Ethers.js: [question] How to convert address/bignumber to hex string?

[shanecarey17]

Hello, trying to implement the following tutorial in ethers https://medium.com/aigang-network/how-to-read-ethereum-contract-storage-44252c8af925 in order to load data from a "private" mapping.

With respect to the mapping example, how would the following code look in ethers?

index = '0000000000000000000000000000000000000000000000000000000000000005'
key =  '00000000000000000000000xbccc714d56bc0da0fd33d96d2a87b680dd6d0df6'
let newKey =  web3.sha3(key + index, {"encoding":"hex"})
console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('DEC: ' + web3.toDecimal(web3.eth.getStorageAt(contractAddress, newKey)))

I have tried the following which raises an error EXCEPTION Error: data out-of-bounds (length=32, offset=64, code=BUFFER_OVERRUN, version=abi/5.0.5):

let key = ethers.getAddress('0x...');
let offset = ethers.BigNumber.from(5);
let mappingKey = ethers.utils.keccak256([key, offset]);
let mappingData = await ethers.provider.getStorageAt(contractAddress, mappingKey, blockNumber);

Specifically, how does one convert the key and offset to appropriately sized hex strings for the hash function? Thanks in advance!

[ricmoo]

Heya!

You are on the right track. To accomplish the same thing in ethers, here is the demo re-written a bit, with more documentation added.

const { BigNumber, ethers, utils } = require("ethers");

(async function() {
    // Connect to Ropsten
    const provider = ethers.getDefaultProvider("ropsten");

    // The contract address
    const addr = "0xf1f5896ace3a78c347eb7eab503450bc93bd0c3b";

    // The key into the mapping which is located at the given Solidity slot
    const key = "0xbccc714d56bc0da0fd33d96d2a87b680dd6d0df6";
    const index = 5;

    // The pre-image used to compute the Storage location
    const newKeyPreimage = utils.concat([
        // Mappings' keys in Solidity must all be word-aligned (32 bytes)
        utils.hexZeroPad(key, 32),

        // Similarly with the slot-index into the Solidity variable layout
        utils.hexZeroPad(BigNumber.from(index).toHexString(), 32),
    ]);

    console.log("New Key Preimage:", utils.hexlify(newKeyPreimage));
    // "0x000000000000000000000000bccc714d56bc0da0fd33d96d2a87b680dd6d0df60000000000000000000000000000000000000000000000000000000000000005"

    const newKey = utils.keccak256(newKeyPreimage);
    console.log("New Key:", newKey);
    // "0xafef6be2b419f4d69d56fe34788202bf06650015554457a2470181981bcce7ef"

    const value = await provider.getStorageAt(addr, newKey);
    console.log("Value:", value);
    // "0x0000000000000000000000000000000000000000000000000000000000000058"
})();

I hope that helps! Let me know if you have any other questions. :)

[shanecarey17]

Thanks for the help! I am still getting the same error with the updated code:

let hexKey = ethers.utils.hexZeroPad(key, 32);
let hexIndex = ethers.utils.hexZeroPad(ethers.BigNumber.from(index).toHexString(), 32);
let mappingKeyPreimage = ethers.utils.concat([hexKey, hexIndex]);
let mappingKey = ethers.utils.keccak256(mappingKeyPreimage);

The error is

[Sat Oct 03 2020 16:42:21] [ERROR]  Error: data out-of-bounds (length=32, offset=64, code=BUFFER_OVERRUN, version=abi/5.0.5)
    at Logger.makeError (/app/node_modules/@ethersproject/logger/src.ts/index.ts:205:28)
    at Logger.throwError (/app/node_modules/@ethersproject/logger/src.ts/index.ts:217:20)
    at Reader._peekBytes (/app/node_modules/@ethersproject/abi/src.ts/coders/abstract-coder.ts:171:24)
    at Reader.readBytes (/app/node_modules/@ethersproject/abi/src.ts/coders/abstract-coder.ts:185:26)
    at Reader.readValue (/app/node_modules/@ethersproject/abi/src.ts/coders/abstract-coder.ts:192:36)
    at NumberCoder.decode (/app/node_modules/@ethersproject/abi/src.ts/coders/number.ts:44:28)
    at /app/node_modules/@ethersproject/abi/src.ts/coders/array.ts:108:31
    at Array.forEach (<anonymous>)
    at Object.unpack (/app/node_modules/@ethersproject/abi/src.ts/coders/array.ts:89:12)
    at TupleCoder.decode (/app/node_modules/@ethersproject/abi/src.ts/coders/tuple.ts:27:41) {
  reason: 'data out-of-bounds',
  code: 'BUFFER_OVERRUN',
  length: 32,
  offset: 64
}

I don't understand what the error is indicating. It looks like there is a fixed sized buffer somewhere. Is this because of my inputs? key is an address from ethers.utils.getAddress(...) and index is just an int.

[shanecarey17]

Also, utils.hexConcat does not appear to be a function as the docs suggest https://docs.ethers.io/v5/api/utils/bytes/#utils-hexConcat, I've tried it as a substitute to utils.concat.

[shanecarey17]

Ah! Your solution worked perfectly, I was dealing with parsing the return of getStorageAt but was confused because the stack trace did not show which function I had called. Obviously this is because its an async function, but I hadnt realized. Thanks!

[ricmoo]

Glad to hear it! :)

[ricmoo]

(I'll also be sure to export hexConcat in the next release too)