Improve help message

[RufusJWB]

Motivation

Improve documentation. Took us ages to find the list of allowed algorithms

Proposed Changes

Link to openssl header file in which the algorithms are defined.

Test Plan

n/a

[DDvO]

Thank you also for this PR.

Hints on which MAC algorithms are available would have been found in the man page or online documentation,
as also written in the README.md:

The CLI use with the available options are documented in cmpClient.pod.

I am not sure if the link https://raw.githubusercontent.com/openssl/openssl/master/include/openssl/obj_mac.h that you suggested to add would be sufficiently simple to use and provides better info than the text given there:

To get the names of supported MAC algorithms use B<openssl list -mac-algorithms>
and possibly combine such a name with the name of a supported digest algorithm,
e.g., hmacWithSHA256.
Defaults to C as per RFC 4210.

Moreover, the CLI help output needs to be very brief and should not have overlong lines.
So I suggest modifying the PR as follows:

• add to cmpClient.c after the existing text for -mac just a line like

    OPT_MORE("See the man page or online doc for hints on available algorithms"),

• as far as you still see value in the providing that above URL, please add it to doc/cmpClient.pod.

[RufusJWB]

@DDvO I updated my PR based on your proposal. Please double check. But honestly, I don't find the man-page helpful. Take for example

Why is one algorithm called hmacWithSHA256 but the other hmac-sha1? And why is hmac-sha256 not working but hmac-sha3-224?

[DDvO]

@DDvO I updated my PR based on your proposal. Please double check.

Looks good - nice that you added it for both -mac and -digest.

But honestly, I don't find the man-page helpful. Take for example

Why is one algorithm called hmacWithSHA256 but the other hmac-sha1? And why is hmac-sha256 not working but hmac-sha3-224?

This is most likely due to the legacy and wonders of IETF & IANA standardization.
As finding a supported combination of names is not straightforward, I hinted at possibly also adding the URL you gave to cmpClient.pod.
I suggest adding there before the line

Defaults to C<hmac-sha1> as per RFC 4210.

something like

L<https://raw.githubusercontent.com/openssl/openssl/master/include/openssl/obj_mac.h>
contains the complete set of combinations of MAC and digest algorithm names.

[RufusJWB]

@DDvO please double check again.

[DDvO]

LGTM - thanks for contributing.