bug in utils.leInt2Buff

[twister-dev]

Description of the Problem

Using leInt2Buff returns inaccurate results depending on the length of the input. I discovered this when constructing a buffer from a small value, eg BigInt(42). When using leInt2Buff(BigInt(42)), the result is Uint8Array(1) [1], whereas the expected result is Uint8Array(1) [42].

My solution in this case is to strictly specify the length passed into leInt2Buff (and to make sure its a multiple of 4). That being said, this issue might go undetected by a consumer of this library, and that could potentially impact the security of a zero knowledge application.

For example, if someone were to write a zk app that uses the pedersenHash of a value that is less than 4 bytes long (up to 2^32). If the primage depends on a Buffer constructed from this function there will be unintentional collisions, because for example leInt2Buff(BigInt(42)) == leInt2Buff(BigInt(216)) == Uint8Array(1) [1] (and there are more repeated patterns up until you exceed 4 bytes, then the pattern changes again).

Steps to Reproduce

Run this script.

const { leInt2Buff } = require('ffjavascript').utils;

const num = BigInt(42);
console.log('num: ', num);
console.log('leInt2Buff len = undefined: ', leInt2Buff(num));
for (let i = 1; i < 33; i++) {
    console.log(`leInt2Buff len = ${i}: `, leInt2Buff(num, i));
}

The truncated results of the above script:

num:  42n
leInt2Buff len = undefined:  Uint8Array(1) [ 1 ]
leInt2Buff len = 1:  Uint8Array(1) [ 1 ]
leInt2Buff len = 2:  Uint8Array(2) [ 0, 1 ]
leInt2Buff len = 3:  Uint8Array(3) [ 0, 1, 1 ]
leInt2Buff len = 4:  Uint8Array(4) [ 42, 0, 0, 0 ]
leInt2Buff len = 5:  Uint8Array(5) [ 42, 0, 0, 0, 1 ]
leInt2Buff len = 6:  Uint8Array(6) [ 42, 0, 0, 0, 0, 1 ]
leInt2Buff len = 7:  Uint8Array(7) [
  42, 0, 0, 0,
   0, 1, 1
]
leInt2Buff len = 8:  Uint8Array(8) [
  42, 0, 0, 0,
   0, 0, 0, 0
]

The pattern of ones at the end of the array continues so on and so forth up until len = 32.

Here's another script, this time comparing 216 vs 42:

const num1 = BigInt(216);
console.log('num1: ', num1);
console.log('leInt2Buff len = undefined: ', leInt2Buff(num1));
console.log('leInt2Buff len = 32: ', leInt2Buff(num1, 32));

const num2 = BigInt(42);
console.log('num2: ', num2);
console.log('leInt2Buff len = undefined: ', leInt2Buff(num2));
console.log('leInt2Buff len = 32: ', leInt2Buff(num2, 32));

num1:  216n
leInt2Buff len = undefined:  Uint8Array(1) [ 1 ]
leInt2Buff len = 32:  Uint8Array(32) [
  216, 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,
    0, 0, 0, 0, 0
]
num2:  42n
leInt2Buff len = undefined:  Uint8Array(1) [ 1 ]
leInt2Buff len = 32:  Uint8Array(32) [
  42, 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,
   0, 0, 0, 0, 0
]

I ran this script in a local project with the following package.json:

{
  "dependencies": {
    "@ethersproject/bignumber": "^5.5.0",
    "@ethersproject/wallet": "^5.5.0",
    "@types/node": "^17.0.19",
    "circomlibjs": "^0.1.1",
    "ffjavascript": "^0.2.48",
    "typescript": "^4.5.5"
  }
}

[phated]

I believe this was solved by #72 - please let us know if you still see a problem.