> For the complete documentation index, see [llms.txt](https://wiki.4thtech.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://wiki.4thtech.io/occ-v1/occ-v1.md).

# Overview

OCC v1 is the first-generation 4thTech protocol for on-chain communication between wallets.

{% hint style="warning" %}
**Status: Legacy (reference).** OCC v1 is the first-generation 4thTech protocol. It is documented for existing integrations and as reference; new agent-to-agent work uses OCC v2 ([AgentLink](/agentlink/agentlink.md)).
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OCC v1 (on-chain communication, version 1) is the first-generation 4thTech protocol. It lets two wallets exchange mail-like messages, short chat messages and files, with a record of every exchange on a blockchain. The code was developed by Block Labs S.à r.l., the licensor of the OCC v1 code.

This section documents OCC v1 for people who already use it and as reference. If you are looking for the newer encrypted agent-to-agent work, go to [AgentLink](/agentlink/agentlink.md).

## What OCC v1 is made of

OCC v1 is a stack of four parts, connected by the [4thTech SDK](/sdk-and-white-label/sdk.md):

1. The OCC v1 smart contracts (the protocol).
2. A blockchain network that runs them.
3. Encryption.
4. Storage for encrypted content.

Read [Architecture: the four layers](/occ-v1/architecture.md) for the picture.

## The idea in one line

One mail, one message or one file transfer is one record on the blockchain. Only the people holding the right keys can read the content. Read [Core concepts](/occ-v1/core-concepts.md).

## Where to go next

| You want to                            | Go to                                                                                            |
| -------------------------------------- | ------------------------------------------------------------------------------------------------ |
| Understand the model                   | [Core concepts](/occ-v1/core-concepts.md)                                                        |
| See how the pieces fit                 | [Architecture](/occ-v1/architecture.md)                                                          |
| Look up the contracts                  | [Protocol contracts](/occ-v1/contracts.md)                                                       |
| See how a mail or chat message travels | [Message flows](/occ-v1/message-flows.md)                                                        |
| Use the apps as an end user            | [BlockMail & BlockChat user manual](/sdk-and-white-label/get-started.md)                         |
| Build on the protocol                  | [4thTech SDK](/sdk-and-white-label/sdk.md) and [Integration paths](/occ-v1/integration-paths.md) |
| Compare with the new protocol          | [From OCC v1 to OCC v2](/occ-v1/occ-v1-to-v2.md)                                                 |

## Pages in this section

* Concepts: [Core concepts](/occ-v1/core-concepts.md), [Architecture](/occ-v1/architecture.md), [Blockchain layer](/occ-v1/blockchain.md), [Storage](/occ-v1/storage.md), [Encryption options](/occ-v1/encryption.md).
* Reference: [Protocol contracts](/occ-v1/contracts.md), [Fees and app registration](/occ-v1/fees.md), [Message flows](/occ-v1/message-flows.md), [Supported networks](/occ-v1/networks.md).
* Guides: [Integration paths](/occ-v1/integration-paths.md).
* Also: [From OCC v1 to OCC v2](/occ-v1/occ-v1-to-v2.md), [FAQ](/occ-v1/faq.md), [Status and roadmap](/occ-v1/status.md), [Licence](/occ-v1/licence.md).

{% hint style="info" %}
The apps built on OCC v1 were earlier called dMail and dChat. They are now called BlockMail & BlockChat.
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