AB9IL.net: Skype's Robust Security

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Initially named "Sky Peer-To-Peer" and intended to be a robust, tamper resistant, and decentralized voice-over-internet system, Skype has become the most popular tool for voice communications over the internet. It is easy to install, requires modest bandwidth (33.6 kbps) for good quality voice connections, and offers video and chat as additional means of contact with other Skype users.

Skype is particularly unique in that it offers a means for people to talk to each other through a medium that is difficult for sophisticated surveillance teams to track or monitor. In fact, Skype has been banned in many countries that insist on total control of its communications within their borders. Therefore Skype is a favorite of all people who value privacy. Presented here is a summary of the strengths and weaknesses of Skype as it relates to defending the users' privacy from surveillance from all others not a party to the communications. It will be shown that Skype is resistant to wiretapping, but the chat and voice data is vulnerable to capture by spyware prior to encryption and entry to the Skype network.

Skype's Main Strengths:

  1. Designed to use decentralized peer-to-peer connections (nodes and supernodes).
    • Skype cannot be disabled by blocking IP addresses.
    • Data flows difficult to trace or capture due to very adaptable routing scheme.
  2. Strong encryption using 256 bit public key and symmetric protocalls.
    • Unlikely, in the history of human existence, to be broken by a brute force attack.
    • Fast key creation on modern hardware.
    • Encryption protocalls well known and found to be quite secure.
  3. Flexible use of ports - diffuicult to block with firewalls.

Skype does have some vulnerabilities, mostly due to its operating environment on computers subject to spyware and malware installations. Users failing to regularly scan and disinfect their computers are especially vulnerable to having their activities monitored or blocked.

Skypes Main Weaknesses:

  1. Skype's inner workings are proprietary and closed sourced.
  2. Skype can't get exclusive access to the audio data within users' computers.
    • Audio or chat can be captured, prior to encryption, by spyware designed to detect skype calls.
    • Captured audio or chat can be archived by spyware for use by adversaries.
  3. Data packets can be detected, blocked or captured using deep packet inspection.
  4. Some activity is easily detected and blocked, such as the login process.

Monitoring a computer's TCP and UDP traffic reveals the login exchange between the local application and the remote server. Skype transmits the following 5 byte message to the server: 0x1603010000. Then the server replies with this 5 byte message: 0x1703010000. Detect and block incoming packets containing the sequence 0x1703010000 and Skype is disabled. Also, there are skype fingerprints in the public key exchange messages at the start of a connection. The client to server key exchange SSL message begins with 0x160301, indicating a key exchange using SSL version 3.1. Receiving this, the server responds with a message using a non-SSL based header: 0x170301. Detect and block packets containing the header 0x170301 and skype is disabled.

Skype Blocking Software

There are Skype filters which detect the skype protocol and / or traffic on ports used by Skype. The most effective use deep packet inspection to find and block the data packets, while simply blocking the most commonly used ports causes skype to fall back on passing data as http packets. Port blocking is the least effective, due to skype's ability to use any of a huge range of ports!

Skype Blocking Software Vendors:

Bitek International Inc.USA
Lynanda CSCountry
Narus Inc.USA
Verso TechnologiesUSA

In some cases, the attacker's goal is not blocking or interrupting the use of Skype, but instead conducting a wiretap. Spyware is covertly installed on the target's computer and gains access to the communications in unencrypted form. Such anti-Skype spyware is useless in situations where the users have encryption devices external to the computer system hosting Skype.

Users in China or making calls to users in China need to be aware that TOM-Skype is compromised software. It enables full wiretapping by the PRC, and is subject to sophisticated tracking and keyword scanning. It contains internal and downloaded Chinese keyword lists. Do not useit under any circumstances. Because Skype's website redirects visitors from China to TOM-Skype, one must use a VPN to get the regular software. Otherwise, download from the Skype beta page which is accessible directly from within China.

Skype Spyware Vendors:

NamePublisherSkype Vulnerability Exploited
FinSpyGamma International UK Ltd.Captures audio directly from audio interface.
TOM-SkypeTOM Group LtdSpecial version of Skype for China, captures and filters text and chat.
HackingTeam SRL (Milan Hackers)Captures audio directly from audio interface.
Gamma (German Hackers)Captures audio directly from audio interface.
DigiTask GmbHCaptures audio directly from audio interface.
ERA IT Solutions AGCaptures audio directly from audio interface.

The number of companies seeking to make huge profits from the sale of anti-Skype software tells us something about human nature: for every tool created to connect people and decentralize connectivity (and the associated empowerment), there are other adversarial tools created by those seeking wealth by catering to governments and organizations interested in disconnecting people while centralizing and controlling communication (thereby taking power and freedom from people). Countries such as Ethiopia have gone so far as to criminalize the use of Skype. In truth, the use of Skype can be effectively concealed and conversations protected even in Skype-hostile countries, if proper measures are taken.

Many measures can be taken to render ineffective the types of anti-Skype software listed above. The focus of defending communications through Skype is to keep spyware / malware off of the Skype user's computer and to make Skype data difficult to detect and block. Below is a list of ways to defend Skype communications from eavesdropping and blocking:

  1. Never, EVER, use TOM-Skype and do not connect to users of TOM-Skype
  2. Use a VPN to prevent detection of Skype calls.
  3. Use a VPN to further protect the data stream from cryptanalysis.
  4. Keep your system free of spyware / malware.
  5. Use Linux or Linux with WINE to run Skype - the spyware / malware is almost exclusively Windows!!
  6. Use a live CD like knoppix or slax, with Skype installed, that can't be infected or altered by spyware or malware.
  7. Use an external audio encryption tool to encode / decode conversations.

After considering the above information on Skype, one can feel secure using it as a means of sharing sensitive information under specific controlled conditions. Specifically, both users should use computers known to be free of spyware or malware. An operating system other than Microsoft Windows is safer, but should still be regularly scanned and purged of rootkits or other unwanted tools of surveillance agencies. Network communications security is only as strong as its weakest link. Except for TOM-Skype, the things putting Skype users at risk are external to Skype and not a factor on a well maintained computer system.

Further Reading on the Strength of Skype Security:

TOM Skype Is Spyware!!
TOM Skype Research
Inferring Speech Activity from Encrypted Skype Traffic
A Study of Skype Blocking Methods
Skype Protocol Analysis
Skype Relay Analysis
Tracking Skype Traffic

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