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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteFirst identify where SSH fails. no matching key exchange method found means the client and server could not agree on connection algorithms; Permission denied (publickey) means SSH reached user authentication but the server did not accept the offered identity. Post-quantum key exchange and the public key that authenticates your account are separate mechanisms, so the right fix depends on the error.
Identify the failure stage from the error
| What you see | Failure stage | What to check |
|---|---|---|
no matching key exchange method found |
Connection negotiation | Whether the client and server have at least one shared key-exchange algorithm enabled. |
Permission denied (publickey) |
User authentication | Which identity the client offers and whether the corresponding public key is authorized for the target account. |
| A warning that the connection is not using a post-quantum key exchange | Negotiation completed, but without a post-quantum method | Whether the server supports a compatible hybrid method; a warning override only suppresses the warning. |
OpenSSH treats connection negotiation and user authentication as distinct stages. Its legacy algorithm guidance explains that successful negotiation requires a shared option for each connection parameter.
Understand what changed when you replaced a post-quantum key
In OpenSSH terminology, post-quantum protection in this context refers to hybrid key agreement, configured through KexAlgorithms. Key agreement establishes the session’s cryptographic keys. It is not the same as the user’s public/private key pair used to log in.
OpenSSH has offered post-quantum key agreement by default since version 9.0, initially using sntrup761x25519-sha512. Version 9.9 added mlkem768x25519-sha256, which became the default in version 10.0. These milestones matter when one peer is older or its configuration disables the relevant methods. See the OpenSSH post-quantum cryptography page.
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Replacing an experimental login identity does not, by itself, change the server’s supported key-exchange algorithms. Conversely, updating key exchange does not install a replacement login public key for your account.
Fix “no matching key exchange method found”
- Record the full error. Confirm that it names key exchange rather than public-key authentication. If it includes a list of algorithms offered by the server, preserve that detail for the server administrator.
- Check the client and server versions. OpenSSH 9.0 and later supports
sntrup761x25519-sha512; 9.9 and later supportsmlkem768x25519-sha256. A peer that predates these releases, or has them disabled, may have no method in common with a client requiring one. - Compare effective algorithm settings. Check the client’s and server’s effective
KexAlgorithmsconfiguration, including any policy that overrides defaults. The connection succeeds only if the peers share an enabled method. - Prefer updating the incompatible peer. If the server offers neither supported post-quantum method, OpenSSH’s recommended path is to update the server rather than weaken the client’s policy. Coordinate with the administrator if you do not manage it.
- Use a legacy exception only when necessary. OpenSSH documents temporary re-enablement of disabled algorithms for legacy connections, but those algorithms are disabled because the project recommends against them. Keep any exception narrowly scoped to the affected host and remove it when compatibility is restored. Do not copy a broad algorithm override from an unrelated error.
Fix “Permission denied (publickey)” after replacing a login key
- Confirm which private key the client is offering. Make sure your SSH client is using the new identity rather than an old key or a different identity. If you have multiple identities configured, check the effective client configuration for the target host.
- Install the matching public key for the correct account. The public half of the replacement key must be present in that account’s
~/.ssh/authorized_keys, or in the server’s configured authorized-key source. A key installed for another account will not authorize this login. - Verify that the installed public key matches the private identity. Do not substitute a different key’s public half or assume that changing the client’s identity automatically updates the server.
- Ask the server administrator to check the authorization source and policy. The server may use a configured key source rather than the account’s default file, or may have authentication policy that rejects the offered identity. The OpenBSD manual explains that the public key must be added to
authorized_keyson machines where that identity is to be used: OpenBSD ssh manual.
If OpenSSH warns that the connection is not post-quantum
OpenSSH 10.1 warns when a connection selects non-post-quantum key exchange. The project’s warning says the session may be vulnerable to “store now, decrypt later” attacks and that the server may need to be upgraded. If the connection otherwise works, this warning is not the same as a rejected login key.
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The preferred remedy is to upgrade a server that offers neither supported hybrid method. When an upgrade is not possible or an administrator accepts the risk, OpenSSH documents WarnWeakCrypto as a way to suppress the warning selectively. Suppression does not add post-quantum protection to the connection; see the OpenSSH release notes and guidance.
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Choose the remedy that matches the symptom
- Negotiation error: compare supported and enabled
KexAlgorithms, then update the incompatible peer where possible. - Public-key denial: confirm the offered private identity and authorize its matching public key for the target account.
- Non-post-quantum warning: arrange a server upgrade for hybrid key exchange; use warning suppression only as a selective acknowledgment, not as a cryptographic fix.
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