The old behavior of generating new DH keys can be maintained for non-OOB
cases and only OOB (in this case, with UFD) will use the pre-configured
DH keys to allow the public key hash to be checked.
WPS spec is not very specific on the presentation used for the DH
values. The Public Key attribute is described to be 192 octets long, so
that could be interpreted to imply that other places use fixed length
presentation for the DH keys. Change the DH derivation to use fixed
length bufferd by zero padding them from beginning if needed. This can
resolve infrequent (about 1/256 chance for both Public Key and Shared
Key being shorter) interop issues.