proofValue is not in MULTIBASE format.

[ellellie]

As per Verifiable Credential Data Integrity 1.0, the proofValue MUST be expressed as in the Multibase encoding format.

This is not done in the code, see https://github.com/mattrglobal/jsonld-signatures-bbs/blob/master/src/BbsBlsSignature2020.ts#L379

Basic reproduction:

import { BbsBlsSignature2020, Bls12381G2KeyPair } from '@mattrglobal/jsonld-signatures-bbs'
import { sign, purposes } from "jsonld-signatures";

const keyPair = new Bls12381G2KeyPair({
    "id": "did:example:489398593#test",
    "controller": "did:example:489398593",
    "privateKeyBase58": "5D6Pa8dSwApdnfg7EZR8WnGfvLDCZPZGsZ5Y1ELL9VDj",
    "publicKeyBase58": "oqpWYKaZD9M1Kbe94BVXpr8WTdFBNZyKv48cziTiQUeuhm7sBhCABMyYG4kcMrseC68YTFFgyhiNeBKjzdKk9MiRWuLv5H4FFujQsQK2KTAtzU8qTBiZqBHMmnLF4PL7Ytu"
});

const documentLoader = async (url: string): Promise<any> => {
    // HACK: Just a quick hack to get the context.
    if (url.startsWith('http')) {
        const res = await fetch(url);
        const linkHeader = res.headers.get('Link')
        if(linkHeader) {
            const parts = linkHeader.split(';')
            const contextUrl = parts[0].replace('<', '').replace('>', '')

            return documentLoader(url + contextUrl)
        }

        return {
            contextUrl: null,
            document: await res.json(),
            documentUrl: url
        }
    }
}

const inputDocument = {
    '@context': [
        'https://schema.org',
        'https://w3id.org/security/bbs/v1'
    ],
    '@type': 'Person',
    'name': 'John Doe'
}

const signedDocument = await sign(inputDocument, {
    suite: new BbsBlsSignature2020({ key: keyPair }),
    purpose: new purposes.AssertionProofPurpose(),
    documentLoader
})

console.log(signedDocument)

Results in:

{
  "@context": [
    "https://schema.org",
    "https://w3id.org/security/bbs/v1"
  ],
  "@type": "Person",
  "name": "John Doe",
  "https://w3id.org/security#proof": {
    "@graph": {
      "type": "BbsBlsSignature2020",
      "created": "2024-01-08T02:17:50Z",
      "proofPurpose": "assertionMethod",
      "proofValue": "rCZ65PkLqdlYHqOJoLtPW/KdfoacLIWZ/EA79DfgygAGRQx5Uiihtno+apCmpMfjWbwMfQxOYveLVux+muOsBGn/CuxGchB3cPozh2rRtO4nvDvkL4XqCp9662Yo9GgwqcuQBY55Y6mTfz3ifaZbKg==",
      "verificationMethod": "did:example:489398593#test"
    }
  }
}

Instead, the proofValue should be expressed as RFC4648 (base64url), with no padding, and prefixed with u to denote the format.