ENHANCED: Safe HMAC verification, using constant time string comparison.

ENHANCED: Safe HMAC verification, using constant time string comparison.

Without this provision, the expected HMAC can be gathered from timing
differences depending on the position where the strings first diverge,
and hence an attacker can forge an authenticated message by supplying
the gathered HMAC.

Test case, using exp(E) to succeed exactly 2^E times:

    exp(E) :-
        N is 2^E,
        between(1, N, _).

yielding:

    ?- Options = [algorithm(sha512),hmac([1,2,3])],
       Ds = "test",
       crypto_data_hash(Ds, H, Options),
       phrase((seq(As),seq(Bs)), H),
       same_length(Bs, Cs),
       maplist(=(a), Cs),
       append(As, Cs, H1),
       time((exp(10),crypto_data_hash(Ds, H1, Options),false)).
    %@    % CPU time: 0.710s, 7_942_187 inferences
    %@    % CPU time: 0.713s, 7_942_187 inferences
    %@    % CPU time: 0.712s, 7_942_187 inferences
    %@    % CPU time: 0.711s, 7_942_187 inferences
    %@    % CPU time: 0.710s, 7_942_187 inferences
    %@    % CPU time: 0.711s, 7_942_187 inferences
    %@    % CPU time: 0.710s, 7_942_187 inferences

    ?- length(_, L), time((exp(10),crypto_data_hash("test", "3caebd1a0a2647930319a660b7d3642eb380fbd43202f9f6d08aabaa9ba50c39522a12ead10f0423f0af613cbc6fea74ad682ee11f563cc2e735722004fda2ba", [algorithm(sha512),hmac([0,L])]),false)).
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.734s, 7_878_699 inferences
    %@    % CPU time: 0.732s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.733s, 7_878_699 inferences
    %@    % CPU time: 0.515s, 5_525_404 inferences
    %@    error('$interrupt_thrown',repl/0).