How to convert Putty-generated public keys to standard OpenSSH format, suitable for insertion in authorized_keys file

convert-putty-pubkey.txt

ferdi@ferdi-U820:~$ cat public_test.ppk 
---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20220214"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---- END SSH2 PUBLIC KEY ----

ferdi@ferdi-U820:~$ ssh-keygen -i -f public_test.ppk 
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCE8JCObbX4s84AYZVHceFaK0F1O2a0ShptHVJh/jjAScNGfw2ViQ5rXXVT2AcJeDjSdhEqiFlSfLgquOv8p7nv2IlQRI03YD9T/FlitPMWQYX72ox1C2B8AG+FpdexqDyduc5y8hNakE5xZy5IQo7VK+LRMwA/5sTDYm20WSw4WCDDUnJdvHoICGab1pPnDf0qsCNL4uSeD7SurAZDeywUKlK0iYlRRnZvUuznY7HWyE6POf56ppyZRCXPkNx8jF2S/8KJz6K/2niS8Q1VqrHSKqxxh/SGyBDerFJC0pHhK3oyHghS8aNTniU4ya+PXqlzY209xcgS7SO7qOYa4goKPuPvllPALzc0YfbzLWZ0ki47FsH2QEJiQ1HkEMg2fjhJQ2zr54YJh/2lUrn9CzO2sfTLxa9Y649QHHF20uyQA/h1tym4Tqgkf9AO9a0jA20HXZevk6npTQg7OZC0c5GObUlnTdKpR1MO9JEM98zfYrTWbMj+cqFBgxTtfcencz0c+WPtTc0clS0krc5u2c4eK/TBbSO6rpMwpGU4KMrauBjEQdoMAp3OWP3XIErMzJBmvTZm8kXxMlPHW+lK0TrCX7014/jAfIDKDqXshb35qrPsaKjVMdbU1QKFbF22gBxBnwcm4SqKMhwhqgbaHJF5nnqm7OXB7J7qKbwxmEOiuw==