31 August 2016
Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space. Received monthly by more than 50,000 subscribers. Maintained by Hanno Böck.
Older block ciphers like Triple-DES and Blowfish encrypt data in blocks of 64 bits. It is well known that the block size limits the amount of data that can be encrypted safely with the same key. After encrypting 2^32 blocks of data with the same key block collisions become very likely due to the birthday paradox.
Karthikeyan Bhargavan and Gaëtan Leurent showed that this weakness can be used to practically attack TLS connections using old cipher modes with Triple-DES and OpenVPN connections using Blowfish. They named their attack SWEET32. Both attack scenarios require several hundred gigabytes of data and take between 20 and 40 hours. These attacks also may be mitigated by factors like limits for Keep-Alive connections in web servers.
In TLS the issue can be completely avoided by disabling Triple-DES-based cipher suites. However Triple-DES is still supported by many servers because it's a compatibility option for old clients. Most notably Windows XP's own TLS implementation and applications using it (like Internet Explorer) support no stronger cipher modes. It remains to be seen whether SWEET32 will cause widespread deprecation of Triple-DES.
Mitigation of this attack is difficult. The implementation of Same-Site Cookies can prevent compression attacks in some scenarios.
WPAD was proposed as a draft to the IETF in 1999, but it never became an official RFC. But all major browsers still support it. Some browser vendors were not vulnerable to the attack because they stripped the URL, others have implemented similar protections in response to the recent findings.
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