How Can I Securely Manage SFTP Host Keys with pysftp and Paramiko?

Managing Host Keys for SFTP Using Pysftp: A Detailed Examination

Verifying the host key is a crucial step when establishing secure SFTP connections. While pysftp is a popular tool for managing SFTP connections, users have often encountered issues with host key handling. This article aims to dissect these issues and provide comprehensive solutions.

Pysftp relies on the Paramiko library for SSH functionality, including host key management. However, it is important to note that pysftp itself has certain limitations with host key handling. Additionally, the pysftp project appears to be inactive, leading some users to consider using Paramiko directly instead. Paramiko offers greater flexibility and control over host key management.

Understanding CnOpts and Trusted Host Keys

One solution to the host key issue is to leverage the CnOpts object in pysftp. CnOpts includes a hostkeys attribute that allows users to manage trusted host keys. By specifying a path to a file containing the server's public key, pysftp can validate the host key during the connection establishment process:

cnopts = pysftp.CnOpts(knownhosts='known_hosts')

with pysftp.Connection(host, username, password, cnopts=cnopts) as sftp:
    # ...

The 'known_hosts' file should contain the public key of the server in the following format:

example.com ssh-rsa AAAAB3NzaC1yc2EAAAADAQAB...

This approach ensures that pysftp verifies the server's host key against a known and trusted list.

Using a Direct Host Key

If maintaining an external host key file is not feasible, you can directly specify the host key in your code using Paramiko:

from base64 import decodebytes
# ...

keydata = b"""AAAAB3NzaC1yc2EAAAADAQAB..."""
key = paramiko.RSAKey(data=decodebytes(keydata))
cnopts = pysftp.CnOpts()
cnopts.hostkeys.add('example.com', 'ssh-rsa', key)

with pysftp.Connection(host, username, password, cnopts=cnopts) as sftp:
    # ...

However, this approach may trigger a warning in pysftp 0.2.9 due to a bug.

Retrieving the Host Key Externally

It is also possible to retrieve the host key automatically using the ssh-keyscan tool:

# Retrieve host key using ssh-keyscan
ssh-keyscan example.com

This command will output the host key in the required format. Beware that redirecting the output to a file in PowerShell may cause issues.

Cautions and Considerations

It is essential to exercise caution when dealing with host key verification. Disabling host key checking (setting cnopts.hostkeys = None) severely compromises security and should be avoided.

Additionally, remotely retrieving host keys can be risky as it makes you vulnerable to potential attacks. For enhanced security, obtain the host key directly from the server's administrator.

In conclusion, managing host keys for SFTP connections using pysftp requires careful consideration of the options available. By leveraging CnOpts and understanding the limitations and bugs associated with pysftp, users can implement secure and reliable host key verification mechanisms.

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