> 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/blockchain.md).

# Blockchain layer

OCC v1 uses a blockchain as the network that keeps a tamper-evident record of each exchange.

{% 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)).
{% endhint %}

In OCC v1 the blockchain is the underlying network. It runs the protocol contracts and keeps an immutable, public record of each exchange.

## What goes on chain

* For a chat message: the encrypted message, a timestamp and the sender's account.
* For a mail: a link to the stored envelope, a checksum, a timestamp and the sender's account.
* For each user: the public key other people use to encrypt to them.
* For each app: the fee settings tied to its app ID.

With encryption on, content is encrypted before it goes on chain. Without encryption, it is stored as sent.

## Why a chain

A chain record cannot be quietly changed afterwards, and anyone can check that an exchange happened. The cost is that records are public and permanent, and that each exchange needs a network fee. Plan for both.

## Which chain

OCC v1 is designed to run on EVM-compatible networks, on layer 1 or layer 2. The SDK 0.5.13 configures a set of networks; see [Supported networks](/occ-v1/networks.md).

## Related

[Protocol contracts](/occ-v1/contracts.md), [Fees and app registration](/occ-v1/fees.md).