SSH key exchange gets a post-quantum upgrade
GitHub is adding a hybrid post-quantum key exchange algorithm to its SSH endpoints for Git data. The change covers github.com and GitHub Enterprise Cloud regions outside the US, with support arriving in GitHub Enterprise Server 3.19 as well. HTTPS access and SSH connections to GitHub Enterprise Cloud in the US region are unaffected.
A defense against future decryption
SSH connections rely on a key exchange algorithm for both parties to agree on a shared secret, which is then used to derive encryption and integrity keys. The existing algorithms are secure against today's threats, but an attacker could capture encrypted sessions now and decrypt them later if a sufficiently powerful quantum computer is ever built. This "store now, decrypt later" scenario is what the new algorithm is designed to prevent.
The algorithm, known as sntrup761x25519-sha512 (also referenced by the older name [email protected]), pairs Streamlined NTRU Prime — a post-quantum secure algorithm — with the classical Elliptic Curve Diffie-Hellman method using the X25519 curve. Because post-quantum algorithms are newer and less battle-tested, the hybrid approach ensures security remains at least as strong as the classical algorithm alone.
The US region is excluded from the rollout because it must use only FIPS-approved cryptography, and this post-quantum algorithm is not FIPS-approved.
Effective dates and timeline
The algorithm will be enabled on September 17, 2025 for GitHub.com and GitHub Enterprise Cloud with data residency, except for the US region. GitHub Enterprise Server 3.19 will also include it.
What SSH users should expect
Most connections won't see any practical change. Clients that support the new algorithm — such as OpenSSH 9.0 and newer — will automatically select it if it's preferred in the client's defaults. Users who have customized their client configuration may need to adjust it to make use of the new algorithm.
Older SSH clients will fall back to previously supported key exchange algorithms, so existing SSH workflows should continue to work without interruption. Those clients simply won't benefit from post-quantum security until they're upgraded.
To check whether your OpenSSH version supports the algorithm, run:
ssh -Q kex
If either sntrup761x25519-sha512 or [email protected] appears in the output, the algorithm is available. To see which key exchange algorithm OpenSSH actually uses when connecting to GitHub, run the following in any Unix-like shell (Linux, macOS, Git Bash):
$ ssh -v [email protected] exit 2>&1 | grep 'kex: algorithm:'Users of other SSH implementations should consult their vendor's documentation.
GitHub says it will continue tracking the evolution of post-quantum cryptography and will provide updates as its SSH libraries add support for more algorithms, including those that comply with FIPS requirements.
