Indirectly and in a fashion that has no direct incidence on your communication security (keys are long enough), Skype makes you pay for security. According to an external audit ordered by Skype whose results are available on Skype security page, Skype discriminates between basic users and those paying for advanced services.

(…) there are two Central Server key pairs, one with a modulus of 1536 bits and the other with a modulus of 2048 bits. The choice of which modulus to use is made by the Central Server. It depends on whether the enrolling user has purchased any Skype premium services, e.g. SkypeOut. If so, the longer modulus is used. If not, the shorter modulus is used. An enrolled user who purchases a premium service for the first time will be issued a new IC, signed with the longer key.

Sorry for that “geeky” post but I was just surfing and one thing led to another so I ended up skimming through this document… well, I’m curious and I can’t deny my past at SafeLogic, creating ConfiMail (an encryption webmail). :)