TLS[edit]

Introduction[edit]

Transport Layer Security (TLS) is a cryptographic protocol that is designed to provide secure communications over a computer network. TLS has replaced the deprecated Secure Sockets Layer (SSL) predecessor and is intended to enforce privacy and data integrity between two or more communicating computer applications. ^[1] TLS is utilized for a host of online activities, such as web browsing, email, instant messaging and VOIP applications. It ensures the client (like a web browser) is securely communicating with a server (such as whonix.org), meaning the connection is private, authenticated and reliable. For a detailed overview of the TLS design, refer to this Wikipedia entry ^[archive].

TLS Attacks[edit]

A significant number of attacks have been demonstrated against the SSL/TLS protocol in the recent past, including: ^[2]

BEAST attack: violation of same origin policy constraints.
ChangeCipherSpec injection attack ^[archive]: a specially crafted handshake forces the use of weak keyring material, allowing decryption and modification of traffic in transit.
Cross protocol attacks: servers are attacked by exploiting their support of obsolete, insecure SSL protocols to leverage attacks on connections using up-to-date protocols.
Heartbleed ^[archive]: private keys are stolen from servers, allowing anyone to read the memory of protected systems.
POODLE attack ^[archive]: padding attacks which reveal the contents of encrypted messages.
Protocol downgrade ^[archive]: web servers are tricked into negotiating connections with earlier versions of TLS that are insecure.
RC4 attack ^[archive]: recovery of plain text relying on the RC4 cipher suite.
Renegotiation attack ^[archive]: plaintext injection attacks via the hijacking of the https connection.
TLS Compression (CRIME attack) ^[archive]: session hijacking of web sessions via recovery of secret authentication cookies.
Truncation attack: victim logout requests are blocked so the user remains logged into a web service.
Unholy PAC attack: URLs are exposed when a user attempts to reach a TLS-enabled web link.

In addition, little trust should be placed in the public TLS certificate authority (CA) system, since it relies on a third-party correctly establishing the authenticity of certificates. If/once the CA is subverted, then the security of the entire system is lost, and potentially all entities relying on the trust of the compromised CA are affected. ^[3]

The Snowden leaks confirmed that CAs were a weakpoint targeted by the IC, allowing for Man-in-the-middle attacks if the CAs were either compromised or cooperative. Examples of CA security breaches include DigiNotar ^[archive], Comodo ^[archive] and Turktrust ^[archive].

Whonix ™ Technical Design[edit]

TLS certificates, especially for https://check.torproject.org ^[archive] (check.tpo) are not yet pinned in Whonix ™; this is a future goal that requires further discussion. How pinning could be technically achieved is documented under Dev/SSL Certificate Pinning. At present this is a low priority for Whonix ™, since:

Not even the Tor Browser Bundle pins the check.tpo TLS certificate (which is a much bigger issue). ^[4]
It is only used when whonixcheck is run with command line parameter --leak-tests which does not happen by default.