[Bitcoin-development] BIP32 - invalidation

[Pavol Rusnak <stick@gk2.sk>]

Hi all!

I would like to discuss invalidation of nodes in BIP32. Currently the
document says:

a) Public CKD

In case I_L >= n or ki = 0, the resulting key is invalid, and one should
proceed with the next value for i.

b) Private CKD

In case I_L >= n or Ki is the point at infinity, the resulting key is
invalid, and one should proceed with the next value for i.

c) Master Key Generation

In case IL is 0 or I_L >= n, the master key is invalid.

(All these cases have probability lower than 1 in 2^127.)

What do you think about the following change for all 3 cases:

In case I_L >= n assign I_L := I_L mod n.

Rationale:

It's easy to say "mark as invalid and proceed with next", but actually
most of the implementations don't do the checking at all, because tjen
it's rather hard at application level to implement skipping logic. OTOH
it's quite straightforward to perform modulo if needed, so we probably
see more implementations doing the checking.

We would still need to deal with cases when I_L = 0 or ki = 0 or ki =
inf, but these have probability around 1 in 2^255.

Does anyone see any concerns when it comes to security of the proposed
change?

-- 
Best Regards / S pozdravom,

Pavol Rusnak <stick@gk2.sk>