[Keybase] Implement an encrypted wrapper for private keys akin to ArmoredJson from V0

[deblasis]

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Objective

Implement a secure wrapper around private keys.

Origin Document

For increased security, we are going to need something like ArmoredJson from v0 to protecting Private keys coming out of a keybase and providing a way of importing new private keys into the keybase.

This issue is dependent on #150 but I believe it's still possible to work on this in parallel by developing the core functionality for encryption/decryption for initial use with the Utility CLI (#112) where, for now, the privatekeys come into play in the form of a clear-text JSON file.
We just have to remember (via TODOs and/or GH issues) that ultimately the keybase should be interfaced with this as well.

Goals

• Determine if the V0 ArmoredJson implementation could be used in V1 as well and if not, propose alternatives with pros/cons comparisons
• Define interfaces and methods to provide access to the underlying private keys given an address and a passphrase
• Define interfaces and methods to allow importing of private keys into the keybase
• Use these new interfaces/methods to access private keys where needed (a good place to start is the CLI, starting from the Utility commands tracked in [Utility] Local Proof of Stake CLI #112 )
• Create import functionality for private keys
• Create export functionality for private keys

Deliverable

• ArmoredJson / ArmoredJson-like implementation with basic Encrypt and Decrypt methods (see v0 ArmoredJson)
• Import account from encrypted and ASCII armored file CLI command
• Export account from encrypted and ASCII armored file CLI command
• Log any unfinished work with TODOs depending on the state of [Keybase] Miniature Keybase Implementation to Enable M1 - Create and Delete #150 and [Utility] Local Proof of Stake CLI #112 at the moment this is completed/ready to merge.

Non-goals / Non-deliverables

• Do any work currently tracked by other Issues (especially [Keybase] Miniature Keybase Implementation to Enable M1 - Create and Delete #150 and [Utility] Local Proof of Stake CLI #112)

General issue deliverables

• Update the appropriate CHANGELOG
• Update any relevant READMEs (local and/or global)
• Update any relevant global documentation & references
• If applicable, update the source code tree explanation
• If applicable, add or update a state, sequence or flowchart diagram using mermaid

[Optional] Testing Methodology

• Task specific tests: make ...
• All tests: make test_all
• LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md
• Write tests that ensure encryption and decryption work as expected, including KO cases (eg: wrong passphrase)

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Creator: @deblasis
Co-Owners: @Olshansk, @andrewnguyen22

[Olshansk]

@h5law completed this as part of the key base work.