use ssh-keygen -i
para converter o formato compatível com SSH2 para o formato compatível com OpenSSH.
de man ssh-keygen
:
-i This option will read an unencrypted private (or public) key file in SSH2-compatible format and print an OpenSSH compatible private (or public) key to stdout. ssh-keygen also reads the RFC 4716 SSH Public Key File Format. This option allows importing keys from several commercial SSH implementations.