Nytro Posted August 24, 2016 Report Posted August 24, 2016 SWEET32: Birthday attacks on 64-bit block ciphers in TLS and OpenVPN CVE-2016-2183, CVE-2016-6329 Cryptographic protocols like TLS, SSH, IPsec, and OpenVPN commonly use block cipher algorithms, such as AES, Triple-DES, and Blowfish, to encrypt data between clients and servers. To use such algorithms, the data is broken into fixed-length chunks, called blocks, and each block is encrypted separately according to a mode of operation. Older block ciphers, such as Triple-DES and Blowfish use a block size of 64 bits, whereas AES uses a block size of 128 bits. It is well-known in the cryptographic community that a short block size makes a block cipher vulnerable to birthday attacks, even if the are no cryptographic attacks against the block cipher itself. We observe that such attacks have now become practical for the common usage of 64-bit block ciphers in popular protocols like TLS and OpenVPN. Still, such ciphers are widely enabled on the Internet. Blowfish is currently the default cipher in OpenVPN, and Triple-DES is supported by nearly all HTTPS web servers, and currently used for roughly 1-2% of HTTPS connections between mainstream browsers and web servers. We show that a network attacker who can monitor a long-lived Triple-DES HTTPS connection between a web browser and a website can recover secure HTTP cookies by capturing around 785 GB of traffic. In our proof-of-concept demo, this attack currently takes less than two days, using malicious Javascript to generate traffic. Keeping a web connection alive for two days may not seem very practical, but it worked easily in the lab. In terms of computational complexity, this attack is comparable to the recent attacks on RC4. We also demonstrate a similar attack on VPNs that use 64-bit ciphers, such as OpenVPN, where long-lived Blowfish connections are the norm. Countermeasures are currently being implemented by browser vendors, OpenSSL, and the OpenVPN team, and we advise users to update to the latest available versions. Our results will appear in the following technical paper at ACM CCS 2016: On the Practical (In-)Security of 64-bit Block Ciphers — Collision Attacks on HTTP over TLS and OpenVPN Karthikeyan Bhargavan, Gaëtan Leurent Link: https://sweet32.info/ Quote
