SSL ciphers and protocols

The most common implementation of SSL is OpenSSL. This document will try to explain a few SSL concepts and show useful commands to test and troubleshoot SSL connections.

Ciphers

A cipher in SSL has a name. A typical name is ECDHE-RSA-AES256-GCM-SHA384. The name is consisted of parts, that in order signify the following:

• Key exchange algorithm (Kx)
• Authentication (Au)
• Encryption algorithm (Enc)
• MAC digest algorithm (Mac)

So for the sample cipher one can deduce that:

• For key exchange it uses ECDHE
• For authentication RSA
• For encryption AES265-GCM (GCM ensures block integrity)
• For MAC SHA384

Lists ciphers supported by OpenSSL:

$ openssl ciphers
ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:SRP-DSS-AES-256-CBC-SHA:SRP-RSA-AES-256-CBC-SHA:SRP-AES-256-CBC-SHA:DH-DSS-AES256-GCM-SHA384:DHE-DSS-AES256-GCM-SHA384...

Protocols

Protocols define how an encrypted connectio is initiated. Typically, one would find the following protocols:

• SSLv2 (insecure and obsoleted)
• SSLv3 (insecure and obsoleted)
• TLSv1.0 (has vulnerabilities and should be avoided)
• TLSv1.1 (has vulnerabilities and should be avoided)
• TLSv1.2 (secure and widely implemented)
• TLSv1.3 (secure but not widely implemented)

Not all protocols support all ciphers. By requesting a ciphers verbose output, the protocols are printed as well:

$yannis@ouranos:~$ openssl ciphers -v
ECDHE-RSA-AES256-GCM-SHA384     TLSv1.2 Kx=ECDH Au=RSA      Enc=AESGCM(256) Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384   TLSv1.2 Kx=ECDH Au=ECDSA    Enc=AESGCM(256) Mac=AEAD
ECDHE-RSA-AES256-SHA384         TLSv1.2 Kx=ECDH Au=RSA      Enc=AES(256)    Mac=SHA384
ECDHE-ECDSA-AES256-SHA384       TLSv1.2 Kx=ECDH Au=ECDSA    Enc=AES(256)    Mac=SHA384
ECDHE-RSA-AES256-SHA            SSLv3   Kx=ECDH Au=RSA      Enc=AES(256)    Mac=SHA1
ECDHE-ECDSA-AES256-SHA          SSLv3   Kx=ECDH Au=ECDSA    Enc=AES(256)    Mac=SHA1
...

Connecting to SSL servers

$ openssl s_client -connect host:port {protocol} {cipher}

• {protocol}:= [-ssl2

  -ssl3

  -tls1

  -tls1_1

  -tls1_2]

• {cipher}:= -cipher cipher_name

To probe a server for accepted protocols and ciphers, the Nmap tool has a really nifty script/command to do so:

nmap --script ssl-enum-ciphers

Useful resources

• OpenSSL ciphers manual page (man ciphers)
• List ciphers used by JVM
• TLS Cipher String Cheat Sheet
• Check supported ciphers script
• SSL server test
• SSL client test