SSL/TLS cipher suite selection and breakdown.

How do I know which cipher suites to select for my web server?

This is a common issue, sysadmins have their web servers up or vpn servers configured. However they are often using older SSL protocols and older cipher suites that are now vulnerable to attack in certain scenarios. We need to understand what a cipher suite is actually doing in order to select the correct ones.

For SSL/TLS connections a cipher suite is selected based on a number of tasks that it has to perform, the client uses a preferred cipher suite list and the server will normally honor this unless it also has a preferred list, set by the sysadmin.

Initial Key Exchange, the Asymmetric Encryption: This will most commonly be RSA, however the following are options; RSA ( Ron Rivest, Adi Shamir, and Leonard Adleman), DH (Diffie-Hellman) orĀ  ECDH (Elliptic Curve Diffie-Hellman).
RSA key length should be 2048 bit minimum. ECDH and others should be an equal strength, note the ECDH key length will be significantly lower due to the way the algorithm works! The Asymmetric Encryption is only being used in the initial key exchange and for the session symmetric encryption key. The Asymmetric encryption method could be used for the data transfer however the computational power needed is far higher than the symmetric Encryption due to the key size.

Session data, the Symmetric Encryption: The most commonly used three ciphers we see in use being RC4, 3DES and AES, careful selection of ciphers is required here:

  • RC4 (Rivest Cipher 4) although used almost everywhere is now considered weak, and being phased out by Microsoft. This should be avoided.
  • 3Des (Triple Data Encryption Standard) uses DES and encrypts three times hence the ‘triple’. The original DES uses a weak key length and is considered weak.
  • AES (Advanced Encryption Standard) 128 bit block size using 128, 192 and 256 bit keys to encrypt data, is all good.

Many other options are available that are not so common include Blowfish, Twofish, Serpent etc. I won’t be going into the different ciphers here or the difference between Block (3DES+ AES) and Stream (RC4) on this page, I’ll save this for another blog.

Digital Signature – The digital signature is used to verify the server.

Integrity check – Here SHA-2 or SHA 256 (Secure Hash Algorithm) should be used. MD5 and SHA1 are being phased out due to weaknesses. SHA1 will still be seen on certificates however Google Chrome will now show a warning for this since October 2014. Microsoft has a deprecation policy indicating SHA1 issued certificates should not be used after 1/1/2017.

With all that being said, lets look at a typical cipher suite. Below is what you might commonly see in the likes of Firefox if you click on the padlock in the address bar and then click on more information.

Cipher suite in use in Firefox
Cipher suite in use in Firefox

Lets look at the cipher suite below for an example. We’ll break down the individual blocks to see what it actually all means.

TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256

TLS – The protocol in use
ECDHE – Elliptic Curve Diffie-Hellman key-exchange using ephemeral keys. More on ephemeral keys later, however this is what is going to give you that all important ‘Perfect Forward Secrecy’. Marked with the E at the front or behind for Ephemeral.
ECDSA – Elliptic Curve Digital Signature Algorithm, used to create the digital signature for authentication.
AES_128 – Advanced Encryption Standard 128 bit key size, used for the session encryption method for data.
GCM – Galois/Counter Mode an operation for block ciphers designed to provide both data authenticity (integrity) and confidentiality. GCMAC – provides authentication only.
SHA256 – Secure hashing Algorithm 256bit used for message integrity.

With the above knowledge and knowing the current vulnerabilities in SSL and TLS we can now make an informed decision and build the cipher suites we would like to use in Windows and Linux.

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