From 771b665321b7f993a1659b2cf81204ff5f90e97c Mon Sep 17 00:00:00 2001
From: Micheal KENECHUKWU <ezemicheal298@gmail.com>
Date: Mon, 11 May 2026 10:05:28 +0100
Subject: [PATCH] Fix embedded mnemonic share indexes

---
 src/functions/bip39_mnemonic.js        |  4 ++--
 src/functions/shamir_secret_sharing.js | 19 ++++++++++++++++---
 test/jsbtc.test.js                     | 15 +++++++++++++++
 3 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/src/functions/bip39_mnemonic.js b/src/functions/bip39_mnemonic.js
index 462579a..592fa61 100644
--- a/src/functions/bip39_mnemonic.js
+++ b/src/functions/bip39_mnemonic.js
@@ -301,7 +301,7 @@ module.exports = function (S) {
             checkSum: A.checkSumVerify, hex: false});
         let bits;
         if (A.embeddedIndex)
-            bits = Math.ceil(Math.log2(total))+1;
+            bits = S.__secret_index_bits(e);
         else
             bits = 8;
 
@@ -366,4 +366,4 @@ module.exports = function (S) {
         return S.entropyToMnemonic(S.__restore_secret(s), A);
     }
 
-};
\ No newline at end of file
+};
diff --git a/src/functions/shamir_secret_sharing.js b/src/functions/shamir_secret_sharing.js
index def4f16..a4af6cf 100644
--- a/src/functions/shamir_secret_sharing.js
+++ b/src/functions/shamir_secret_sharing.js
@@ -85,11 +85,24 @@ module.exports = function (S) {
     };
 
 
-    S.__split_secret = (threshold, total,  secret, indexBits=8) => {
+    S.__secret_index_bits = (secret) => {
+        const checksumBitsByEntropyLength = {
+            16: 4,
+            20: 5,
+            24: 6,
+            28: 7,
+            32: 8,
+        };
+
+        return checksumBitsByEntropyLength[secret.length] || 8;
+    };
+
+    S.__split_secret = (threshold, total,  secret, indexBits=null) => {
         if (threshold > 255) throw new Error("threshold limit 255");
         if (total > 255) throw new Error("total limit 255");
+        if (indexBits === null) indexBits = S.__secret_index_bits(secret);
         let index_mask = 2**indexBits - 1;
-        if (total > index_mask) throw new Error("index bits is to low");
+        if (total > index_mask) throw new Error("index bits is too low");
         if (threshold > total) throw new Error("invalid threshold");
         let shares = {};
         let sharesIndexes = [];
@@ -167,4 +180,4 @@ module.exports = function (S) {
     };
 
     S.__precompute_GF256_expLog(S);
-};
\ No newline at end of file
+};
diff --git a/test/jsbtc.test.js b/test/jsbtc.test.js
index 99a4ce8..af06fbc 100644
--- a/test/jsbtc.test.js
+++ b/test/jsbtc.test.js
@@ -256,6 +256,21 @@ describe(`${(browser) ? 'Browser' : 'Node'} test jsbtc library`, function () {
             equal(combineMnemonic(shares), m);
 
         });
+        it('split mnemonic with embedded indexes uses checksum-sized share indexes', () => {
+            let m = 'gift unfold street machine glue spring spin energy assist stereo liar ramp';
+            let shares = splitMnemonic(3, 15, m, {embeddedIndex: true});
+
+            equal(shares.length, 15);
+
+            for (let share of shares) {
+                let index = getMnemonicCheckSumData(share);
+                equal(index > 0, true);
+                equal(index <= 15, true);
+            }
+
+            equal(combineMnemonic(shares.slice(0, 3)), m);
+            expect(() => splitMnemonic(3, 16, m, {embeddedIndex: true})).to.throw('index bits is too low');
+        });
         it('split mnemonic with embedded indexes extensive test', () => {
 
             let m = entropyToMnemonic(generateEntropy());