Handling BIP32 Edge Case about corebitcoin HOT 4 CLOSED

Good point.

My interpretation of BIP was this: if i = X yields an invalid key, user should proceed with i = X + 1.

Alternatively, if the library silently returns a key for X + 1, then when the user supplies i = X + 1 himself, he'll get exactly the same key which may not be expected or could be dangerous (in a protocol where keys must never be reused).

Interestingly, I haven't found a single implementation that does ++i in such case. Some do explicitly wrap private key around curve order, others do nothing. I think it's generally safer to return nil, crash or force user to deal with it than to accidentally produce a duplicate key. Another option is to standardize the way to retry, but in a safer way.

Keep in mind that this case will never be triggered in practice, but even if it does, current implementation will not compromise user's security. I would prefer to have a standard way to resolve this issue, but doing ++i does not look safe to me.

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@oleganza -- that is a valid interpretation, I went back and forth. For reference, here's an implementation that does ++1:

https://github.com/bitcoinjs/bitcoinjs-lib/blob/ca0c56fa3bba142d0fb590186ac1496f6f89fc8a/src/hdnode.js#L230

Of course, other interpretations raise or return nil:

https://github.com/BitVault/money-tree/blob/00cce46fb58ffa337895c96150942db02f42e525/lib/money-tree/node.rb#L88

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So what do you think we should do? Mind that in 99,99..99% of cases it does not matter what we do. In the remaining 0.00..001% we may silently compromise user's security or privacy by reusing the key when it's not expected.

I would personally follow Nick Szabo's advice to "leave small holes unplugged" (http://blog.oleganza.com/post/69174500046/leaving-small-holes-unplugged) and keep things as they are until we have a really good argument for ++i. Even better, to guarantee that we won't have nasty effects if the user forgets to check for nil result, I'd rather throw an exception to guarantee shutdown.

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@oleganza Agreed -- I meant to show the bitcoin-js lib that does ++i, which is where I initially started wondering what CoreBitcoin did in that case. Was only showing the money-tree library as a reference to another implementation that doesn't ++i. Not suggesting to throw exceptions.

I say leave everything as is.

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