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description
This page is currently a draft spec. Please don't use yet.

Polkadot / Substrate (draft)

Mailchain users can send messages to any Substrate account address. This section details how the Substrate implementation of the Message Flow is achieved.

Networks

Mailchain plans to support substrate networks that have the Contract Substrate Runtime Module Library enabled (srml_contracts).

Public Key Finder

To send an encrypted message, a public key is required. Mailchain uses the public key associated with an address to encrypt the data, and if specified by the envelope, the message contents. This ensures the recipient is also the owner of the private key and only they can decrypt data. The public key finder decodes a recipient address from base58 and extracting the public key (dropping the first byte and last two bytes before prepending '0x').

Example

Field Example
Address (base58 encoded) 5EU2BwdEH8rojoKuJ16p3t1Gs8uUUp6SoaomKud2FmXBvdbp
Decoded Address (from Base58) 2a6a40d2c0edf1dfa67a639599dd3de2f49eb55fcd73991ba33a67debfb04b5846459b
Extracting Public Key [2a]6a40d2c0edf1dfa67a639599dd3de2f49eb55fcd73991ba33a67debfb04b5846[459b]
Public Key 0x6a40d2c0edf1dfa67a639599dd3de2f49eb55fcd73991ba33a67debfb04b5846

{% hint style="info" %} The public key for an account address can either be calculated from an address or supplied by that account holder.

Using the same private key to generate addresses on different networks produces the same public key, but a different public address. {% endhint %}

Sender

Mailchain uses the functionality provided by the Contract module (srml_contracts) `call` function which is similar to a standard Substrate transaction (signed extrinsic) with the addition of a data field (Vec<u8>). A substrate parachain (or parathread) with the Contract module enabled permits sending transactions with an envelope containing a message.

Envelope

Contract transactions (signed calls) contain a data field that stores the envelope. Bytes can be stored in the data field and must be hexadecimal encoded, prefixed with 0x. Data stored in a transaction must follow the Mailchain standard encoding format:[protocol-prefix]+[mailchain-prefix]+[envelope].


| Field | Example | Notes |
| :--- | :--- | :--- |
| protocol-prefix | 0x | Required Contract SRML `call` function data prefix |
| mailchain-identifier | `6d61696c636861696e` | "mailchain" encoded as hexadecimal |
| envelope | `010a82012ee10c59024c836d7ca12470b5ac74673002127ddedadbc6fc4375a8c086b650060ede199f603a158bc7884a903eadf97a2dd0fbe69ac81c216830f94e56b847d924b51a7d8227c80714219e6821a51bc7cba922f291a47bdffe29e7c3f67ad908ff377bfcc0b603007ead4bfd87ff0acc272528ca03d6381e6d0e1e2c5dfd24d521` | Envelope encoded as hexadecimal |

TODO

An example of transaction data for a Mailchain message sent on \[INSERT TESTNET HERE\] is as follows:

```
?????
```

The same transaction and data can be viewed on **here** on Polkascan.

/TODO

####  Transaction/ Extrinsic Fields

To send a message the following transaction fields must be specified.

| Field | Type | Example | Notes |
| :--- | :--- | :--- | :--- |
| origin | [AccountId](https://substrate.dev/rustdocs/v1.0/srml_system/trait.Trait.html#associatedtype.AccountId) |  | TODO |
| dest | [AccountId](https://substrate.dev/rustdocs/v1.0/srml_system/trait.Trait.html#associatedtype.AccountId) |  | TODO |
| value | [BalanceOf](https://substrate.dev/rustdocs/v1.0/srml_contract/type.BalanceOf.html) |  | TODO |
| gas\_limit | [Gas](https://substrate.dev/rustdocs/v1.0/srml_contract/trait.Trait.html#associatedtype.Gas) |  | TODO |
| data | [Vec](https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html)&lt;[u8](https://doc.rust-lang.org/nightly/std/primitive.u8.html)&gt; |  | TODO |

Once the fields have been populated, the function must be signed using the sender private key. The signed transaction bytes can then be transmitted to the substrate network over JSON-RPC using the Contract module `call` function.

### Receiver

To [read](../concepts/read.md) Mailchain messages for a specific address, Contract calls sent to that address need to be identified. Substrate does not natively support an address index or similar functionality that identifies all transactions sent to or from a specific address. Third parties have API's that provide this functionality, including Polkascan \[TODO: confirm polkascan api reqs; needs testnet example\].

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