Incrementing hardened nodes overflows to non-hardened ones and vice-versa

[quapka]

Hi,
While looking around the KeyPath implementation, because of #1258, I've stumbled upon the Increment method.

Consider the following examples:

Console.WriteLine(masterPrivateKey.Derive(KeyPath.Parse("2147483647'").Increment()) == masterPrivateKey.Derive(KeyPath.Parse("0"))); //
Console.WriteLine(masterPrivateKey.Derive(KeyPath.Parse("2147483647").Increment()) == masterPrivateKey.Derive(KeyPath.Parse("0'"))); //

Incrementing a child key seems like a benign and usable feature. However, crossing non-hardened and hardened boundaries silently seems like asking for an unexpected behavior somewhere down the line. Generally, key-reuse and thus address reuse is bad for privacy. Just the API cannot defend against an address reuse, but still I would definitely err on the side of caution and stop incrementing, when crossing both boundaries.

Increment is being used. However, I did not find it being used in any of the top projects listed in your README.md.

[lontivero]

Good catch.