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When you share a single key instance across multiple threads, the underlying signing operations (handled by the cryptography backend) aren’t thread safe. This can result in the "key cannot be used for signing" error when concurrent threads try to use the same key instance simultaneously.

Why It Happens Non-Thread-Safe Operations: The cryptographic signing operation on a loaded key maintains internal state. When multiple threads try to sign with the same key concurrently, they interfere with each other, causing the signing method to fail.

Shared Key Instance: Even though the key itself is valid and supported, sharing the same key object across threads leads to race conditions. Each thread expects to work with a fresh, unaltered key state.

How to Fix It You have two main options to resolve this:

Load a Separate Key Instance Per Thread: Instead of sharing a single key instance, load the key separately in each thread. For example:

python
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import paramiko

def create_ssh_client(key_path, host, username):
    key = paramiko.RSAKey.from_private_key_file(key_path)
    client = paramiko.SSHClient()
    client.load_system_host_keys()
    client.connect(hostname=host, username=username, pkey=key)
    return client

Each thread calling create_ssh_client gets its own key instance, avoiding concurrent access issues.

Use a Lock to Serialize Key Usage: If reloading the key is too expensive or impractical, you can synchronize access to the shared key using a threading lock:

python
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import threading
import paramiko

key_lock = threading.Lock()
shared_key = paramiko.RSAKey.from_private_key_file("path/to/key")

def thread_safe_connect(host, username):
    with key_lock:
        client = paramiko.SSHClient()
        client.load_system_host_keys()
        client.connect(hostname=host, username=username, pkey=shared_key)
    return client

This ensures that only one thread uses the key at a time, though it may reduce parallelism.

Summary Even though your key is valid, the concurrent use of a single key instance leads to thread-safety issues during signing operations. The recommended approach is to either load a new key instance in each thread or to protect the shared key with a lock if you must reuse it.

By addressing the underlying thread-safety issue, you should be able to avoid the "key cannot be used for signing" error when making parallel SSH connections.

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