Skype

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Skype is an internet-based instant messaging service. It was introduced in 2003 and has been owned by Microsoft since 2011. The service provides video telephony, video conferencing, IP telephony, instant messaging, file transfer and screen sharing. It can be used with a special application program or with a browser via the Skype website. Data transmission is based on a copyright protected network protocol. For the encryption of messages, calls and video calls, users are provided with the use of the free signal protocol, which is classified by security experts as secure.

Internet calls with customers of other online services are not possible. Fees are charged for connections to fixed and mobile networks. In order to receive calls from the conventional telephone network, a landline telephone number must be purchased. It is possible to set up such a number for around 25 countries without being physically present in these countries. For some countries, proof of residence (but not actual presence) is required for legal reasons: these are currently Germany, Switzerland, the Netherlands, France and South Korea. In the current versions for Windows and macOS, conference calls with up to 50 participants are possible.

Technical characteristics
The software works without problems behind most firewalls and NAT routers, as a variant of the STUN protocol for connection is used for communication. In addition, Skype can use TCP ports 80 and 443 for connection establishment, which are usually used for surfing the World Wide Web.

For data compression, Skype uses the codecs SVOPC (16 kHz), AMR-WB (16 kHz), G.729 (8 kHz), G.711, previously also ISAC and ILBC. Since 2009, the in-house audio codec SILK has been used, the source code of which was submitted to the IETF in March 2010.

Skype works with any standard headset or microphone speaker configuration – the device integrated into the operating system is recognized.

For connections from device to device, encryption with AES-256 is used, the keys would be transmitted with 1,536 to 2,048 bit RSA. However, since this is a closed source program, this information cannot be checked by everyone. However, they are informed by a security evaluation commissioned by Skype by renowned cryptology expert Tom Berson on the 18th. Confirmed October 2005. On the other hand, the developer of Pretty Good Privacy, Phil Zimmermann, expressed criticism in connection with plans for their own VoIP software: Zfone. Since mid-2010, Skype’s encryption methods have also been publicly exposed. Reverse Engineering determined that in addition to AES-256, three variants of RC4 were used whose implementations were vulnerable. Skype has previously been investigated