TL;DR: You handed the key to the wrong loader: an Ed25519 or ECDSA key given to RSAKey, or a PuTTY .ppk file. Use the matching key class (Ed25519Key, ECDSAKey, RSAKey) or convert the key with ssh-keygen/puttygen first.

```text
paramiko.ssh_exception.SSHException: not a valid RSA private key file
```

## Fix it

1. Identify the key type: head -1 ~/.ssh/[keyfile]. Expected: BEGIN OPENSSH PRIVATE KEY (could be any type), BEGIN RSA PRIVATE KEY, or PuTTY-User-Key-File-2.
2. Load with the right class: from paramiko import Ed25519Key, ECDSAKey, RSAKey; pick by type. Or let paramiko choose: key_filename=... in connect() tries each class automatically. Expected: loads cleanly.
3. For .ppk files: convert with puttygen key.ppk -O private-openssh -o key_rsa. Expected: an OpenSSH-format file paramiko reads.
4. If the key is encrypted, pass your passphrase via the password argument of RSAKey.from_private_key_file. Expected: loads.

## When this applies
- The SSHException names key validity at load time.
- The same file works with ssh (which sniffs the type).

## When it doesn't
- The error is AuthenticationException: the key loaded; the server rejected it.
- The error mentions bad passphrase/decryption: the file is valid but locked; supply the password.

## Compatibility
- paramiko 2.x/3.x. Ed25519 needs the bcrypt/nacl extras installed.

## Why it happens
Each key class parses only its own format. Guessing RSAKey for everything was common in old examples, and it breaks on modern default Ed25519 keys.

## Edge cases
- paramiko 3 dropped DSSKey entirely; DSA keys from ancient setups will not load at all.
- pip install paramiko[ed25519] if Ed25519Key import fails for missing deps.