Hello,

I was thinking that ANSI-X9.63-KDF could be a good candidate for this crate. It is the KDF function to use for CMS with ECC as per RFC 5753.

From what I gathered, it seems to be essentially the same as concat-kdf, but with a different concatenation order.
I have local branch (based on/adapting the crate's existing concat-kdf implementation) that I am using for CMS with ECC.

Would you consider a PR?

Thank you.