defaultAbiDecoder doesn't work for encodePacked

[novaknole]

Hey @ricmoo @zemse

In remix:

function awesome(bytes32  nice, bytes memory good) public view returns(bytes memory){
        return abi.encodePacked(nice, good);
}

nice = 0x1010101010101010101010101010101010101010101010101010101010101010
good = 0x00000000000000000000

This gives back 0x101010101010101010101010101010101010101010101010101010101010101000000000000000000000

Then I do

const metadataABI = [
    'bytes32',
    'bytes'
];

const data = defaultAbiCoder.decode(metadataABI, "0x101010101010101010101010101010101010101010101010101010101010101000000000000000000000")

and error is:

Error: data out-of-bounds (length=42, offset=64, code=BUFFER_OVERRUN, version=abi/5.0.13)
    at Logger.makeError (index.js:195)
    at Logger.throwError (index.js:205)
    at Reader._peekBytes (abstract-coder.js:166)
    at Reader.readBytes (abstract-coder.js:181)
    at Reader.readValue (abstract-coder.js:189)
    at array.js:81
    at Array.forEach (<anonymous>)
    at unpack (array.js:77)
    at TupleCoder.decode (tuple.js:66)
    at AbiCoder.decode (abi-coder.js:126)
    at decode (Default.js:12)
    at decodeMetadata (dispute-metadata.js:10)
    at decode (subscription-hooks.js:220)
    at subscription-hooks.js:225
    at commitHookEffectListMount (react-dom.development.js:19607)
    at commitPassiveHookEffects (react-dom.development.js:19644)
    at HTMLUnknownElement.callCallback (react-dom.development.js:189)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:238)
    at invokeGuardedCallback (react-dom.development.js:291)
    at flushPassiveEffectsImpl (react-dom.development.js:22708)
    at unstable_runWithPriority (scheduler.development.js:656)
    at runWithPriority$1 (react-dom.development.js:11076)
    at flushPassiveEffects (react-dom.development.js:22676)
    at performSyncWorkOnRoot (react-dom.development.js:21591)
    at react-dom.development.js:11130
    at unstable_runWithPriority (scheduler.development.js:656)
    at runWithPriority$1 (react-dom.development.js:11076)
    at flushSyncCallbackQueueImpl (react-dom.development.js:11125)
    at flushSyncCallbackQueue (react-dom.development.js:11113)
    at scheduleUpdateOnFiber (react-dom.development.js:21053)
    at dispatchAction (react-dom.development.js:15633)
    at _ref5 (urql.es.js:67)
    at wonka.mjs:227
    at wonka.mjs:650
    at d (wonka.mjs:181)
    at d (wonka.mjs:71)
    at wonka.mjs:547
    at d (wonka.mjs:181)
    at wonka.mjs:577
    at wonka.mjs:595
    at wonka.mjs:779
    at wonka.mjs:369
    at wonka.mjs:660
    at Array.forEach (<anonymous>)
    at b (wonka.mjs:659)
    at wonka.es.js:340
    at wonka.es.js:281
    at wonka.es.js:914
    at b (wonka.mjs:119)
    at wonka.mjs:159

[zemse]

Heya! I don't think it is possible to properly decode all kinds of packed encodings as it is ambigious, take the following example:

function awesome(bytes32  nice, bytes memory good, bytes memory good2) public view returns(bytes memory){
        return abi.encodePacked(nice, good, good2);
}

nice = 0x1010101010101010101010101010101010101010101010101010101010101010
good = 0x00000000000000000000
good2 = 0x10000000000000000000

returnded value: 0x10101010101010101010101010101010101010101010101010101010101010100000000000000000000010000000000000000000

How since good and good2 are dynamic lengths, and their lengths are not encoded, there are multiple possible decodings that would encode to the same encoded value.

What you can do is:

1. manually implement that decoding, since you know that you're encoding one fixed length and one dynamic length.
2. don't encode packed, just use abi.encode, it would also encode the length information with ethers js can use to do the decode.

[ricmoo]

Exactly correct. You cannot (in general), decode non-standard packed data. Consider the following example:

> utils.solidityPack([ "string", "string" ], [ "Hello", "World" ])
"0x48656c6c6f576f726c64"
> utils.solidityPack([ "string", "string" ], [ "Hell", "oWorld" ])
"0x48656c6c6f576f726c64"

Both yield the same output, but any combination of strings which concatenate to "HelloWorld" collides.

This is one key reason why it cannot be used for many security applications (as creating colliding pre-images is trivial) and why it is generally recommended against. See the code example here in the docs for a slightly more complex example. Also see the solidity docs here for more details.