I found a private key which seems to be in the putty private key format.
The key has the following header:
---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ---- Subject: <sensitive> Comment: <sensitive> P2/56wAAA/MAAAA3a... ...Yhp ---- END SSH2 ENCRYPTED PRIVATE KEY ----
When opened in the putty keygen tool it prompts for a password which it does not do for other files so I assume some sort of validation is done on the PPK file. The same behavior with a PPK file that I generated using putty keygen.
I tried the PHRASENDRESCHER tool to guess private key passwords but this tool gives no output in case of an invalid/unknown file format. I tried generating my own PPK with a weak password and it did not crack that private key. So I assume the tool does not understand this type of private key.
Since putty keygen has no CLI, it is not possible to guess the password automatically. Does anyone have cracking tool that is able to guess the password (brute-force and/or word list) for PPK files?