800 characters for "hello world", not including assertion method:

```
{
"@context": [
"https://www.w3.org/ns/activitystreams",
"https://w3id.org/security/data-integrity/v1"
],
"id": "https://server.example/activities/1",
"type": "Create",
"actor": "https://server.example/users/alice",
"object": {
"id": "https://server.example/objects/1",
"type": "Note",
"attributedTo": "https://server.example/users/alice",
"content": "Hello world"
},
"proof": {
"@context": [
"https://www.w3.org/ns/activitystreams",
"https://w3id.org/security/data-integrity/v1"
],
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-jcs-2022",
"verificationMethod": "https://server.example/users/alice#ed25519-key",
"proofPurpose": "assertionMethod",
"proofValue": "...",
"created": "2023-02-24T23:36:38Z"
}
}
```