improved the manpage

[calestyo]

This includes:

• Added the note from the RFC, that the Argon2id is recommended for all
  environments.
• Added some formatting.
• The complete usage modes.
• For the 3 Argon variants, added some basic description of these, taken from
  the RFC, with the exception that Argon2i isn’t decribed as preferred.
• Mentioned which options are mutually exclusive.
• Added the missing -i and -k options.
• Some further minor textual changes.
• References to the RFC.

Signed-off-by: Christoph Anton Mitterer [email protected]

[calestyo]

Someone should double check especially the following:

• I took some quotes from the RFC (describing the differences of the variants). I would have said that these snippets are too small to deserve any copyright - but I'm not a lawyer.
• I also modified the portion from the RFC, that describes the i variant as preferred:

Argon2i uses data-independent memory access, which is preferred for password hashing and password-based key derivation.

and rather added a note in the beginning that the id variant is generally preferred.

[calestyo]

@dkg ... since you wrote the main part of the manpage... maybe you want to have a look at this.

[dkg]

This all looks good to me. as for the argon2i vs. argon2id choices and documentation, RFC 9106 is a little funny in that its introduction says:

Argon2i uses data-independent memory access, which is preferred for password hashing and password-based key derivation.

but the recommendation section says:

The Argon2id variant with t=1 and 2 GiB memory is the FIRST RECOMMENDED option and is suggested as a default setting for all environments.

I think the proposed changes are fair, because arguably the sentence in the introduction is saying that 2i is "preferred…" in these contexts over 2d, due to 2d's vulnerability to side-channel attacks.

I don't have the ability to merge this change, of course, but i have no objections to it.

[calestyo]

@veorq Just wondered whether this is still maintained? Last commit is already quite some time ago, and the number of MRs is rising ;-)