it’s subject old as world (password-protected world, that is). i had to do some of cleanup on my devices and i hit a problem with 4096 bit keys.
so, just as a reference that may be helpful somewhere for someone - you import keys to Cisco IOS without any special problems:
for Cisco ASA, keys that are longer than 2048 bits need to be prepared using pkf format, as command line has limit of 512 bytes. so, to move key in OpenSSH compliant format like this one:
to new one, you need to convert it, by simply typing:
now this can be imported to Cisco ASA:
and you should be happy :)