Protection Against Side-Channel Attacks Added to OpenSSH

Protection against Spectre, Meltdown, Rowhammer, RAMBleed and other side-channel attacks has been added to OpenSSH.

These recently uncovered attack methods can allow malicious actors to obtain passwords, encryption keys and other sensitive information from memory by leveraging bit flips or speculative execution.

OpenSSH, the free and open source version of the Secure Shell (SSH) tools designed for securely controlling devices and transferring data, now includes a mechanism designed to protect private keys at rest in memory, said OpenSSH developer Damien Miller.

Private keys, when not in use, are now encrypted with a symmetic key derived from a large “prekey” consisting of random data, Miller said. In order to obtain the protected private key, an attacker would first need to recover the entire prekey, which is not an easy task due to the bit error rates in current side-channel attack methods.

“Implementation-wise, keys are encrypted ‘shielded’ when loaded and then automatically and transparently unshielded when used for signatures or when being saved/serialised,” Miller explained. “Hopefully we can remove this in a few years time when computer architecture has become less unsafe.”

In the case of the recently disclosed RAMBleed, researchers demonstrated the impact of the attack by targeting OpenSSH and leaking a 2048-bit RSA key. However, they have highlighted that OpenSSH was merely a convenient target for demonstrating RAMBleed and it’s not more vulnerable compared to other software.

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