From 98362d5449082756cdf9b217228629db8321025f Mon Sep 17 00:00:00 2001
From: Guillem <guillem.moreno.garcia@estudiantat.upc.edu>
Date: Mon, 11 May 2026 09:37:37 +0200
Subject: [PATCH] fix embedded mnemonic share index bits

---
 src/functions/bip39_mnemonic.js | 12 ++++++++----
 test/jsbtc.test.js              |  4 ++++
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/functions/bip39_mnemonic.js b/src/functions/bip39_mnemonic.js
index 462579a..d6f9a41 100644
--- a/src/functions/bip39_mnemonic.js
+++ b/src/functions/bip39_mnemonic.js
@@ -297,12 +297,16 @@ module.exports = function (S) {
                  sharesVerify: false,
                  embeddedIndex: false,
                  hex: true});
+        let wordCount = m.trim().split(/\s+/).length;
         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 availableBits = e.length * 8 / 32;
+            bits = Math.ceil(Math.log2(total + 1));
+            if (bits > availableBits)
+                throw new Error(`Maximum ${2 ** availableBits - 1} shares allowed for ${wordCount} mnemonic words`);
+        } 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..4cc4096 100644
--- a/test/jsbtc.test.js
+++ b/test/jsbtc.test.js
@@ -273,6 +273,10 @@ describe(`${(browser) ? 'Browser' : 'Node'} test jsbtc library`, function () {
             s = splitMnemonic(3, 8, m, {embeddedIndex: true});
 
             equal(m, combineMnemonic(s));
+            s = splitMnemonic(3, 15, m, {embeddedIndex: true});
+            equal(m, combineMnemonic(s.slice(0, 3)));
+            expect(() => splitMnemonic(3, 16, m, {embeddedIndex: true}))
+                .to.throw('Maximum 15 shares allowed for 12 mnemonic words');
 
             for (let q = 0; q < 10; q++) {
                 let t =  Math.floor(Math.random() * (10  )) + 2;