Bug when using new reduced-size SPHINCS+ parameter sets

[crypto4a]

There is an issue with the existing reference implementation when experimenting with the new reduced-signature size SPHINCS+ parameter sets due to the fact that some of the new parameter sets utilize large subtree sizes (i.e., > 16) that result in the address.c::set_keypair_addr() function not setting all of the necessary bits as it only copies over the first two bytes.

In general, you may want to modify all ADDR-handling functions to ensure they copy over, or set, all of the bytes of each of the ADDR fields to ensure this doesn't happen.

[bwesterb]

Feel free to send a PR.

[kste]

I will send a pull request. There is actually another issue, as

sphincsplus/ref/hash_sha2.c

Line 186 in 035b394

*tree &= (~(uint64_t)0) >> (64 - SPX_TREE_BITS);

can lead to undefined behaviour for additional parameters if there is only a single tree (d=1) [SPX_TREE_BITS becomes zero in that case].

[bwesterb]

Thanks @kste