propose did-onion also for local verifications

## Type

Discussion
## Abstract

This issue documents how to extract Ed25519 public key from an onion address which could be used for local verification.

A Tor onion service requires an `Ed25519` keypair to operate. In fact, Tor v3 onion address is essentially an `Ed25519` public key. This property allows us to use `did:onion` (e.g. concatenated with #allow_local_verification or without any id) as a `verification method` that does not require Tor or Internet. Alternatively, we can convert `did:onion` to `did:key`.

## Introduction

Onion v3 address is 62 characters long (`fscst5exmlmr262byztwz4kzhggjlzumvc2ndvgytzoucr2tkgxf7mid.onion`) and is
defined as

```bash
onion_address = base32(Ed25519_pubkey | CHECKSUM | VERSION) + ".onion"
CHECKSUM = sha3_256(".onion checksum" | Ed25519_pubkey | VERSION)[:2]
```

where | represents byte concatenation, + string concatenation, version is 3
and the checksum is the first two bytes `[:2]` of the `sha3_256` digest.

So, to extract the `Ed25519_pubkey` we decode the first 56 characters of the onion address according to `base32` with `RFC4648
alphabet` and `no padding`. `Ed25519_pubkey` is represented by the first 32 bytes which can be converted into `JWK` or any other format.

## Example of a Key Conversion

Decoding `fscst5exmlmr262byztwz4kzhggjlzumvc2ndvgytzoucr2tkgxf7mid.onion` into `Ed25519` public key results in

```bash
[44, 133, 41, 244, 151, 98, 217, 29, 123, 65, 198, 103, 108, 241, 89, 57, 140, 149, 230, 140, 168, 180, 209, 212, 216, 158, 93, 65, 71, 83, 81, 174]
```

From here we can convert into different formats:

```json
"publicKeyBase58": "3zni9pmFAWvFvuGCnnMVSRxYkjv5gkFMKsMsAtqX1SJq"
```

```json
"publicKeyJwk": {
"crv": "Ed25519",
"kty": "OKP",
"x": "LIUp9Jdi2R17QcZnbPFZOYyV5oyotNHU2J5dQUdTUa4"
}
```

## Example of a VC

[TODO: make a smaller example and a good signature]

```json
{
"@context": [
"https://www.w3.org/2018/credentials/v1",
{
"SoftwareSourceCode": "https://schema.org/SoftwareSourceCode",
"creator": "https://schema.org/creator",
"codeRepository": "https://schema.org/codeRepository",
"version": "https://schema.org/version",
"programmingLanguage": "https://schema.org/programmingLanguage"
}
],
"id": "urn:uuid:3f68f17e-c019-4193-9899-c1a8f2cafebd",
"type": [
"VerifiableCredential",
"SoftwareSourceCode"
],
"credentialSubject": {
"id": "urn:uuid:0c3049ec-433f-47cf-b502-6e25d73d2b04",
"programmingLanguage": "C++",
"creator": {
"id": "did:onion:fscst5exmlmr262byztwz4kzhggjlzumvc2ndvgytzoucr2tkgxf7mid",
"type": "Person|Organization"
},
"codeRepository": "https://github.com/BlockchainCommons/bc-seedtool-cli.git",
"version": "6b5b64afd16db4b075bfe68d06f6d58f3189f13a"
},
"issuer": "did:onion:fscst5exmlmr262byztwz4kzhggjlzumvc2ndvgytzoucr2tkgxf7mid",
"issuanceDate": "2021-01-10T19:23:24Z",
"proof": {
"type": "Ed25519Signature2018",
"proofPurpose": "assertionMethod",
"verificationMethod": "did:onion:fscst5exmlmr262byztwz4kzhggjlzumvc2ndvgytzoucr2tkgxf7mid",
"created": "2021-01-27T03:00:50.565Z",
"jws": "eyJhbGciOiJFZERTQSIsImNyaXQiOlsiYjY0Il0sImI2NCI6ZmFsc2V9..s8OLNCqDkwVhnRbyWewr3jNiPnhMoUdsp3-eOSGK6JlfrhsXDnTARZEhdIDaUqZ1iOBWRctWm0ecVvnq4I6xAw"
}
}
```

## Advantages

Besides online verification this method can also allow a verification without a Tor protocol or internet connection which is useful for many devices, including embedded devices.

## Downside

Tor daemon automatically generates an `Ed25519` keypair for an onion service. The private key is stored in an `expanded format` ([1](https://github.com/dalek-cryptography/ed25519-dalek/blob/925eb9ea56192053c9eb93b9d30d1b9419eee128/src/secret.rs#L200)) though, which makes it inconvertible to `JWK` format ([2](https://tools.ietf.org/html/rfc7517)). If a user wants to be able to make such a conversion it is recommended to generate a keypair outside of Tor and do conversions into any format from there.

EDIT: the major downside is the did method cannot be revoked. So probably this option is not viable.