To use an ECDSA SSH key, generate a key pair with ssh-keygen, install the public key for the correct remote account, and keep the private key on your own device. If login fails, first check which key the client offers, where the server looks for authorized keys, and whether the public-key line is intact.
Generate an ECDSA key pair
On a system with OpenSSH, run:
ssh-keygen -t ecdsa -b 256 -C "your-label"When prompted, accept the default file path or enter a different one. The usual files are ~/.ssh/id_ecdsa (private key) and ~/.ssh/id_ecdsa.pub (public key). You can set a passphrase to encrypt the private key. OpenBSD’s ssh-keygen manual documents ECDSA sizes of 256, 384, or 521 bits; these are the available curve-size choices, not arbitrary values for -b.
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The server receives only the public key. Keep the private key readable only by your user; do not copy it into a server account, share it, or include its contents in logs or support requests. The public key is designed to be installed on the server. OpenBSD’s ssh manual describes the distinction: “The server knows the public key, and only the user knows the private key.”
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Install the public key for the right account
Copy the entire contents of ~/.ssh/id_ecdsa.pub into the authorized-key file for the remote account you intend to use. The conventional location is ~/.ssh/authorized_keys in that account’s home directory. The client manual describes this public-key workflow, while the server’s sshd_config manual documents that AuthorizedKeysFile can change the location or disable file-based key lookup.
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- Identify the destination account. The account in your SSH command must be the same account whose authorized-key file you edit. For example,
ssh [email protected]requires the key to be authorized foralice, not for another user on that host. - Transfer the public key. Use a trusted method to append the complete single line from the
.pubfile to the account’s configured authorized-key file. If the server administrator specifies a different path, use that instead of assuming the default. - Preserve the line exactly. An authorized-key entry includes a key type and encoded public-key data, with optional options and a comment. Do not wrap, truncate, retype, or substitute the private-key file. The sshd manual lists ECDSA nistp256, nistp384, and nistp521 key types and describes authorized-key syntax.
- Check server-side access rules if it still fails. Ownership and permission requirements can depend on the operating system, account layout, ACLs, and sshd configuration. Check the server’s authentication logs and the effective sshd settings rather than applying one chmod recipe blindly.
Choose an ECDSA size with compatibility in mind
The documented ECDSA choices for ssh-keygen are 256, 384, and 521 bits. The cited manual establishes the available sizes, not a universally best choice. Use the size permitted by your organization’s cryptographic policy and supported by both the client and server. A hardware-backed ECDSA security key, such as [email protected], is a separate setup from an ordinary software ECDSA key and requires compatible hardware and software.
Diagnose a rejected or ignored key
Work from client-side selection toward server-side policy. OpenSSH’s ssh manual documents verbose mode for examining public-key authentication attempts.
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- See what identity the client offers. Run
ssh -v user@hostand inspect the output for the intended key. Increase verbosity if needed. If the key is not offered, specify its private-key path withssh -i /path/to/private_key user@host; confirm the client user can read that file. - Verify the username and host. Make sure the connection targets the server and account where the public key was installed. A key authorized for a different account will not grant access to the requested one.
- Confirm the configured key-file path. The default may include
~/.ssh/authorized_keysand~/.ssh/authorized_keys2, but the server’sAuthorizedKeysFilesetting can use another path or disable file-based lookup. Check the effective server configuration if you administer the host; otherwise ask its administrator to verify it. - Recheck the public-key entry. Compare the server entry with the full line in the local
.pubfile. A wrapped, incomplete, edited, or wrong key line will not match the private identity being offered. - Use server logs to identify permission or policy refusals. If the client offers the intended key and the path and entry are correct, inspect authentication logs and sshd configuration for the reason the server rejects it. Apply the ownership and permission requirements for that server rather than assuming one universal fix.
- Check implementation and version compatibility last. Compare the algorithms supported by the installed client and server. OpenSSH’s release notes record changes over time, so old advice about enabling or disabling an algorithm may not apply to a current installation.
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