deterministic-rfc6979 DSS signature of a HMAC object.

[koolfy]

Hello, this is a continuation on my attempts over at https://github.com/Legrandin/pycryptodome/issues/414#issuecomment-643824612

I hesitated for a very long while before opening a new issue, but I couldn't find any email info to reach you in a softer way.

The tl;dr is that even with deterministic signature there were some technical roadblocks to sign a HMAC object, in the linked comment I tried to provide as much explanation as I could on what's blocking the signature internally in pycryptodome's data structures, but I don't feel confident producing a pull request as the implementation design decisions I would chose might really not meet your criterias.

I apologize for the second issue, if you deem it invalid don't hesitate to close it, or maybe reopen the previous one, or just point to me what other obvious thing I might have missed again :(

Thanks