Clearing unused sensitive data from memory

[defacedbeef]

A mnemonic generated in two HDWallet c-tors is not cleared properly.
Proposed change clears out the sensitive data from memory right after std::string copy-ctor.

Description

This commit clears out sensitive data from the memory.

Testing instructions

Types of changes

Checklist

• Prefix PR title with [WIP] if necessary.
• Add tests to cover changes as needed.
• Update documentation as needed.
• I have read the guidelines for adding a new blockchain
• If there is a related Issue, mention it in the description (e.g. Fixes #<issue_number> ).

[vikmeup]

Is there a way to add a unit test for this?

[optout21]

Accepted. I changed memset to memzero.

[optout21]

This zeroing is valid, but keep in mind that it is not critical, as the mnemonic is stored in memory if the HDWallet object anyhow (field mnemonic).

[optout21]

Is there a way to add a unit test for this?

In general it's very hard. I tried to add tests for zeroing out local variables, but after zeroing out, memory may be deallocated, in whch case it may be reallocated and overwritten with other values, unpredictable.

[defacedbeef]

Is there a way to add a unit test for this?

Not sure if I would call it a unit test. But yes, doable.