Matrix Signup

Matrix Signup

Get yourself onto a Matrix homeserver so the operator can DM you and invite you to rooms.

Prerequisites

Ask the operator for:

• Homeserver URL — e.g. https://matrix.example.com or tailnet-local http://matrix:8008
• Desired username — e.g. myagent (your local part, not the full @myagent:example.com)
• Password — pick one, or ask the operator

Optional, depending on the homeserver:

• Registration token (for invite-only public servers)
• Shared registration secret (for admin-locked homelab Synapse/Dendrite)

If the operator says "I'll just create the account for you" — skip to the Fallback section at the bottom.

Step 1: probe the homeserver

Figure out what registration flows the server accepts:

curl -s -X POST "$HOMESERVER/_matrix/client/v3/register" \
  -H 'Content-Type: application/json' -d '{}' | jq .

Look at the flows array in the response. Pick the first matching path below.

Step 2: register

Path A — open registration (m.login.dummy)

Simplest case. Just register with username + password:

curl -s -X POST "$HOMESERVER/_matrix/client/v3/register" \
  -H 'Content-Type: application/json' \
  -d '{
    "auth": {"type": "m.login.dummy"},
    "username": "myagent",
    "password": "...",
    "inhibit_login": false
  }' | jq .

Response includes user_id and access_token. Skip to Step 3.

Path B — registration token (m.login.registration_token)

Operator gave you a token:

curl -s -X POST "$HOMESERVER/_matrix/client/v3/register" \
  -H 'Content-Type: application/json' \
  -d '{
    "auth": {"type": "m.login.registration_token", "token": "TOKEN_FROM_OPERATOR"},
    "username": "myagent",
    "password": "..."
  }' | jq .

Path C — shared-secret admin registration

Used when POST /v3/register fails with M_FORBIDDEN or asks for recaptcha. Requires MATRIX_REGISTRATION_SHARED_SECRET from operator.

Endpoint depends on the homeserver:

• Synapse: /_synapse/admin/v1/register
• Dendrite: /_dendrite/admin/v1/register

Probe both if unsure — one will 404.

SECRET='...'  # from operator
USER='myagent'
PASS='...'
ADMIN='false'  # true only if the operator explicitly wants you to be admin

NONCE=$(curl -s "$HOMESERVER/_synapse/admin/v1/register" | jq -r .nonce)

MAC=$(printf '%s\x00%s\x00%s\x00%s' "$NONCE" "$USER" "$PASS" "$ADMIN" | \
  openssl dgst -sha1 -hmac "$SECRET" | awk '{print $2}')

curl -s -X POST "$HOMESERVER/_synapse/admin/v1/register" \
  -H 'Content-Type: application/json' \
  -d "{
    \"nonce\": \"$NONCE\",
    \"username\": \"$USER\",
    \"password\": \"$PASS\",
    \"admin\": $ADMIN,
    \"mac\": \"$MAC\"
  }" | jq .

The HMAC input is NUL-separated: nonce\0user\0pass\0admin where admin is the literal string true or false.

If Synapse 404s, retry with /_dendrite/admin/v1/register.

Step 3: log in to get an access token

If the register response didn't already give you one (or you want a fresh one):

curl -s -X POST "$HOMESERVER/_matrix/client/v3/login" \
  -H 'Content-Type: application/json' \
  -d '{
    "type": "m.login.password",
    "identifier": {"type": "m.id.user", "user": "myagent"},
    "password": "..."
  }' | jq .

Grab user_id (looks like @myagent:example.com) and access_token (looks like syt_...).

Step 4: write credentials to .kern/.env

Append — don't overwrite — these three lines:

MATRIX_HOMESERVER=https://matrix.example.com
MATRIX_USER_ID=@myagent:example.com
MATRIX_ACCESS_TOKEN=syt_...

Use edit or a small bash append with >>. Don't commit .kern/.env — it's gitignored.

Step 5: ask the operator to restart

The Matrix interface is only initialized on startup — it won't pick up the new env vars until the agent restarts.

Tell the operator:

I'm signed up as @myagent:example.com. Type /restart and I'll connect to Matrix. Then invite me to a room from your client.

Fallback — operator creates the account

If all registration paths fail (locked-down server, no secret, no token), ask the operator to do it themselves:

On Synapse: register_new_matrix_user -c homeserver.yaml On Dendrite: create-account --config dendrite.yaml --username myagent --password ... then hit /v3/login yourself to get a token.

Or ask them to log in as the new user once in Element and copy the access token from Settings → Help & About → Advanced.

Either way, you end up with MATRIX_USER_ID + MATRIX_ACCESS_TOKEN. Go to Step 4.

Troubleshooting

• M_USER_IN_USE — pick a different username
• M_FORBIDDEN on open register — registration is disabled; fall back to shared-secret or ask operator
• M_UNKNOWN_TOKEN after restart — token was revoked or server re-keyed; log in again to get a fresh one
• Homeserver URL vs server name — MATRIX_HOMESERVER is the HTTP base URL (with scheme + port). MATRIX_USER_ID uses the server's declared server_name, which may differ. Use whatever the login response returns.