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Jul 23, 2025
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fix(core/cbor): calculation of bigInteger offset #1651

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5 changes: 5 additions & 0 deletions .changeset/clever-shirts-prove.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"@smithy/core": patch
---

fix offset calculation when decoding bigInteger in CBOR
36 changes: 29 additions & 7 deletions packages/core/src/submodules/cbor/cbor-decode.ts
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
import { NumericValue } from "@smithy/core/serde";
import { nv } from "@smithy/core/serde";
import { toUtf8 } from "@smithy/util-utf8";

import {
Expand Down Expand Up @@ -129,22 +129,44 @@ export function decode(at: Uint32, to: Uint32): CborValueType {
for (let i = start; i < start + length; ++i) {
b = (b << BigInt(8)) | BigInt(payload[i]);
}

_offset = offset + length;
// the new offset is the sum of:
// 1. the local major offset (1)
// 2. the offset of the decoded count of the bigInteger
// 3. the length of the data bytes of the bigInteger
_offset = offset + _offset + length;
return minor === 3 ? -b - BigInt(1) : b;
} else if (minor === 4) {
const decimalFraction = decode(at + offset, to);
const [exponent, mantissa] = decimalFraction;
const s = mantissa.toString();
const numericString = exponent === 0 ? s : s.slice(0, s.length + exponent) + "." + s.slice(exponent);
const normalizer = mantissa < 0 ? -1 : 1;
const mantissaStr = "0".repeat(Math.abs(exponent) + 1) + String(BigInt(normalizer) * BigInt(mantissa));

let numericString: string;
const sign = mantissa < 0 ? "-" : "";

numericString =
exponent === 0
? mantissaStr
: mantissaStr.slice(0, mantissaStr.length + exponent) + "." + mantissaStr.slice(exponent);
numericString = numericString.replace(/^0+/g, "");
if (numericString === "") {
numericString = "0";
}
if (numericString[0] === ".") {
numericString = "0" + numericString;
}
numericString = sign + numericString;

return new NumericValue(numericString, "bigDecimal");
// the new offset is the sum of:
// 1. the local major offset (1)
// 2. the offset of the decoded exponent mantissa pair
_offset = offset + _offset;
return nv(numericString);
} else {
const value = decode(at + offset, to);
const valueOffset = _offset;

_offset = offset + valueOffset;

return tag({ tag: castBigInt(unsignedInt), value });
}
}
Expand Down
92 changes: 88 additions & 4 deletions packages/core/src/submodules/cbor/cbor.spec.ts
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@@ -1,4 +1,4 @@
import { NumericValue } from "@smithy/core/serde";
import { NumericValue, nv } from "@smithy/core/serde";
import * as fs from "fs";
// @ts-ignore
import JSONbig from "json-bigint";
Expand Down Expand Up @@ -219,12 +219,19 @@ describe("cbor", () => {
name: "object containing big numbers",
data: {
map: {
items: [BigInt(1e80)],
items: [BigInt(1e80), BigInt(1e80), nv("0.0000000001234000000001234"), nv("0.0000000001234000000001234")],
bigint: BigInt(1e80),
bigDecimal: nv("0.0000000001234000000001234"),
},
},
cbor: allocByteArray([
161, 99, 109, 97, 112, 161, 101, 105, 116, 101, 109, 115, 129, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
161, 99, 109, 97, 112, 163, 101, 105, 116, 101, 109, 115, 132, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 194, 88, 34, 3, 95, 157, 234, 62,
31, 107, 224, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 196, 130, 56, 24,
27, 0, 4, 98, 81, 3, 167, 36, 210, 196, 130, 56, 24, 27, 0, 4, 98, 81, 3, 167, 36, 210, 102, 98, 105, 103, 105,
110, 116, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 106, 98, 105, 103, 68, 101, 99, 105, 109, 97, 108, 196, 130, 56, 24, 27, 0, 4, 98, 81,
3, 167, 36, 210,
]),
},
];
Expand Down Expand Up @@ -305,6 +312,83 @@ describe("cbor", () => {
}
});

it("should round-trip sequences of big numbers", () => {
const sequence = {
map: {
items1: [
BigInt(1e20),
BigInt(2e30),
BigInt(3e40),
BigInt(4e50),
BigInt(5e60),
BigInt(6e70),
BigInt(7e80),
BigInt(8e90),
],
items2: [
nv("111.00000000000000000001"),
nv("0.000000000000000000002"),
nv("333.0000000000000000000003"),
nv("0.00000000000000000000004"),
nv("-555.000000000000000000000005"),
nv("-0.0000000000000000000000006"),
nv("-777.00000000000000000000000007"),
nv("-0.000000000000000000000000008"),
],
items3: [nv("0.0000000001234000000001234"), nv("0.00000000678678001234"), BigInt(1e20), BigInt(2e30)],
items4: [
BigInt(1e20),
BigInt(2e30),
nv("0.0000000001234000000001234"),
nv("0.0000067867867801234"),
BigInt(1e20),
BigInt(2e30),
nv("0.0000000001234000000001234"),
nv("1.000000000123678678678234"),
],
items5: [
nv("0.0000000001234000000001234"),
nv("0.00006786781234678678678"),
BigInt(1e20),
BigInt(2e30),
nv("0.0000000001234000000001234"),
nv("0.000000000123400000087678634"),
BigInt(1e20),
BigInt(2e30),
],
items6: [
nv("10469069930579305970359073950793057903597035970395069240692049609"),
nv("99130490139501395091035901395031950.4928053468045683958609485649280534680456839586094856"),
nv("1.000135135000103501305000000000004928053468045683958609485649280534680456839586094856"),
nv("1.00013513500010350130500000000000"),
nv("0.0000000001234000000001234"),
nv("0.0000001"),
nv("0.00001"),
nv("0.001"),
nv("0.000000"),
nv("0.0000"),
nv("0.00"),
nv("0.0"),
nv("0"),
nv("-0.1"),
nv("-0.01"),
nv("-0.000000000123400000087678634"),
nv("-0.0000000001234000000876786340000000000000000000000000000"),
nv("-1.000135135000103501305000000000004928053468045683958609485649280534680456839586094856"),
nv("-1.00013513500010350130500000000000"),
nv("-100305096350939057390735093.0000000001234000000001234"),
nv("-104695047960794069730590793057.0"),
nv("-104695047960794069730590793057"),
],
},
};

const serialized = cbor.serialize(sequence);
const deserialized = cbor.deserialize(serialized);

expect(deserialized).toEqual(sequence);
});

it("should throw an error if serializing a tag with missing properties", () => {
expect(() =>
cbor.serialize({
Expand Down
8 changes: 6 additions & 2 deletions packages/core/src/submodules/serde/value/NumericValue.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -53,15 +53,19 @@ export class NumericValue {
return this.string;
}

public [Symbol.hasInstance](object: unknown) {
public static [Symbol.hasInstance](object: unknown) {
if (!object || typeof object !== "object") {
return false;
}
const _nv = object as NumericValue;
const prototypeMatch = NumericValue.prototype.isPrototypeOf(object.constructor?.prototype);
if (prototypeMatch) {
return prototypeMatch;
}
if (typeof _nv.string === "string" && typeof _nv.type === "string" && _nv.constructor?.name === "NumericValue") {
return true;
}
return false;
return prototypeMatch;
}
}

Expand Down
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