature.
     * @throws Exception if decryption or signing fails.
     */
    public static byte[] sign(
            EncryptedEnvelope envelope,
            SecretKey masterKey,
            byte[] message
    ) throws Exception {
        
        byte[] plaintextDek = null;
        byte[] plaintextPrivateKey = null;
        
        try {
            // 1. Decrypt the Data Encryption Key (DEK) using the Master Key
            plaintextDek = Vault.decrypt(
                    masterKey, 
                    envelope.getDekIv(), 
                    envelope.getEncryptedDek()
            );
            SecretKey dek = new SecretKeySpec(plaintextDek, "AES");

            // 2. Decrypt the Private Key using the DEK
            plaintextPrivateKey = Vault.decrypt(
                    dek, 
                    envelope.getDataIv(), 
                    envelope.getEncryptedData()
            );

            // 3. Perform Ed25519 signature
            // The Ed25519 implementation in gm-java uses internal scratch pads 
            // that are also wiped upon completion.
            return Ed25519.sign(plaintextPrivateKey, message);

        } finally {
            // 4. Memory Hygiene: Wipe sensitive data from the heap
            if (plaintextDek != null) {
                CryptoUtils.wipe(plaintextDek);
            }
            if (plaintextPrivateKey != null) {
                CryptoUtils.wipe(plaintextPrivateKey);
            }
        }
    }
}