From be323056e11b29fc1d9416b2238d33300df6e4ee Mon Sep 17 00:00:00 2001
From: digzrow-coder <digzrow@gmail.com>
Date: Mon, 11 May 2026 18:27:04 +0800
Subject: [PATCH 1/2] Enhance embedded index logic in bip39_mnemonic.js

Refactor embedded index handling to include checksum bit length calculation and maximum shares validation.
---
 src/functions/bip39_mnemonic.js | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/functions/bip39_mnemonic.js b/src/functions/bip39_mnemonic.js
index 462579a..29c5c1b 100644
--- a/src/functions/bip39_mnemonic.js
+++ b/src/functions/bip39_mnemonic.js
@@ -300,9 +300,14 @@ module.exports = function (S) {
         let e =  S.mnemonicToEntropy(m, {wordList: A.wordList,
             checkSum: A.checkSumVerify, hex: false});
         let bits;
-        if (A.embeddedIndex)
-            bits = Math.ceil(Math.log2(total))+1;
-        else
+        if (A.embeddedIndex) {
+            let mWords = m.trim().split(/\s+/);
+            let checkSumBitLen = (mWords.length * 11) % 32;
+            let maxEmbeddedIndex = 2 ** checkSumBitLen - 1;
+            if (total > maxEmbeddedIndex)
+                throw new Error(`Maximum ${maxEmbeddedIndex} shares allowed for ${mWords.length} mnemonic words`);
+            bits = Math.ceil(Math.log2(total + 1));
+        } else
             bits = 8;
 
         let shares = S.__split_secret(threshold, total, e, bits);
@@ -366,4 +371,4 @@ module.exports = function (S) {
         return S.entropyToMnemonic(S.__restore_secret(s), A);
     }
 
-};
\ No newline at end of file
+};

From 645f1fd1abbf120c4c203f3fdf160c5a6ad1005c Mon Sep 17 00:00:00 2001
From: digzrow-coder <digzrow@gmail.com>
Date: Mon, 11 May 2026 18:28:02 +0800
Subject: [PATCH 2/2] Enhance tests for splitMnemonic function

Add test for splitMnemonic with embedded indexes and checksum capacity
---
 test/jsbtc.test.js | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/test/jsbtc.test.js b/test/jsbtc.test.js
index 99a4ce8..850780c 100644
--- a/test/jsbtc.test.js
+++ b/test/jsbtc.test.js
@@ -256,6 +256,14 @@ describe(`${(browser) ? 'Browser' : 'Node'} test jsbtc library`, function () {
             equal(combineMnemonic(shares), m);
 
         });
+        it('split mnemonic with embedded indexes honors checksum capacity', () => {
+            let m = 'stage amused wasp estate tomorrow outer satoshi version verb pudding ghost slender';
+            let shares = splitMnemonic(3, 15, m, {embeddedIndex: true});
+            equal(shares.length, 15);
+            equal(combineMnemonic(shares), m);
+            expect(() => splitMnemonic(3, 16, m, {embeddedIndex: true}))
+                .to.throw('Maximum 15 shares allowed for 12 mnemonic words');
+        });
         it('split mnemonic with embedded indexes extensive test', () => {
 
             let m = entropyToMnemonic(generateEntropy());