Your conversation history includes all the instant messages you’ve sent to and received from your contacts or groups, as well as details of any calls you’ve had on Skype.
How to export/backup your Skype history?
Sign-in Skype > click Skype Settings
In the Settings menu, click Messaging > Export chat history
In the Export your chat history, check the profile you wish to export then click Next
Exporting your Skype history
Skype support open history on the browser once export complete.
As you can see the attached image below, it can easily view on a web browser so an unauthorized user can read your chat history easily. Just back it up then save it into a safe place. Hoping it will be fixed in the next Skype release such as enhance password protection or encryption for history chat.