Firefox blocks websites with vulnerable encryption keys

To protect users from cryptographic attacks that can compromise secure web connections, the popular Firefox browser will block access to HTTPS servers that use weak Diffie-Hellman keys.

Diffie-Hellman is a key exchange protocol that is slowly replacing the widely used RSA key agreement for the TLS  (Transport Layer Security) protocol. Unlike RSA, Diffie-Hellman can be used with TLS's ephemeral modes, which provide forward secrecy -- a property that prevents the decryption of previously captured traffic if the key is cracked at a later time.

However, in May 2015, a team of researchers devised a downgrade attack that could compromise the encryption connection between browsers and servers if those servers supported DHE_EXPORT, a version of Diffie-Hellman key exchange imposed on exported cryptographic systems by the National Security Agency in the 1990s and which limited the key size to 512 bits. In May 2015 around 7 percent of websites on the internet were vulnerable to the attack, which was dubbed LogJam.

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