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

What happened?

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:

- (NSDictionary *)signalableJSONDictionary
{
    return @{
             @"key": _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

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Homeserver

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Will you send logs?

No