# HDTP

> HDTP in five lines, and where the specification, its MUSTs and its test vectors are published.

The normative text of the protocol is published at [hdtp.io/spec](https://hdtp.io/spec/), with every MUST listed at [/spec/musts](https://hdtp.io/spec/musts/) and the test vectors at [/spec/vectors](https://hdtp.io/spec/vectors/). What follows is the protocol repository's own summary.

1. **Identity** — a self-signed root certificate you hold, pinned by its fingerprint. The host you choose serves you under a leaf your root issued for its address, valid for as long as you choose up to 398 days; contacts learn each renewed leaf from the chain, carried once and named by fingerprint after. Every call is mTLS with the leaf key.
2. **Contacts** — standard vCards with `X-HDTP-CERT`; shared over channels people already use; always mutual, always human-approved.
3. **Invites** — short URLs/QRs whose settings (expiry, max uses, auto-accept, preset) live server-side, so they're revocable at the protocol level.
4. **Capabilities** — everything a contact may do is an MCP tool, filtered per caller via `tools/list`; new integrations are just new tools.
5. **Delivery** — direct HTTPS, always; there is no relay. A person who must be reachable while their own machine is off is hosted by a provider under a leaf they issued and can leave (§9).

## Learn the protocol

The guided pages on hdtp.io, one per topic:

- [Certificates and renewal](https://hdtp.io/learn/certificates-and-renewal/)
- [Contacts and invites](https://hdtp.io/learn/contacts-and-invites/)
- [HDTP, MCP and A2A](https://hdtp.io/learn/hdtp-mcp-and-a2a/)
- [Hosting and moving](https://hdtp.io/learn/hosting-and-moving/)
- [Identity: root, leaf, passkey](https://hdtp.io/learn/identity/)
- [Messages and threads](https://hdtp.io/learn/messages-and-threads/)
- [Permissions and tiers](https://hdtp.io/learn/permissions-and-tiers/)
- [Sealed envelopes](https://hdtp.io/learn/sealed-envelopes/)
- [Security model and trade-offs](https://hdtp.io/learn/security-model/)
- [What is HDTP](https://hdtp.io/learn/what-is-hdtp/)