From 9856d30741820aefe9daec30461909469193e193 Mon Sep 17 00:00:00 2001
From: leo1987820 <leo1987820@users.noreply.github.com>
Date: Sat, 27 Jun 2026 00:37:48 +0800
Subject: [PATCH] fix reveal substring LessThan bit widths

---
 .../circuits/helpers/reveal-substring.circom  |  8 +++----
 .../tests/reveal-substring-bit-width.test.ts  | 22 +++++++++++++++++++
 2 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 packages/circuits/tests/reveal-substring-bit-width.test.ts

diff --git a/packages/circuits/helpers/reveal-substring.circom b/packages/circuits/helpers/reveal-substring.circom
index f3197e71..f6fd9d8c 100644
--- a/packages/circuits/helpers/reveal-substring.circom
+++ b/packages/circuits/helpers/reveal-substring.circom
@@ -20,16 +20,16 @@ template RevealSubstring(maxLength, maxSubstringLength, shouldCheckUniqueness) {
     signal output substring[maxSubstringLength];
 
     // Substring start index should be less than maxLength
-    signal isSubstringStartIndexValid <== LessThan(log2Ceil(maxLength))([substringStartIndex, maxLength]);
+    signal isSubstringStartIndexValid <== LessThan(log2Ceil(maxLength + 1))([substringStartIndex, maxLength]);
     isSubstringStartIndexValid === 1;
 
     // Substring length should be less than maxSubstringLength + 1
-    signal isSubstringLengthValid <== LessThan(log2Ceil(maxSubstringLength + 1))([substringLength, maxSubstringLength + 1]);
+    signal isSubstringLengthValid <== LessThan(log2Ceil(maxSubstringLength + 2))([substringLength, maxSubstringLength + 1]);
     isSubstringLengthValid === 1;
 
     // substring index + substring length should be less than maxLength + 1
     signal sum <== substringStartIndex + substringLength;
-    signal isSumValid <== LessThan(log2Ceil(maxLength + 1))([sum, maxLength + 1]);
+    signal isSumValid <== LessThan(log2Ceil(maxLength + 2))([sum, maxLength + 1]);
     isSumValid === 1;
 
     // Extract the substring
@@ -47,4 +47,4 @@ template RevealSubstring(maxLength, maxSubstringLength, shouldCheckUniqueness) {
     }
 
     substring <== selectSubArray.out;
-}
\ No newline at end of file
+}
diff --git a/packages/circuits/tests/reveal-substring-bit-width.test.ts b/packages/circuits/tests/reveal-substring-bit-width.test.ts
new file mode 100644
index 00000000..30a38bac
--- /dev/null
+++ b/packages/circuits/tests/reveal-substring-bit-width.test.ts
@@ -0,0 +1,22 @@
+import fs from "fs";
+import path from "path";
+
+describe("RevealSubstring LessThan bit widths", () => {
+  it("allocates one extra value for exclusive upper-bound constants", () => {
+    const source = fs.readFileSync(
+      path.join(__dirname, "../helpers/reveal-substring.circom"),
+      "utf8",
+    );
+    const compactSource = source.replace(/\s+/g, "");
+
+    expect(compactSource).toContain(
+      "LessThan(log2Ceil(maxLength+1))([substringStartIndex,maxLength])",
+    );
+    expect(compactSource).toContain(
+      "LessThan(log2Ceil(maxSubstringLength+2))([substringLength,maxSubstringLength+1])",
+    );
+    expect(compactSource).toContain(
+      "LessThan(log2Ceil(maxLength+2))([sum,maxLength+1])",
+    );
+  });
+});