Add support for message encryption and decryption

[slaweet]

Expected behaviour

Users should be able to encrypt messages with someone public key and decrypt messages with their private key. The overall style should be similar to sign/verify message.

Actual behaviour

Not implemented

[slaweet]

Lisk-js supports this https://liskhq.github.io/lisk-js/example/experiment.html#encrypt

[pcdinh]

How large in size can the message be?

[pcdinh]

Is the message here stored in "data" field implemented in the pull request? LiskArchive/lisk-sdk#470

[slaweet]

This message encryption feature in this issue is not meant to store the message onto blockchain. Therefore I believe that there is no size limit.

Support for "data" field LiskArchive/lisk-sdk#470 will be implemented to Lisk Nano here: #595

It might be worth to integrate these two features together, but that would be another issue.

[pcdinh]

Thanks for clarification. So what does this ticket intend to solve?

[slaweet]

The goal of this issue is to implement the same form as lisk-js provides here:
https://liskhq.github.io/lisk-js/example/experiment.html#encrypt
https://liskhq.github.io/lisk-js/example/experiment.html#decrypt
Except for the "secret" field, because in Lisk Nano that is already known from login.

The intended use case is that message is encrypted in Nano and sent by some other means (email, etc.).