With the release of OpenSSH 9 and the adoption of the hybrid Streamlined NTRU Prime + x25519 key exchange technique, post-quantum cryptography has become the default.
"The NTRU algorithm is thought to be resistant to assaults enabled by future quantum computers, and it is combined with the X25519 ECDH key exchange (the former default) as a backstop against any future flaws in NTRU Prime. The combination assures that the hybrid exchange provides at least as much security as the current system "According to the release notes.
"We are making this modification now (i.e. before cryptographically relevant quantum computers) to prevent 'capture now, decrypt later' attacks in which an adversary who can record and store SSH session ciphertext can decrypt it once a sufficiently capable quantum computer is available."
As progress on quantum computers has been made, so has the need to protect against future attacks. Because of the tremendous parallelism envisaged from viable quantum computers, classical cryptography is projected to be straightforward to crack once such a machine is developed.
The NATO Cyber Security Centre tested its quantum-proof network last month.
"Securing NATO's communications for the quantum era is critical to our capacity to function efficiently and without fear of interception," said chief scientist Konrad Wrona at the time.
"The trial began in March 2021. The experiment concluded in early 2022. Quantum computing is becoming more inexpensive, scalable, and useful. All organisations, including NATO, are planning to respond to the threat of 'harvest now, decrypt later."
Elsewhere in the mostly bug-fixing OpenSSH release, the SCP command has been switched from its default legacy protocol to SFTP, despite the fact that it introduces several incompatibilities, such as not supporting wildcards with remote filenames or expanding a user path, though the latter is supported via an extension.
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