Noose around Internet’s TLS system tightens with 2 new decryption attacks: The noose around the neck of the Internet’s most widely used encryption scheme got a little tighter this month with the disclosure of two new attacks that can retrieve passwords, credit card numbers and other sensitive data from some transmissions protected by secure sockets layer and transport layer security protocols.
Both attacks work against the RC4 stream cipher, which is estimated to encrypt about 30 percent of today’s TLS traffic. Cryptographers have long known that some of the pseudo-random bytes RC4 uses to encode messages were predictable, but it wasn’t until 2013 that researchers devised a practical way to exploit the shortcoming. The result was an attack that revealed small parts of the plaintext inside an HTTPS-encrypted data stream. It required attackers to view more than 17 billion (234) separate encryptions of the same data. That was a high bar, particularly given that the attack revealed only limited amounts of plaintext. Still, since the researchers demonstrated the attack could decrypt HTTPS-protected authentication cookies used to access user e-mail accounts, Google and other website operators immediately took notice.
Now, researchers have figured out refinements that allow them to recover RC4-protected passwords with a 50-percent success rate using slightly more than 67 million (226) encryptions, a two-order of magnitude reduction over the previous attack used to recover secure cookies. The exploits—laid out in a paper published last week titled Attacks Only Get Better: Password Recovery Attacks Against RC4 in TLS—work against both Basic access authentication over HTTPS and the widely used IMAP protocol for retrieving and storing e-mail.