Create EncryptedMultisigDescriptor.java

This enhances multisig privacy by encrypting the descriptor with public-key-derived symmetric keys, preventing balance exposure. Advantages: Simplifies setup for individuals without private keys or Shamir sharing; enables wallet access via any two zpubs; maintains security as decryption requires matching pairs; avoids revealing full setup to single parties.

src/main/java/com/sparrowwallet/sparrow/wallet/EncryptedMultisigDescriptor.java

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+// Import necessary Java libraries for encoding, cryptography, and utilities
+import java.nio.charset.StandardCharsets;  // For UTF-8 charset handling
+import java.security.MessageDigest;  // For SHA-256 hashing
+import java.security.SecureRandom;  // For generating secure random IVs
+import java.util.Arrays;  // For array operations like sorting
+import java.util.Base64;  // For Base64 encoding/decoding
+import javax.crypto.Cipher;  // For encryption/decryption operations
+import javax.crypto.SecretKeySpec;  // For creating AES keys
+import javax.crypto.spec.GCMParameterSpec;  // For GCM mode parameters
+import javax.crypto.AEADBadTagException;  // For handling authentication failures in GCM
+
+// Define the public class for encrypted multisig descriptors
+public class EncryptedMultisigDescriptor {
+
+    // Constants for IV and authentication tag lengths in GCM mode
+    private static final int IV_LENGTH = 12;  // Standard IV size for AES-GCM
+    private static final int TAG_LENGTH = 16;  // Standard tag size for authentication
+
+    // Method to generate a symmetric key from two zpubs by hashing their sorted concatenation
+    public static byte[] getPairKey(String zpub1, String zpub2) throws Exception {
+        String[] pair = {zpub1, zpub2};  // Create array of the two zpubs
+        Arrays.sort(pair);  // Sort to ensure consistent order regardless of input
+        String concat = pair[0] + pair[1];  // Concatenate sorted zpubs
+        MessageDigest md = MessageDigest.getInstance("SHA-256");  // Get SHA-256 digest instance
+        return md.digest(concat.getBytes(StandardCharsets.UTF_8));  // Hash concatenation and return bytes
+    }
+
+    // Method to encrypt a descriptor string using AES-GCM with the given key
+    public static String encrypt(String descriptor, byte[] key) throws Exception {
+        SecretKeySpec skey = new SecretKeySpec(key, "AES");  // Create AES key from byte array
+        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");  // Get AES-GCM cipher instance
+        byte[] iv = new byte[IV_LENGTH];  // Create IV array
+        new SecureRandom().nextBytes(iv);  // Fill IV with secure random bytes
+        GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH * 8, iv);  // Create GCM spec with IV and tag bits
+        cipher.init(Cipher.ENCRYPT_MODE, skey, spec);  // Initialize cipher for encryption
+        byte[] ct = cipher.doFinal(descriptor.getBytes(StandardCharsets.UTF_8));  // Encrypt descriptor to ciphertext
+        byte[] tag = cipher.getAuthenticationTag();  // Get authentication tag (unused here, but typically from doFinal)
+        byte[] blob = new byte[IV_LENGTH + TAG_LENGTH + ct.length];  // Create output blob array
+        System.arraycopy(iv, 0, blob, 0, IV_LENGTH);  // Copy IV to start of blob
+        System.arraycopy(tag, 0, blob, IV_LENGTH, TAG_LENGTH);  // Copy tag after IV
+        System.arraycopy(ct, 0, blob, IV_LENGTH + TAG_LENGTH, ct.length);  // Copy ciphertext after tag
+        return Base64.getEncoder().encodeToString(blob);  // Base64 encode blob and return as string
+    }
+
+    // Method to decrypt an encrypted blob string using AES-GCM with the given key
+    public static String decrypt(String blobStr, byte[] key) throws Exception {
+        byte[] blob = Base64.getDecoder().decode(blobStr);  // Decode Base64 blob to bytes
+        if (blob.length < IV_LENGTH + TAG_LENGTH) throw new IllegalArgumentException();  // Check minimum length
+        byte[] iv = Arrays.copyOfRange(blob, 0, IV_LENGTH);  // Extract IV from blob
+        byte[] tag = Arrays.copyOfRange(blob, IV_LENGTH, IV_LENGTH + TAG_LENGTH);  // Extract tag
+        byte[] ct = Arrays.copyOfRange(blob, IV_LENGTH + TAG_LENGTH, blob.length);  // Extract ciphertext
+        SecretKeySpec skey = new SecretKeySpec(key, "AES");  // Create AES key
+        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");  // Get cipher instance
+        GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH * 8, iv);  // Create GCM spec
+        cipher.init(Cipher.DECRYPT_MODE, skey, spec);  // Initialize for decryption
+        cipher.setAuthenticationTag(tag);  // Set expected tag for verification (GCM-specific)
+        try {
+            return new String(cipher.doFinal(ct), StandardCharsets.UTF_8);  // Decrypt and return string
+        } catch (AEADBadTagException e) {
+            return null; // Failed decryption due to tag mismatch
+        }
+    }
+
+    // Comment block: Example usage for encryption and decryption in a 2-of-3 setup
+    // Usage example: String[] zpubs = {...}; String descriptor = "...";
+    // String[] blobs = new String[3];
+    // blobs[0] = encrypt(descriptor, getPairKey(zpubs[0], zpubs[1]));
+    // etc.
+    // To decrypt: given zpubA, zpubB, try decrypt each blob with getPairKey(A,B), return first non-null.
+}