From Electronic Freedom Foundation:

This security limitation has concerned us for a long time. Last year, Chris Soghoian argued that, for this reason, “Skype is in a position to give the government sufficient data to perform a man in the middle attack against Skype users.” Soghoian argued that Skype should change its design to eliminate this ability, or else disclose the risk more prominently. One way of limiting man-in-the-middle attacks would be for Skype to introduce a way for users to do their own encryption key verification, without relying on the Skype service. As Soghoian notes, that’s what many other encrypted communications tools do—but such a verification option is missing from Skype.