Add memory wiping functionality

[ionut-arm]

This commit improves the security of the library by wiping out all
sensitive data before it is dropped.

Signed-off-by: Ionut Mihalcea ionut.mihalcea@arm.com

[hug-dev]

I know that we came to the conclusion that it was ok to have Secret and Zeroizing on the interface but I am now thinking about it. Is it such a good idea? I definitely agree that we need to internally use them to clear our memory but should we do impose that on the client? Maybe in its threat model, the client does not need memory wiping and having those type in the interface will be come a burden.

I am also thinking about #31: could we kill two birds with one stone? As in both:

1. use references for sensitive data (references of Vec<u8>)
2. let the user handle zeroizing of memory of its owned structures
3. whenever we clone it ourselves, we do zeroize it

For 1, if we have in our interface &Vec<u8>, can a client use it with Zeroizing or Secret? I think so both with the Deref implementation and with expose_secret.

[ionut-arm]

Yep, totally agree with that, we can, in the future, add the Zeroizing and Secret as a layer in our client, the BasicClient can just stay... basic.

[ionut-arm]

Should we still use Secret in the authentication token, though? I'd say yes, in which case we have to export the secrecy crate as well. It' just the BasicClient that stays the same.

[hug-dev]

Should we still use Secret in the authentication token, though? I'd say yes, in which case we have to export the secrecy crate as well. It' just the BasicClient that stays the same.

Yes, I think so too because this data is really Parsec specific.

[hug-dev]

Thanks!