Google Online Security Blog: Disabling SSLv3 and RC4

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Disabling SSLv3 and RC4

2015年9月17日

Posted by Adam Langley, Security Engineer
As the previously announced transition to SHA-256 certificates is nearing completion, we are planning the next changes to Google’s TLS configuration. As part of those changes, we expect to disable support for SSLv3 and RC4 in the medium term.
SSLv3 has been obsolete for over 16 years and is so full of known problems that the IETF has decided that it must no longer be used. RC4 is a 28 year old cipher that has done remarkably well, but is now the subject of multiple attacks at security conferences. The IETF has decided that RC4 also warrants a statement that it too must no longer be used.
Because of these issues we expect to disable both SSLv3 and RC4 support at Google’s frontend servers and, over time, across our products in general, including Chrome, Android, our webcrawlers and our SMTP servers. (Indeed, SSLv3 support has already been removed from Chrome.) The SSL Pulse survey of the top 200,000 HTTPS sites finds that, already, 42% of sites have disabled RC4 and 65% of sites have disabled SSLv3.
If your TLS client, webserver or email server requires the use of SSLv3 or RC4 then the time to update was some years ago, but better late than never. However, note that just because you might be using RC4 today doesn’t mean that your client or website will stop working: TLS can negotiate cipher suites and problems will only occur if you don’t support anything but RC4. (Although if you’re using SSLv3 today then things will stop working when we disable it because SSLv3 is already a last resort.)
Minimum standards for TLS clients
Google's frontend servers do a lot more than terminate connections for browsers these days; there are also lots of embedded systems talking to Google using TLS. In order to reduce the amount of work that the deprecation of outdated cryptography causes, we are also announcing suggested minimum standards for TLS clients today. This applies to TLS clients in general: certainly those that are using TLS as part of HTTPS, but also, for example, SMTP servers using STARTTLS.
We can't predict the future, but devices that meet these requirements are likely to be able to continue functioning without changes to their TLS configuration up to 2020. You should expect these standards to be required in cases where Google runs certification programs, but it’s a very good idea to meet them anyway.
Devices that don’t meet these standards aren’t going to stop working anytime soon (unless they depend on RC4 or SSLv3—see above), but they might be affected by further TLS changes in the coming years.
Specifically, we are requiring:

TLS 1.2 must be supported.

A Server Name Indication (SNI) extension must be included in the handshake and must contain the domain that's being connected to.

The cipher suite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 must be supported with P-256 and uncompressed points.

At least the certificates in https://pki.google.com/roots.pem must be trusted.

Certificate handling must be able to support DNS Subject Alternative Names and those SANs may include a single wildcard as the left-most label in the name.

In order to make testing as easy as possible we have set up https://cert-test.sandbox.google.com, which requires points 1–3 to be met in order to make a successful connection. Thus, if your TLS client can’t connect to that host then you need to update your libraries or configuration.
No longer serving a cross-sign to Equifax
At the moment the certificate chains that Google properties serve most often include a cross-sign from our CA, GeoTrust, to our previous CA, Equifax. This allows clients that only trust our previous CA to continue to function. However, this cross-sign is only a transitional workaround for such clients and we will be removing it in the future. Clients that include our required set of root CAs (at https://pki.google.com/roots.pem) will not be affected, but any that don’t include the needed GeoTrust root may stop working.

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