From bb263cf89d95366d921f23be1991bcc9420f3f8c Mon Sep 17 00:00:00 2001
From: Randy-sin <randyxian08@gmail.com>
Date: Mon, 11 May 2026 15:26:51 +0800
Subject: [PATCH] Fix embedded mnemonic share index bits

---
 src/functions/bip39_mnemonic.js | 12 ++++++++----
 test/jsbtc.test.js              | 17 ++++++++++++++++-
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/functions/bip39_mnemonic.js b/src/functions/bip39_mnemonic.js
index 462579a..0ca7cc2 100644
--- a/src/functions/bip39_mnemonic.js
+++ b/src/functions/bip39_mnemonic.js
@@ -300,9 +300,13 @@ 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 checkSumBitLen = Math.ceil(e.length * 8 / 32);
+            let maxEmbeddedIndex = 2 ** checkSumBitLen - 1;
+            if (total > maxEmbeddedIndex)
+                throw new Error(`Maximum ${maxEmbeddedIndex} shares allowed for ${m.trim().split(/\s+/).length} mnemonic words`);
+            bits = Math.ceil(Math.log2(total + 1));
+        } else
             bits = 8;
 
         let shares = S.__split_secret(threshold, total, e, bits);
@@ -366,4 +370,4 @@ module.exports = function (S) {
         return S.entropyToMnemonic(S.__restore_secret(s), A);
     }
 
-};
\ No newline at end of file
+};
diff --git a/test/jsbtc.test.js b/test/jsbtc.test.js
index 99a4ce8..764a334 100644
--- a/test/jsbtc.test.js
+++ b/test/jsbtc.test.js
@@ -256,6 +256,22 @@ describe(`${(browser) ? 'Browser' : 'Node'} test jsbtc library`, function () {
             equal(combineMnemonic(shares), m);
 
         });
+        it('split mnemonic embedded indexes fit in checksum bits', () => {
+            let m = "stage amused wasp estate tomorrow outer satoshi version verb pudding ghost slender";
+            let shares = splitMnemonic(3, 15, m, {embeddedIndex: true, sharesVerify: true});
+            equal(shares.length, 15);
+            equal(combineMnemonic(shares), m);
+
+            for (let share of shares) {
+                let index = getMnemonicCheckSumData(share);
+                assert.isAtLeast(index, 1);
+                assert.isAtMost(index, 15);
+            }
+
+            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());
@@ -2926,4 +2942,3 @@ describe(`${(browser) ? 'Browser' : 'Node'} test jsbtc library`, function () {
 
 
 });
-