A SWF file playing in a browser has many of the same security concerns as an HTML page being displayed in a browser. This includes the security of the SWF file while it is being loaded into the browser, as well as the security of communication between Flash and the server after the SWF file has loaded and is playing in the browser. In particular, data communication between the browser and the server is susceptible to being intercepted by third parties. The solution to this issue in HTML is to encrypt the communication between the client and server to make any data captured by third parties undecipherable and thus unusable. This encryption is done by using an SSL-enabled browser and server.
Because a SWF file running within a browser uses the browser for almost all of its communication with the server, it can take advantage of the browser's built-in SSL support. This lets communication between the SWF file and the server be encrypted. Furthermore, the actual bytes of the SWF file are encrypted while they are being loaded into the browser. Thus, by playing a SWF file within an SSL-enabled browser through an HTTPS connection with the server, you can ensure that the communication between Flash Player and the server is encrypted and secure.
The one exception to this security is the way Flash Player uses persistent sockets (through the ActionScript XMLSocket object), which does not use the browser to communicate with the server. Because of this, SWF files that use sockets cannot take advantage of the built-in encryption capabilities of the browser. However, you can use one-way encryption algorithms written in ActionScript to encrypt the data being communicated.