How to fix key_load_public: invalid format in SSH

Check what format your key file is in. The first line should indicate the format:

head -1 ~/.ssh/id_rsa.pub

Expected first lines:
- OpenSSH public key: ssh-rsa AAAAB3NzaC1yc2E... (or ssh-ed25519 AAAA...)
- OpenSSH private key: -----BEGIN OPENSSH PRIVATE KEY-----
- Old PEM private key: -----BEGIN RSA PRIVATE KEY-----

If you see something else, such as ---- BEGIN SSH2 PUBLIC KEY ---- (PuTTY/RFC 4716 format), the key needs to be converted or regenerated into OpenSSH format.

If you still have a valid private key but the public key file is corrupted or missing, regenerate the public key from it:

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

This extracts the public key from your private key in the correct single-line OpenSSH format. The new public key file will start with ssh-rsa (or ssh-ed25519). If the private key is passphrase-protected, you will be prompted for the passphrase.

If the key was edited on Windows or copied through certain tools, it may have a UTF-8 BOM (an invisible 3-byte sequence at the start of the file). Detect it:

hexdump -C ~/.ssh/id_rsa.pub | head -1

A leading ef bb bf indicates a BOM. Strip it with sed:

sed -i '1s/^\xef\xbb\xbf//' ~/.ssh/id_rsa.pub

After removing it, re-check the key and test SSH.

If you transferred the key from Windows, it may have CRLF (\r\n) line endings instead of LF (\n). Convert them:

# Using dos2unix
dos2unix ~/.ssh/id_rsa ~/.ssh/id_rsa.pub

# Or with sed
sed -i 's/\r$//' ~/.ssh/id_rsa
sed -i 's/\r$//' ~/.ssh/id_rsa.pub

Verify with file:

file ~/.ssh/id_rsa.pub

It should report ASCII text (or OpenSSH ... public key). If it reports ASCII text, with CRLF line terminators, the conversion did not take effect.

PuTTY's .ppk files are not in OpenSSH format. Use PuTTYgen (puttygen), the correct tool for converting them. ssh-keygen -i does not read .ppk files — it only imports RFC 4716 / PKCS#8 *public* keys — so do not use it here.

Using puttygen on the command line (Linux/macOS/WSL):

# Export the private key in OpenSSH format
puttygen id_rsa.ppk -O private-openssh -o ~/.ssh/id_rsa

# Export the matching public key
puttygen id_rsa.ppk -O public-openssh -o ~/.ssh/id_rsa.pub

Install puttygen first if needed (sudo apt install putty-tools on Debian/Ubuntu, brew install putty on macOS).

Using the PuTTYgen GUI (Windows):
1. Open PuTTYgen.
2. Click File -> Load private key and select your .ppk file (enter the passphrase if prompted).
3. Click Conversions -> Export OpenSSH key and save it as id_rsa (no .ppk extension).
4. Copy the contents of the "Public key for pasting into authorized_keys file" box into id_rsa.pub.

After converting, set the correct permissions (see the permissions step) before using the key.

If a tool rejects your private key because it expects the older PEM format (-----BEGIN RSA PRIVATE KEY-----) rather than the newer OpenSSH format (-----BEGIN OPENSSH PRIVATE KEY-----), convert it in place. ssh-keygen -p changes the on-disk format with the -m flag. The valid values for -m are RFC4716, PKCS8, and PEM.

Convert an existing private key to old PEM format:

ssh-keygen -p -f ~/.ssh/id_rsa -m PEM

You will be prompted for the current passphrase (if any) and a new one; press Enter to keep it unchanged. Make a backup of the key first (cp ~/.ssh/id_rsa ~/.ssh/id_rsa.bak) so you can recover if the conversion is not what you wanted.

If instead you need to (re)generate a key directly in PEM format:

ssh-keygen -t rsa -b 4096 -m PEM -f ~/.ssh/id_rsa

Note: generating a new key creates a *different* key pair, so you must re-add the new public key to every server's authorized_keys. Avoid -N "" (empty passphrase) for keys that protect real systems; use a passphrase and let ssh-agent cache it.

SSH refuses to use key files with overly permissive modes. Even when the format is correct, wrong permissions can cause failures:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub

Verify:

ls -la ~/.ssh/

The private key should show -rw------- (600) and the public key -rw-r--r-- (644).

After fixing the format, confirm the key parses cleanly:

ssh-keygen -l -f ~/.ssh/id_rsa.pub

This prints the key fingerprint, confirming the file is in a valid format. If it still reports "invalid format", the file is likely truncated or corrupted beyond repair and the key pair should be regenerated:

# Preferred: Ed25519
ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519 -C "your_email@example.com"

# Or RSA (4096-bit)
ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa -C "your_email@example.com"

Use a passphrase when prompted. Then add the new public key to each server's ~/.ssh/authorized_keys (for example with ssh-copy-id) and test the connection with ssh -v to confirm the key is now offered and accepted.