Enlarge / From an upcoming paper laying out a new attack against 64-bit block ciphers used by HTTPS and OpenVPN.Karthikeyan Bhargavan and Gaëtan Leurent
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Researchers have devised a new attack that can decrypt secret session cookies from about 1 percent of the Internet’s HTTPS traffic and could affect about 600 of the Internet’s most visited sites, including nasdaq.com, walmart.com, match.com, and ebay.in.
The latter must be done either by actively manipulating an HTTP response on the wire or by hosting a malicious website that the user is tricked into visiting.
A related attack against OpenVPN requires 18 hours and 705GB of data to recover a 16-byte authentication token.
Impractical no more
Despite the difficulty in carrying out the attack, the researchers said it works in their laboratory and should be taken seriously.
They are calling on developers to stop using legacy 64-bit block-ciphers.
For transport layer security, the protocol websites use to create encrypted HTTPS connections, that means disabling the Triple DES symmetric key cipher, while for OpenVPN it requires retiring a symmetric key cipher known as Blowfish.
Ciphers with larger block sizes, such as AES, are immune to the attack.
“It is well-known in the cryptographic community that a short block size makes a block cipher vulnerable to birthday attacks, even if the[re] are no cryptographic attacks against the block cipher itself,” the researchers wrote in a blog post explaining the attacks. “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.”
A birthday attack is a type of cryptographic exploit that is based on the mathematical principle known as the birthday paradox.
It holds that in a room of 23 randomly selected people, there is a 50-percent chance two of them will share the same birthday, and there’s a 99.9 percent chance when the number is increased to 70 people.
The same principle can be used by cryptographers to find so-called collisions, in which the output of two chunks of encrypted text is the same.
Collisions, in turn, easily return the plaintext.
By collecting hundreds of gigabytes worth of HTTPS or VPN data and carefully analyzing it, the attackers are able to recover the sensitive cookie.
In response to the new attack, which the researchers have dubbed Sweet32, OpenVPN developers on Tuesday released a new version of the program that actively discourages the use of 64-bit ciphers. OpenSSL maintainers, meanwhile, said in a blog post that they plan to disable Triple DES in version 1.1.0, which they expect to release on Thursday.
In versions 1.0.2 and 1.0.1, they downgraded Triple DES from the “high” to “medium,” a change that increases the chances that safer ciphers are used to encrypt data traveling between servers and end users.
The precise cipher choice is made dynamically and is based on a menu of options supported by both parties. While stripping Triple DES out of all versions would be the safest course, it also would leave some people unable to browse certain HTTPS sites altogether.
“A matter of good hygiene”
“When you have a large installed base, it is hard to move forward in a way that will please everyone,” Rich Salz, a senior architect at Akamai Technologies and a member of the OpenSSL developer team, wrote. “Leaving triple-DES in ‘DEFAULT’ for 1.0.x and removing it from 1.1.0 is admittedly a compromise. We hope the changes above make sense, and even if you disagree and you run a server, you can explicitly protect your users through configuration.”
Browser makers are also in the process of making changes that prioritize safer ciphers over Triple DES.
The Sweet32 attack will be presented in October at the 23rd ACM Conference on Computer and Communications Security. While the time and data-collection requirements present a significant barrier, it works as described on sites that support Triple DES and allow long-lived HTTPS connections.
As of May, about 600 websites in the Alexa 100,000 were identified, including those mentioned at the beginning of this article. Karthikeyan Bhargavan and Gaëtan Leurent—the researchers behind Sweet32—estimate that about 1 percent of the Internet’s HTTPS traffic is vulnerable. OpenSSL team member Viktor Dukhovni summed things up well in an e-mail.
“We’re not making a fuss about the 3DES issue, and rating it ‘LOW,” Dukhovni wrote. “The 3DES issue is of little practical consequence at this time.
It is just a matter of good hygiene to start saying goodbye to 3DES.”