Dictionary: you'd need to guess my complex passphrase
Totally doable. You don't brute force the key, you brute force the passphrase that protects the key. Ask five eyes if password-protected GPG keys are impenetrable. A very large computer, smart algorithm, and good sigint can make short work of a "complex" passphrase. The difference between the password manager and memorized passwords is, if you crack the password manager, you have all the keys. If you memorize passwords, they have to intercept each key to compromise it. The most basic attack vector goes from "exfiltrate data one time" to "intercept all logins for a month".
Cold boot and evil maid also work better against someone who unlocks their gpg key for longer than a second, and extra code = extra possibility for bugs.
Comments
Totally doable. You don't brute force the key, you brute force the passphrase that protects the key. Ask five eyes if password-protected GPG keys are impenetrable. A very large computer, smart algorithm, and good sigint can make short work of a "complex" passphrase. The difference between the password manager and memorized passwords is, if you crack the password manager, you have all the keys. If you memorize passwords, they have to intercept each key to compromise it. The most basic attack vector goes from "exfiltrate data one time" to "intercept all logins for a month".
Cold boot and evil maid also work better against someone who unlocks their gpg key for longer than a second, and extra code = extra possibility for bugs.