> For the complete documentation index, see [llms.txt](https://assay.gitbook.io/assay-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://assay.gitbook.io/assay-docs/integrations/mera.md).

# Mera

How one passkey keeps your salts encrypted, gives each app its own co-signer key, and opens one receipt at a time.

**Where:** `web/src/mera.ts` and the Vault tab of the [web app](/assay-docs/for-developers/web-app.md). Onchain, per-app keys are checked by `ReceiptAnchor.cosignK`.

Mera turns one passkey's PRF output into separate keys. Assay uses it for three namespaces, each a different primitive.

## Problem

A requester must keep the salt of every request. It's the only way to later prove that an output answered a prompt without publishing the prompt. Salts can't go to the host or sit in plain `localStorage`, because anyone holding a salt can brute-force a short prompt. Lose them and your receipts can't be opened.

There is a second problem. If one passkey co-signs receipts for every app, one public key links all your AI activity across apps.

## Why Mera

The same passkey, rpId and PRF salt always give the same 32 bytes, on every synced device, with nothing stored. A password-derived key can be forgotten or phished. A cloud KMS means a platform holds your keys. A seed phrase makes it a wallet.

## What we built

| Label                        | Primitive                                            | Used for                                                                         |
| ---------------------------- | ---------------------------------------------------- | -------------------------------------------------------------------------------- |
| `assay:vault:v1`             | HKDF-SHA256, then AES-256-GCM                        | Encrypts receipts, salts and outputs. Only ciphertext is stored                  |
| `assay:requester:<appId>`    | BIP-39 entropy, BIP-32 `m/44'/60'/0'/0/0`, secp256k1 | One unlinkable co-signer per app. Signs `receiptHash` with EIP-191 for `cosignK` |
| `assay:reveal:<receiptHash>` | HKDF-SHA256, then AES-256-GCM                        | Opens one receipt's entry and nothing else                                       |

| Rule                                                       | How it holds                                                                          |
| ---------------------------------------------------------- | ------------------------------------------------------------------------------------- |
| Salts are namespaced                                       | Each PRF salt is `sha256(label)`                                                      |
| Encryption keys never sign, and signing keys never encrypt | Separate namespaces                                                                   |
| Keys live briefly                                          | The PRF output is used once and zeroed. The signing session ends with `session.end()` |
| Ciphertext can't move between namespaces                   | The label is AES-GCM associated data                                                  |

## How to try it

1. Open the web app on HTTPS or `localhost` with a PRF-capable passkey provider (iCloud Keychain, 1Password or Google Password Manager).
2. In Vault, click "Create a vault passkey", then save a receipt from Ask & co-sign.
3. Click "Show this browser's ciphertext" and copy it to a second device.
4. On that device, "Replace with pasted ciphertext" and unlock with the same synced passkey.
5. Click "Share this receipt" to make a disclosure key, and open it with "Open a disclosure" elsewhere.

## Where else this shows up

| Place                                                       | What it covers         |
| ----------------------------------------------------------- | ---------------------- |
| [Passkey co-signatures](/assay-docs/how-it-works/cosign.md) | `cosign` and `cosignK` |
| [Privacy](/assay-docs/how-it-works/privacy.md)              | What the salt protects |

{% hint style="warning" %}
On desktop Chrome, only passkeys saved to Google Password Manager return PRF. A passkey in the local profile fails with `PRF_UNAVAILABLE`. Hardware keys don't sync, so the second-device test needs a synced platform passkey.
{% endhint %}

Next: [MonadGuard and Mandate](/assay-docs/integrations/other-teams.md)


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