[Dovecot] patch for ssl_prefer_server_ciphers in dovecot 2.1

[Adi Kriegisch]

Hi!

> > PS: I need that feature to enable PFS while allowing Outlook to still
> > connect and the others not to fall back to a different cipher; I was
> > unable to find a PFS cipher that is supported by Outlook and OpenSSL
> 
> ssl_cipher_list =
> EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH+aRSA+RC4:EECDH:EDH+aRSA:RC4:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!SSLv2:@STRENGTH
> ssl_prefer_server_ciphers = yes
> 
> Outlook, at least on WinXP any version, continues to use RC4 ciphers
> but any sane mail client is using PFS ciphers
Thanks for sharing; I opted for disabling RC4 completely and came up with
the following (formatted for readability):
  HIGH:EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:
       EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:EDH+aRSA:ECDHE-RSA-AES256-SHA:
       +DHE-RSA-AES256-SHA:!AES256-SHA256:!AES256-GCM-SHA384:!CAMELLIA256-SHA:
       !AES128:!CAMELLIA128:
       !aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!SSLv2:!RC4:!SEED:
       +AES256-SHA 
which disables every cipher with less than 256bit and leaves AES256-SHA as
a last resort for Outlook... (and is except for that pretty similar to your
cipher string).
This gives (openssl ciphers -V 'theabovestring'):
0xC0,0x30 - ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA  Enc=AESGCM(256) Mac=AEAD
0xC0,0x2C - ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(256) Mac=AEAD
0xC0,0x28 - ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA Enc=AES(256)  Mac=SHA384
0xC0,0x24 - ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(256)  Mac=SHA384
0xC0,0x14 - ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA Enc=AES(256)  Mac=SHA1
0xC0,0x0A - ECDHE-ECDSA-AES256-SHA  SSLv3 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA1
0x00,0x9F - DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA Enc=AESGCM(256) Mac=AEAD
0x00,0x6B - DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA Enc=AES(256)  Mac=SHA256
0x00,0x88 - DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH       Au=RSA Enc=Camellia(256) Mac=SHA1
0xC0,0x32 - ECDH-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(256) Mac=AEAD
0xC0,0x2E - ECDH-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AESGCM(256) Mac=AEAD
0xC0,0x2A - ECDH-RSA-AES256-SHA384  TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA384
0xC0,0x26 - ECDH-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256)  Mac=SHA384
0xC0,0x0F - ECDH-RSA-AES256-SHA     SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA1
0xC0,0x05 - ECDH-ECDSA-AES256-SHA   SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256)  Mac=SHA1
0x00,0x39 - DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA Enc=AES(256)  Mac=SHA1
0x00,0x35 - AES256-SHA              SSLv3 Kx=RSA      Au=RSA Enc=AES(256)  Mac=SHA1

...and as I have no DSA keys (aka | grep -v DSA):
0xC0,0x30 - ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA  Enc=AESGCM(256) Mac=AEAD
0xC0,0x28 - ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA Enc=AES(256)  Mac=SHA384
0xC0,0x14 - ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA Enc=AES(256)  Mac=SHA1
0x00,0x9F - DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA Enc=AESGCM(256) Mac=AEAD
0x00,0x6B - DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA Enc=AES(256)  Mac=SHA256
0x00,0x88 - DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH       Au=RSA Enc=Camellia(256) Mac=SHA1
0xC0,0x32 - ECDH-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(256) Mac=AEAD
0xC0,0x2A - ECDH-RSA-AES256-SHA384  TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA384
0xC0,0x0F - ECDH-RSA-AES256-SHA     SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA1
0x00,0x39 - DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA Enc=AES(256)  Mac=SHA1
0x00,0x35 - AES256-SHA              SSLv3 Kx=RSA      Au=RSA Enc=AES(256)  Mac=SHA1

There is probably still room for improvement, but Android devices now use
some DHE key exchange, Thunderbird uses the CAMELLIA cipher and so on; only
Outlook -- which should have mitigated the BEAST attack uses AES256-SHA.
I hope -- here too -- I didn't miss a thing. Probably the ordering should
be changed a little...

all the best!
    -- Adi