I have noticed this problem in a few sites. For example, EFF.org is with forward secrecy in its openSSL implementation. When I load it from Tor Browser Bundle and check the "Padlock" icon on address bar, it only says "AES256, 256 bit keys". When I load the same EFF site on plain Firefox, it says "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA_256" ..

From what I found and read online, "ECDHE" part means Perfect forward secrecy. So why is Tor browser bundle is disabling it if PFS will be providing better security?


Both browsers using the same cipher suite with PFS for EFF site. ESR-24 decodes used cipher and showing information partially while fresh vanilla Firefox showing cipher suite without decoding (by accident). https://bugzilla.mozilla.org/show_bug.cgi?id=897615


Tor Browser doesn't actively disable PFS, but just uses an old Firefox version. The current version of Tor Browser's Firefox is 24.4 and plain Firefox is at 28.

If I recall correctly, Firefox enabled PFS by default in version 27. So when the Firefox version from Tor Browser is updated you'll get PFS by default. If you want to use PFS right now, you can navigate to about:config and look for tls.version. You'll find a minimum and maximum setting. When you set maximum to 3 this means you want to use TLS 1.2 with PFS. I explained it step by step in a blog posting more in detail.

If you want to see if your browser supports PFS test your client at the Qualys SSL Labs site.

  • I Opened about:config and set tls.version maximum to 3. I quit TBB and started TBB again. When I tried loading eff.org, it still uses "AES 256" but not using its Perfect forward secrecy.
    – user1552
    Apr 16 '14 at 17:23

AES256, 256 bit keys is just the type of encryption used. Firefox ESR/Tor browser only shows the encryption used, nothing more. You can't see the key-exhange(for example DHE, ECDHE, RSA.)

Newer versions of Firefox show the full cipher: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA_256 This also shows the key-exchange, in this case ECDHE.

FPS ciphers are already supported in TLS 1.0, so enabling TLS 1.2 does not change this.


If you enter about:config in the address bar and then search for "ssl3" you will get a list of all approved cypher suites in your FF. If you disable all of them except those that contain the strings DHE (including ECDHE) then it SHOULD force PFS on any ssl connection, assuming of course that the website supports PFS.

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