The Heartbleed bug once again raised a very serious question — what happens if someone steals our SSL private key. The future actions are all clear — we revoke the key, change it on the server and we are secure. However any recorded past SSL encrypted sessions which the attacker posses can be decrypted to plain text using the stolen SSL private key.
The article “SSL: Intercepted today, decrypted tomorrow” explains the problem in great details. It also suggests who may take advantage of your stolen SSL private key.
Fortunately there is a solution to this attack vector. It is called Forward Secrecy and solves the problem by using a different private key to encrypt each new SSL session. If an attacker wanted to decrypt all your SSL sessions, the attacker would need to brute-force the private keys of each of your SSL sessions. While this attack vector still exists, current computing power is too small to solve such a task in a reasonable time. Note that Forward Secrecy is not new at all and was invented in 1992, pre-dating the SSL protocol by two years, as stated in the “SSL: Intercepted today, decrypted tomorrow” article.
Many websites and blogs recommend their own cipher list which needs to be used, in order to enable Perfect Secrecy, and at the same time to not be vulnerable to the BEAST attack. Trusting such long lists of ciphers without understanding all of them makes me a bit insecure. Luckily the OpenSSL vendors have created a special string which we can use to identify the TLS v1.2 specific ciphers — “TLSv1.2”. This makes your Apache configuration very readable and leaves the control in the OpenSSL developers who can update the ciphers accordingly:
SSLProtocol -ALL -SSLv2 +SSLv3 +TLSv1 +TLSv1.1 +TLSv1.2 SSLHonorCipherOrder on SSLCipherSuite TLSv1.2:RC4:HIGH:!aNULL:!eNULL:!MD5 SSLCompression off TraceEnable Off
The effect of this configuration is that for TLS v1.2 connections you will prefer the strong TLS v1.2 ciphers. For all other connections, like SSLv3 or TLS v1.0, the RC4 ciphers will be used which are required to protect against the BEAST attack. Note that TLS v1.2 does not suffer from the BEAST attack.
This configuration won’t give you Forward Secrecy for Internet Explorer. The explanation here says:
“Internet Explorer, in all versions, does not support the ECDHE and RC4 combination (which has the benefit of supporting Forward Secrecy and being resistant to BEAST). But IE has long patched the BEAST vulnerability and so we shouldn’t worry about it.”
It’s worth mentioning that using those strong TLS v1.2 ciphers may increase the CPU time required to establish a new SSL connection 3 times.
Real-time SSL test:
- Mitigating the BEAST attack on TLS
- Is BEAST Still a Threat?
- RC4 in TLS is Broken: Now What?
- SSL/TLS & Perfect Forward Secrecy
- SSL Labs: Deploying Forward Secrecy
- Forcing IE11 to use perfect forward secrey on AWS ELB