If servers biggest problem is cipher strength then no info is provided in Summary why grade was downgraded to B.
As mentioned in SSL Labs rating guide
Cipher Strength ratings
0 bits (no encryption) = 0%
< 128 bits (e.g., 40, 56) = 20%
< 256 bits (e.g., 128, 168) = 80%
>= 256 bits (e.g., 256) = 100%
Cipher Strength Score = (Score of the Strongest Supported Cipher + Score of the Weakest Supported Cipher ) / 2
when server supports (< 128 bits Weak ciphers) + (strong >=256) cipher grade = (20+100) / 2 = 60
when server supports (< 128 bits Weak ciphers) + (strong <256) cipher grade = (20+80) / 2 = 50
And then overall rating grade is calculated as
final server rating = (protocol * 0.3) + (key exch * 0.3) + (cipher * 0.4)
In your example key exch grade = 90, protocol = 95, cipher grade = 60So, final server rating = (95 * 0.3) + (90 * 0.3) + (60 * 0.4) = 79.5 (Grade B)
If server has low cipher grade still It can still get grade A if it scores good enough in other two categories.
eg. In below example Server score is (97.5*0.3) + (90*0.3) + (60*0.4) = 80.25 (grade A) and its biggest problem is cipher strength problem (Ignore the trust issues)
So basically Grade is not capped to B. In future above Cipher strength ratings (based on key size) might change and we could get different grade. Generally we show warning message if Grading is capped to lower grade because of some security issue. And we are not showing any warning messages if overall calculation leads to some lower grade (eg.B,C etc).
Forward secrecy warning is there because it is some exception to the general grading algorithm, In case of Forward secrecy issue only impact on grading is when server gets A, it is reduced to A-.
"Grade reduced to A-" message is displayed only when Server actually gets A-, that can be changed. Currently we point more info links to the external pages with details about that security issue, Same link for more info used in both sections for forward secrecy issue (check on dev.ssllabs.com).
Thanks a lot for explaining this. It looks like in some cases ssllabs.com grading got little bit too complicated to my taste...
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