Final answer:
The passphrase for an SSH key is set during key creation and is not retrievable if forgotten. A new key pair must be generated and the public key updated wherever necessary. Some managed services may offer recovery options through their support.
Step-by-step explanation:
The passphrase for an SSH key is typically created by the user when they generate the key pair using a tool like ssh-keygen. If you have lost or forgotten the passphrase, it cannot be retrieved because it is not stored anywhere for security reasons. The purpose of the passphrase is to secure the private key so that even if someone gains access to the key, they won't be able to use it without the passphrase.
To regain access, you would have to generate a new key pair and replace the public key on the systems you want to access. However, if the SSH key is being managed by software or a service (like a cloud provider or a corporate management system), the passphrase might be recoverable through their recovery processes or support teams.
Note that finding the passphrase is a matter of remembering it, using a password manager where it might have been saved, or consulting with the system administrator or service which manages the SSH keys if it's part of a larger infrastructure.