Substrate (Polkadot)

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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
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.
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]. 
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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
 Transaction/ Extrinsic Fields
To send a message the following transaction fields must be specified.
Field
Type
Example
origin
​AccountId​
​
dest
​AccountId​
​
value
​BalanceOf​
​
gas_limit
​Gas​
​
data
​Vec<u8>
​
Once the fields have been populated, the extrinsic must be signed using the sender private key. The signed extrinsic bytes can then be transmitted to the substrate network over JSON-RPC using the Contract module call function.
Receiver
To read 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 provide some of this information through a web interface (including Subscan.io) but as of August 2020, no APIs consistently provide a transaction index for received transactions and their data fields.
Transaction Indexer
Mailchain provides a simple transaction indexer which listens for transactions and filters mailchain messages for the recipient. Further details and instructions for running it can be found here: Substrate Transaction Indexer.
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Reference - Previous
Ethereum
Substrate Transaction Indexer
Last modified 6mo ago
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