Unable to establish Olm outbound session from fallback key (BAD_MAC error when verifying signature)

[BillCarsonFr]

Steps to reproduce

Try to establish an olm session from an OTK of type fallback

Intended result and actual result

Signature will fail, because the signable json is only using 'key' where it should use the all json and only remove unsigned and signatures.

Only happens with fallback key as a new fallback field is present in the dict:

{
    "fallback": true,
    "key": "dQp6YuFhLbZ4...wBq2deKNmdpewSzc",
    "signatures": {
        "@XXXX:matrix.org": {
            "ed25519:EXRRRRRR": "R1NiKUQd+ZhvxMPZiFgcG/6Lm5.....h6RkF2+mdopOw+W8UB4YRBA"
        }
    }
}

Current:

fun signalableJSONDictionary(): Map<String, Any> {
        return mapOf("key" to value)
    }

Expected from JS sdk:

    // prepare the canonical json: remove unsigned and signatures, and stringify with anotherjson
    const mangledObj = Object.assign({}, obj);
    if ("unsigned" in mangledObj) {
        delete mangledObj.unsigned;
    }
    delete mangledObj.signatures;
    const json = anotherjson.stringify(mangledObj);

Your phone model

No response

Operating system version

No response

Application version and app store

No response

Homeserver

No response

Will you send logs?

No

[BillCarsonFr]

iOS equivalent element-hq/element-ios#5120