synapse/docs/admin_api/register_api.md

# Shared-Secret Registration

This API allows for the creation of users in an administrative and
non-interactive way. This is generally used for bootstrapping a Synapse
instance with administrator accounts.

To authenticate yourself to the server, you will need both the shared secret
(`registration_shared_secret` in the homeserver configuration), and a
one-time nonce. If the registration shared secret is not configured, this API
is not enabled.

To fetch the nonce, you need to request one from the API:

```
> GET /_synapse/admin/v1/register

< {"nonce": "thisisanonce"}
```

Once you have the nonce, you can make a `POST` to the same URL with a JSON
body containing the nonce, username, password, whether they are an admin
(optional, False by default), and a HMAC digest of the content. Also you can
set the displayname (optional, `username` by default).

As an example:

```
> POST /_synapse/admin/v1/register
> {
   "nonce": "thisisanonce",
   "username": "pepper_roni",
   "displayname": "Pepper Roni",
   "password": "pizza",
   "admin": true,
   "mac": "mac_digest_here"
  }

< {
   "access_token": "token_here",
   "user_id": "@pepper_roni:localhost",
   "home_server": "test",
   "device_id": "device_id_here"
  }
```

The MAC is the hex digest output of the HMAC-SHA1 algorithm, with the key being
the shared secret and the content being the nonce, user, password, either the
string "admin" or "notadmin", and optionally the user_type
each separated by NULs. For an example of generation in Python:

```python
import hmac, hashlib

def generate_mac(nonce, user, password, admin=False, user_type=None):

    mac = hmac.new(
      key=shared_secret,
      digestmod=hashlib.sha1,
    )

    mac.update(nonce.encode('utf8'))
    mac.update(b"\x00")
    mac.update(user.encode('utf8'))
    mac.update(b"\x00")
    mac.update(password.encode('utf8'))
    mac.update(b"\x00")
    mac.update(b"admin" if admin else b"notadmin")
    if user_type:
        mac.update(b"\x00")
        mac.update(user_type.encode('utf8'))

    return mac.hexdigest()
```