Private key format problems
What “Private key format problems” usually means
When PuTTY reports Unable to use key file, the client has already attempted a network or protocol step and failed before a normal interactive shell is available. This guide explains what the message usually means on Windows clients, which local settings to verify first, and when the problem is almost certainly on the server, firewall, or DNS path rather than inside the PuTTY GUI.
PuTTY could not use the selected key file—often because it is OpenSSH format, truncated, wrong passphrase, or not a PPK. The Auth panel needs a format PuTTY understands unless you convert first.
Independent resource: Putty.info is educational only and is not affiliated with the official PuTTY project. Compare symptoms against the official documentation for your installed version.
Symptoms you will see
Operators usually notice this failure in one of a few repeatable ways. Capture the exact dialog text, the hostname and port from the Session panel, and whether the failure happens before or after a username prompt—those details decide which checklist below applies.
- Unable to use key file dialog or similar.
- Connection falls back to password or fails entirely.
Likely causes ranked by frequency
Several independent conditions can produce the same client-facing wording. Work from the outside in: reachability first, protocol selection second, authentication third. Changing key files will not fix a TCP refusal, and opening a wider firewall will not fix a rejected public key.
- Selecting id_rsa / OpenSSH private key without conversion.
- PPK version newer than your ancient PuTTY build supports.
- File corruption or incomplete download of the key.
- Wrong passphrase entered repeatedly.
Step-by-step client checks
Use this ordered checklist on a workstation you control. Prefer saved sessions so Host Name, Port, and Connection type stay consistent while you isolate one variable at a time.
- Open the file in PuTTYgen; if it offers conversion, convert and save as PPK.
- Upgrade PuTTY/PuTTYgen from official hosts to read modern PPK versions.
- Confirm you pointed Connection → SSH → Auth at the private PPK, not the .pub file.
- Restore from encrypted backup if the file is truncated.
- See /ssh-keys/convert-openssh-to-ppk/ for conversion steps.
- Never paste private keys into web tools to “fix format.”
Do not paste private keys, passphrases, or production passwords into Putty.info tools or contact forms. Diagnose locally with PuTTY, Plink, PSCP, or PSFTP from an official install.
When it is a server or network issue
Servers only need the public key. Format problems are almost entirely client-side conversion/version issues.
If TCP connects from another network path but fails from yours, involve the network team with traceroute/path evidence rather than repeatedly regenerating keys. If authentication fails after a banner appears, collect the server sshd logs for the same timestamp and username—client-side retries alone rarely reveal AccountLocked, Match blocks, or AllowUsers denials.