The easiest way to find poor implementations of SSL and TLS encryption algorithms is to run the SSL Scan tool.  If you run it against this website you will get something along the lines of:

sslscan --no-failed www.yg.ht:443
_
___ ___| |___  ___ __ _ _ __
/ __/ __| / __|/ __/ _` | '_ \
\__ \__ \ \__ \ (_| (_| | | | |
|___/___/_|___/\___\__,_|_| |_|

Version 1.8.2
http://www.titania.co.uk
Copyright Ian Ventura-Whiting 2009

Testing SSL server www.yg.ht on port 443

Supported Server Cipher(s):
Accepted  SSLv3  256 bits  DHE-RSA-AES256-SHA
Accepted  SSLv3  256 bits  DHE-RSA-CAMELLIA256-SHA
Accepted  SSLv3  256 bits  AES256-SHA
Accepted  SSLv3  256 bits  CAMELLIA256-SHA
Accepted  SSLv3  168 bits  EDH-RSA-DES-CBC3-SHA
Accepted  SSLv3  168 bits  DES-CBC3-SHA
Accepted  SSLv3  128 bits  DHE-RSA-AES128-SHA
Accepted  SSLv3  128 bits  DHE-RSA-SEED-SHA
Accepted  SSLv3  128 bits  DHE-RSA-CAMELLIA128-SHA
Accepted  SSLv3  128 bits  AES128-SHA
Accepted  SSLv3  128 bits  SEED-SHA
Accepted  SSLv3  128 bits  CAMELLIA128-SHA
Accepted  SSLv3  128 bits  RC4-SHA
Accepted  TLSv1  256 bits  DHE-RSA-AES256-SHA
Accepted  TLSv1  256 bits  DHE-RSA-CAMELLIA256-SHA
Accepted  TLSv1  256 bits  AES256-SHA
Accepted  TLSv1  256 bits  CAMELLIA256-SHA
Accepted  TLSv1  168 bits  EDH-RSA-DES-CBC3-SHA
Accepted  TLSv1  168 bits  DES-CBC3-SHA
Accepted  TLSv1  128 bits  DHE-RSA-AES128-SHA
Accepted  TLSv1  128 bits  DHE-RSA-SEED-SHA
Accepted  TLSv1  128 bits  DHE-RSA-CAMELLIA128-SHA
Accepted  TLSv1  128 bits  AES128-SHA
Accepted  TLSv1  128 bits  SEED-SHA
Accepted  TLSv1  128 bits  CAMELLIA128-SHA
Accepted  TLSv1  128 bits  RC4-SHA

Prefered Server Cipher(s):
SSLv3  256 bits  DHE-RSA-AES256-SHA
TLSv1  256 bits  DHE-RSA-AES256-SHA

In that long list you are looking for any old cipher suites that have known cryptographic weaknesses.  For example any which use “SSLv2”.  You possibly want to also identify any symmetric algorithms that have a bit length smaller than 128 and a-symmetric algorithms that have a bit length smaller than 2048 as well.

It is important to keep relatively up-to-date with the various cipher suites available as over time weaknesses are found in older suites making them vulnerable to attack.  There are, for example, currently questions and indeed vulnerabilities for some of the encryption algorithms that are in popular use today.   Lookup the BEAST attack and the RC4 problems and decide for yourself what you would do.

Keep in mind that the list of known “good” and “bad” ciphers changes pretty often.

*** UPDATE *** The tool demonstrated above has changed significantly since this article was published as a result of many additional cryptographic qualities that need checking. The current version of SSLScan can be found here

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