The private key is stored password-protected in the registry. DPAPI, the API used to protect the private key, encrypted based on the user's Windows account pass

The private key is stored password-protected in the registry. DPAPI, the API used to protect the private key, encrypted based on the user's Windows account password, and allows decryption without further password entry by the already-logged-on user. There isn't an equivalent API in the Unix world that I'm aware of. When you fire-up the Windows SSH agent no password entry is necessary, because the password used to logon to Windows is also used to decrypt the stored private key. In a Unix environment you'd need to enter a passphrase to allow the SSH agent to decrypt the key.