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fix: correct RevealSubstring LessThan bit widths #321

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fix: correct RevealSubstring LessThan bit widths #321

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8 changes: 4 additions & 4 deletions packages/circuits/helpers/reveal-substring.circom
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -47,4 +47,4 @@ template RevealSubstring(maxLength, maxSubstringLength, shouldCheckUniqueness) {
}

substring <== selectSubArray.out;
}
}
22 changes: 22 additions & 0 deletions packages/circuits/tests/reveal-substring-bit-width.test.ts
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Original file line number Diff line number Diff line change
@@ -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])",
);
});
});